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UBC Theses and Dissertations

Some aspects of the population dynamics of the mountain pine beetle, Dendroctonus ponderosae in lodgepole… Peterman, Randall Martin 1974

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SOKE ASPECTS OF THE POPULATION DYNAMICS OF THE MOUNTAIN PINE B E E T L E , DEBCRCCTGNUS P C J E J B C S A E , IN LODGEPOLE P INE FORESTS OF B R I T I S H COLUMBIA by RANBAIL MARTIN PETERKAN B . S c , U n i v e r s i t y c f C a l i f o r n i a , D a v i s , U S A , 1970 A T H E S I S SUBMITTED IN PARTIAL F U L F I L L M E N T CF THE REQUIREMENTS FOB THE DEGREE CF DOCTOR OF PHILOSOPHY i n t h e D e p a r t m e n t o f Z o o l o g y We a c c e p t t h i s t h e s i s a s c o n f o r m i n g t o the r e q u i r e d s t a n d a r d THE UNIVERSITY OF B R I T I S H COLOMBIA JULY , 1974 In presenting th is thes is in par t ia l fu l f i lment of the requirements for an advanced degree at the Univers i ty of B r i t i s h Columbia, I agree that the L ibrary sha l l make it f ree ly ava i lab le for reference and study. I fur ther agree that permission for extensive copying of th is thesis for scho la r ly purposes may be granted by the Head of my Department or by his representat ives . It is understood that copying or pub l i ca t ion of th is thesis fo r f inanc ia l gain shal l not be allowed without my wri t ten permission. Depa rtmen t The Univers i ty of B r i t i s h Columbia Vancouver 8, Canada i Supervisor: Dr. Carl J. Walters ABSTRACT Outbreaks of mountain pine b e e t l e (Dendrcctonus £2£^5£osae Hopk.) are common i n lodgepole pine f o r e s t s cf western North America. C h a r a c t e r i s t i c s of both the tark b e e t l e and i t s host t r e e were compared using f i e l d r e p l i c a t e s c f epidemic and endemic areas to t e s t f o r any p o s s i b l e i n t r i n s i c d i f f e r e n c e s between p o p u l a t i o n s cf t r e e s cr i n s e c t s in these two d i f f e r e n t s t a t e s . l a b o r a t o r y s t u d i e s were conducted on b e e t l e d i s p e r s a l c h a r a c t e r i s t i c s and cn e f f e c t s of a t t a c k d e n s i t y and female parent s i z e on b e e t l e r e p r o d u c t i v e success and o f f s p r i n g s i z e . R e s u l t s are as f o l l o w s : Trees i n outbreak areas are o l d e r than i n endemic r e g i o n s , and t r e e s of a given s i z e and b e e t l e attack d e n s i t y are more l i k e l y t o be overcome and to permit s u c c e s s f u l b e e t l e r e p r o d u c t i o n i n epidemic than i n endemic areas. However, t r e e s p a t i a l d i s t r i b u t i o n s , average a t t a c k d e n s i t i e s and p r o p o r t i o n s of t r e e s u n s u c c e s s f u l l y attacked by b e e t l e s dc net d i f f e r c o n s i s t e n t l y between epidemic and endemic areas. A method ( i n which b l u e - s t a i n i n g f u n g i were i n o c u l a t e d i n t o trees) of measuring p o t e n t i a l of t r e e s t c r e s i s t mountain pine b e e t l e was t e s t e d and found to be inadequate. i i Epidemic and endemic bark beetles did not d i f f e r consistently in dispersal, size, cr reproductive c h a r a c t e r i s t i c s . However, early emerging beetles were larger than late emergers and females had a larger c o e f f i c i e n t of variation in size than males. F i e l d and laboratory data shew that the number of offspring emerging per parent decreases with increasing attack density. Breeding experiments further indicated that, 1) small female parents produce fewer and smaller offspring than large females, 2) small female parents produce female of f s p r i n g with more strongly bimcdal size d i s t r i b u t i o n s than large females, and 3) high parental attack densities r e s u l t in smaller offspring. Dispersal studies on the insect using chemical extracts of lodgepole pine bark showed that early emerging beetles are more l i k e l y to respond p o s i t i v e l y to tree chemicals than late emergers with the same f l i g h t history. Increasing lengths of f l i g h t increase female but not male responses to these chemicals. Evidence from a simulation model i s presented tc support the hypothesis that the age at which lodgepcle pine normally becomes susceptible to mountain pine beetle attack i s clcse to the age at which certain tree fitness measures are maximized. i i i TABLE OF CONTENTS ABSTRACT i TABLE OF CONTENTS i i i L IST OF FIGURES V LIST OF TABLES v i i i ACKNOWLEDGEMENTS X I. INTRODUCTION 1 A . Background 1 B. The Study 5 II. THE BARK BEETLES 8 A . G e n e r a l . . . . 8 B. The Mountain P ine B e e t l e 11 I I I . THE HOST TREE SPECIES, LODGEPOLE PINE 18 6. S y s t e m a t i c s And D i s t r i b u t i o n , 18 B . L i f e H i s t o r y 18 C . N a t u r a l Enemies 20 D. N a t u r a l Defenses 21 1. Types Of R e s i s t a n c e 21 2. F a c t o r s A f f e c t i n g R e s i s t a n c e Mechanisms . . . . . . . . . . . . 25 E . V a r i a b i l i t y In Host A t t r a c t i v e n e s s 29 IV. STUDY AREAS 34 V. FIELD EVALUATIONS OF TREES 39 A. Age 39 B. D e t e r m i n a t i o n Of Tree S t a t u s 39 i v C . S p a t i a l D i s t r i b u t i o n 41 D. A t tack D e n s i t i e s 4U E . P r e d i c t i n g T r e e S t a t u s 52 F . P r o p o r t i o n S u c c e s s f u l l y A t t a c k e d . . . . . . . . . . . . . . . . . . . . . 60 G. Funga l S a t i n g System 64 H. Summary 71 VI. SAMPLES OF FIELD BEETLES 73 V I I . BREEDING EXPERIMENTS . 84 I n t r o d u c t i o n 84 Methods 85 R e s u l t s 9 4 A . A t t a c k D e n s i t y E f f e c t On R e p r o d u c t i v e S u c c e s s . . . . . . . . 94 B . A t t a c k D e n s i t y E f f e c t On O f f s p r i n g S i z e . . . . . . . . . . . . . . 101 C . P a r e n t a l Female S i z e E f f e c t On R e p r o d u c t i v e S u c c e s s . . 101 D. P a r e n t a l Female S i z e E f f e c t s On O f f s p r i n g ~ S ^ z e . . . . . . . 109 E . Summary 129 V I I I . DISPERSAL 131 A . I n t r o d u c t i o n 131 B. Methods 135 C . R e s u l t s 145 D. D i s c u s s i o n 159 IX. GENERAL DISCUSSION 168 X. SPECULATIONS ON LODGEPOLE FITNESS 174 LITERATURE CITED , , 197 V LIST OF FIGURES FIGURE PAGE 1 A t y p i c a l mountain pine b e e t l e g a l l e r y system ....... 15 2 Map l o c a t i o n of the fou r study areas ................ 35 3 R e l a t i o n between p r o b a b i l i t y of t r e e s dying and att a c k d e n s i t y of b e e t l e s 48 4 D i s t r i b u t i o n of a t t a c k d e n s i t i e s on t r e e s by b e e t l e r e p r o d u c t i v e success c l a s s 50 5 The use of t r e e age and b e e t l e a t t a c k d e n s i t y f o r p r e d i c t i n g b e e t l e r e p r o d u c t i v e success .............. 53 6 The use of t r e e circumference and b e e t l e a t t a c k d e n s i t y f o r p r e d i c t i n g b e e t l e r e p r o d u c t i v e success .. 56 7 Data of F i g . 6 broken down by epidemic and endemic areas 58 8 Labor a t o r y emergence p a t t e r n s of a d u l t mountain pine b e e t l e s from l o g s sampled i n the f i e l d .............. 75 9 S i z e s of female mountain pine b e e t l e s i n r e l a t i o n to emergence date 77 10 S i z e s of male mountain pine b e e t l e s i n r e l a t i o n t o emergence date •• 79 11 Experimental design f o r 1973 breeding experiments ... 87 12 S p a t i a l arrangement of a t t a c k holes d r i l l e d f o r breeding experiment females 92 13 R e l a t i o n between numbers of b e e t l e s emerging and numbers of e x i t holes 97 1U F i e l d r e l a t i o n between a t t a c k d e n s i t y and numbers of emergence holes per a t t a c k ... 99 15 S i z e d i s t r i b u t i o n s o f o f f s p r i n g c l a s s e d by sex and attack d e n s i t y 104 v i 16 S i z e d i s t r i b u t i o n s of o f f s p r i n g c l a s s e d by sex and attack d e n s i t y , f o r average s i z e d parents 106 17 S i z e d i s t r i b u t i o n s of o f f s p r i n g c l a s s e d by sex and s i z e of female parents 113 18 S i z e d i s t r i b u t i o n s of female o f f s p r i n g c l a s s e d by s i z e of female parent and a t t a c k d e n s i t y 115 19 S i z e d i s t r i b u t i o n s of male o f f s p r i n g c l a s s e d by s i z e of female parent and a t t a c k d e n s i t y 117 20 S i z e s of female o f f s p r i n g i n r e l a t i o n to emergence date. Logs 1 -4 119 21 S i z e s of female o f f s p r i n g i n r e l a t i o n to emergence date. Logs 5-8 121 22 S i z e s of male o f f s p r i n g i n r e l a t i o n to emergence date, Logs 1-4 123 23 S i z e s of male o f f s p r i n g i n r e l a t i o n to emergence date. Logs 5-8 125 24 Test chamber f o r measuring responses of b e e t l e s t o chemical e x t r a c t s 136 25 Examples of paths followed i n the t e s t chamber by three t y p i c a l b e e t l e s 140 26 R e l a t i o n between response to t r e e e x t r a c t s and emergence date of b e e t l e 153 27 R e l a t i o n between d i s t a n c e flown and p r o p o r t i o n o f p o p u l a t i o n responding to t r e e chemicals 161 28 H y p o t h e t i c a l sequence of a t t a c k s by D. ponderosae and the D o u g l a s - f i r b e e t l e 163 29 R e l a t i o n f o r lodgepole pine between age and p r o p o r t i o n of t r e e s b e a r i n g cones ................... 178 30 R e l a t i o n f o r lodgepole pine between t r e e diameter and number of cones per t r e e 180 31 R e l a t i o n between average stand diameter and p r o p o r t i o n of t r e e s bearing cones ................... 182 v i i 32 R e l a t i o n between stand d e n s i t y and average diameters at given ages 184 33 Simulated r e l a t i o n between numbers of s a p l i n g s produced per grandparent and age at which stand becomes s u s c e p t i b l e .....189 34 Simulated r e l a t i o n between numbers of s a p l i n g s produced per t r e e - y e a r and s u s c e p t i b l e age ..........191 35 S a p l i n g s per reproducing grandparent using two d i f f e r e n t random sequences of f i r e s ................. 194 v i i i LIST OF TABLES TABLE PAGE 1 C h a r a c t e r i s t i c s of the fo u r study areas 37 2 S p a t i a l d i s t r i b u t i o n s of s u c c e s s f u l l y a t t a c k e d t r e e s 43 3 Comparisons of average a t t a c k d e n s i t i e s by area and by t r e e type 46 4 Status of a t t a c k e d t r e e s , d e f i n e d i n terms of b e e t l e r e p r o d u c t i v e s uccess, broken down by study area ..... 61 5 L o g i s t i c s of f u n g a l assay experiment, 1973 .......... 67 6 R e s i s t a n c e r a t i n g s of lodgepole pine t r e e s based on f u n g a l assay 68 7 B e e t l e b i o a s s a y of f u n g a l r a t i n g s of t r e e r e s i s t a n c e 70 8 Regression data f o r b e e t l e s i z e as a f u n c t i o n of emergence date ...................................... 82 9 S i z e d i s t r i b u t i o n s o f female parents used i n breeding experiments ................................ 89 10 Attack d e n s i t y e f f e c t s on r e p r o d u c t i v e success 95 11 Regression s t a t i s t i c s f o r f i e l d data on attack d e n s i t y e f f e c t s on r e p r o d u c t i v e success 102 12 A n a l y s i s of v a r i a n c e data f o r s i z e as a f u n c t i o n o f at t a c k d e n s i t y and r e p l i c a t e l o g number 103 13 E f f e c t s of s i z e of female parent on r e p r o d u c t i v e success 108 11 Table 13 broken down by a t t a c k d e n s i t i e s ............110 15 R e l a t i o n between o f f s p r i n g s i z e and female parent s i z e 111 16 Comparisons of s i z e d i s t r i b u t i o n s of o f f s p r i n g between r e p l i c a t e l o g s 128 i x 17 F l i g h t times used i n d i s p e r s a l experiments 143 18 Data on h a b i t u a t i o n of i n s e c t s t o d i s p e r s a l t e s t chamber ............................................. 146 19 M u l t i p l e r e g r e s s i o n data f o r d i s p e r s a l 148 20 Regression data f o r female d i s p e r s a l as a f u n c t i o n of emergence date ................................... 150 21 Regression data f o r male d i s p e r s a l as a f u n c t i o n of emergence d a t e . . . . . . . * 151 22 Data on d i f f e r e n c e s between r e g r e s s i o n l i n e s f o r female data i n F i g u r e 26 156 23 Data on d i f f e r e n c e s between r e g r e s s i o n l i n e s f o r male data i n F i g u r e 26 158 X ACKNOWLEDGEMENTS I would l i k e to extend my a p p r e c i a t i o n to the f o l l o w i n g people who helped me at some stage i n my f i e l d or l a b o r a t o r y work: C a r l Whitney, Akbar Syed, Mike M o r r e l l , and Karen Hudson. In a d d i t i o n , numerous members of the B.C. F o r e s t S e r v i c e and Canadian F o r e s t S e r v i c e lended a s s i s t a n c e . In p a r t i c u l a r , Drs. D.M. Shrimpton, L. S a f r a n y i k and H.S. Whitney proved i n v a l u a b l e as sources of background knowledge and i n f o r m a t i o n . Dr. K. Graham very k i n d l y permitted the use of his l a b o r a t o r y f o r the d i s p e r s a l t e s t s . No thanks would be too much f o r my s u p e r v i s o r . Dr. C.J. Walters, who made h e l p f u l suggestions at v a r i o u s p o i n t s i n the study. J. Anderson, and Drs. Walters, Graham, and W.G. W e l l i n g t o n commented on a t h e s i s d r a f t and these f o u r c o l l e a g u e s , along with Dr. C.S. H o l l i n g , provided numerous ideas and c o n s t r u c t i v e c r i t i c i s m throughout the study. F i n a n c i a l support f o r the f i a l d and l a b o r a t o r y work was pro v i d e d by N a t i o n a l Research C o u n c i l Operating Grant #675869-270 to Dr. C.J. Walters. 1 I. INTRODUCTION A. Background Ecologists have been interested for many years in animal populations that exhibit cycles in numbers. Numerous physical and biotic factors have been related to these population changes but no explanation has been completely convincing. Most of these theories have been related to changes in numbers of predators or parasites, food quality and quantity, or other limiting resources. Since the mid-1960's, increasing research effort has been put into two different topics related to population changes: environmental heterogeneity and within-population variation in characteristics. Morris (1971) and Morris and Fulton (1970a, 1970b) have shown that population genetic changes are closely related to population density in the f a l l webworra, H^£hantria cunea. In addition, the consequences of changes in maternal parent types for population trends have been illustrated by Wellington (1957, 1959, 1960, 196U) with the western tent caterpillar, and by Campbell (1962) with the spruce budworm. These workers have shown that certain important characteristics are heritable, either in the s t r i c t genetic sense or through maternally transmitted physiological effects. They suggest that in some species i t i s necessary to consider variation 2 within p o p u l a t i o n s i n order to o b t a i n a b e t t e r understanding of p o p u l a t i o n trends. Host t e r r e s t r i a l animals l i v e i n s p a t i a l l y and/or temporally heterogeneous environments. However, u n t i l r e c e n t l y , most e c o l o g i s t s have e i t h e r ignored h e t e r o g e n e i t y or t r i e d to c o n t r o l f o r i t i n the l a b o r a t o r y . Many animal adaptations might be more c l e a r l y understood by e x p l i c i t l y talcing t h i s h e t e r o g e n e i t y i n t o c o n s i d e r a t i o n . Some beginning t h e o r e t i c a l c o n s i d e r a t i o n s of the t o p i c have been made by MacArthur (1968), MacArthur and L e v i n s (1964), MacArthur and Pianka (1966), Pimentel et a l . (1963 , 1965) , Levins (1962, 1965, 1968), den Boer (1968), and L e v i n s and MacArthur (1966). A l l of these authors conclude that the p a t c h i n e s s or " g r a i n i n e s s " of the environment i n time and/or space can have a profound i n f l u e n c e on d e t e r m i n a t i o n of s t r a t e g i e s f o r s u r v i v a l . Some of these i n v e s t i g a t o r s d i s c u s s the s i g n i f i c a n c e of the d i s p e r s a l process f o r u t i l i z i n g heterogeneous environments. D i s p e r s a l (the a c t of moving to a new o v i p o s i t i o n or f e e d i n g s i t e before reproduction) has r e c e i v e d mainly d e s c r i p t i v e study. I t has been very w e l l d e s c r i b e d f o r many animals but i t has r a r e l y been looked a t i n terms of i t s e v o l u t i o n a r y c o n t e x t , i . e . , the s e l e c t i o n p r e s s u r e s that have caused the p a r t i c u l a r d i s p e r s a l c h a r a c t e r i s t i c s of the animal group to evolve as they have. In a d d i t i o n , l i t t l e i s known about how r a p i d l y d i s p e r s a l c h a r a c t e r i s t i c s change as a r e s u l t 3 of s e l e c t i o n p r e s s u r e s . W e l l i n g t o n (1957, 1959, 1960, 1964) Huffaker (1958), Huffaker, Shea, and Herman (1963), Huf f a k e r (1966 ) , and White and Huf faker (1969) have been among the few e c o l o g i s t s to use t h i s e v o l u t i o n a r y viewpoint i n de s i g n i n g f i e l d s t u d i e s . These works show t h a t d i s p e r s a l i s a key f e a t u r e i n determining how p o p u l a t i o n s w i l l behave through the course of time. In a d d i t i o n , Huffaker*s work p o i n t s out the importance o f s p a t i a l l y heterogeneous environments i n c r e a t i n g unique s i t u a t i o n s f o r the predator-prey system. The works of W e l l i n g t o n , H u f f a k e r , B i r c h ( 1 9 7 1 ) , den Boer(1968, 1971), and Green (1974) suggest t h a t g r o s s e r system behaviors such as o v e r a l l s t a b i l i t y of predator-prey systems can be more c l e a r l y understood by t a k i n g i n t o account d i s p e r s a l and heterogeneous environments. K i t c h i n g ( 1 9 7 1 ) , through the use of a s i m u l a t i o n model, has a l s o e x p l o r e d the r e l a t i o n s h i p s between some c h a r a c t e r i s t i c s of d i s p e r s a l , h eterogeneity i n the environment, and success a t c o l o n i z a t i o n . He too has found t h a t the r e s u l t s o f d i s p e r s a l are s i g n i f i c a n t l y a f f e c t e d by environmental h e t e r o g e n e i t y . Because of these i n d i c a t i o n s t h a t d i s p e r s a l processes and environmental h e t e r o g e n e i t y are l i n k e d , i t might be u s e f u l to look a t d i s p e r s a l as one ad a p t a t i o n of animals to s u r v i v i n g i n changing environments. Atkins(1966a), Southwood (1962), and Brown(1951), among ot h e r s , have shown t h a t those i n s e c t s p e c i e s which occupy temporary h a b i t a t s are g e n e r a l l y b e t t e r adapted to d i s p e r s i n g t o new h a b i t a t s than those s p e c i e s 4 i n h a b i t i n g more p e r s i s t e n t environments. For those adaptations whose s e l e c t i v e advantages are a f f e c t e d by s p a t i a l or temporal p a t t e r n i n g i n the environment, some wi t h i n - p o p u l a t i o n v a r i a b i l i t y i n those a d a p t a t i o n s may be r e q u i r e d t o i n s u r e p o p u l a t i o n p e r s i s t e n c e . An example of t h i s might be the s e a r c h i n g p a t t e r n of i n s e c t p r e d a t o r s . I f prey are u s u a l l y randomly d i s t r i b u t e d i n space, and i f p r e d a t o r s always search i n a random p a t t e r n , then the capture r a t e w i l l decrease i f prey should become h i g h l y clumped. In such a case, those i n d i v i d u a l p r e d a t o r s with s e a r c h i n g p a t t e r n s t h a t enabled e f f i c i e n t u t i l i z a t i o n of clumped prey would be at an advantage. Thus, t h e r e may be a s e l e c t i v e advantage i n m a i n t a i n i n g some phenotypes which are l e s s e f f i c i e n t under normal c o n d i t i o n s of prey d i s t r i b u t i o n i n order to i n s u r e some f l e x i b i l i t y f o r coping with f u t u r e changes i n s e l e c t i o n pressures or l i m i t i n g r e s o u r c e a v a i l a b i l i t y . The same might be true f o r d i s p e r s a l behaviors. Marked v a r i a t i o n i n d i s p e r s a l c h a r a c t e r i s t i c s has been observed w i t h i n i n s e c t p o p u l a t i o n s (e.g. A t k i n s , 1966b; Rose, 1972; Syad, 1972; W e l l i n g t o n , 1957, 1960) and t h i s v a r i a t i o n has been i n t e r p r e t e d as being advantageous f o r u t i l i z i n g s p a t i a l l y d i s c o n t i n u o u s and temporally unstable environments (Wellington, 1964). Some c h a r a c t e r i s t i c s of i n s e c t d i s p e r s a l t h a t might have evolved as a r e s u l t of c e r t a i n environmental p a t t e r n s a r e : y e a r l y t i m i n g of movement, d i r e c t i o n a l i t y and speed of 5 movement, t o t a l d i s t a n c e covered, and amount of a c t i v e c o n t r o l as opposed to p a s s i v e movement. A f a m i l i a r and extreme example i s the spruce budworm which d i s p e r s e s l a r g e l y p a s s i v e l y by wind. These and s i m i l a r i n s e c t s have not had to evolve a c t i v e d i s p e r s a l mechanisms because of high r e p r o d u c t i v e r a t e s and because of the p a r t i c u l a r s p a t i a l and temporal d i s t r i b u t i o n of t h e i r host t r e e s and of a p p r o p r i a t e weather c o n d i t i o n s . These p a t t e r n s i n time and space, though p r o b a b i l i s t i c i n occurrence, are c o n s i s t e n t enough to enable spruce budworm p o p u l a t i o n s to p e r s i s t . At another extreme are i n s e c t s p e c i e s t h a t have some a c t i v e p a r t i n determining t h e i r long range movements and t h a t r e l y on a t t r a c t i n g pheromones f o r l o c a t i o n of mates and s u i t a b l e hosts. B. The Study In order to i n v e s t i g a t e the e f f e c t of s p a t i a l l y and temporally heterogeneous environments on d i s p e r s a l s t r a t e g i e s , i t was necessary to choose a s i t u a t i o n where h a b i t a t changes which are s e l e c t i v e l y important f o r the animal occur with about the same p e r i o d i c i t y as the l i f e s p a n of the animal. I f the frequency of such temporal or s p a t i a l h a b i t a t changes were mush s h o r t e r than the l i f e s p a n , the animals would respond p h y s i o l o g i c a l l y and/or b e h a v i o r a l l y (for i n s t a n c e , f i s h a c c l i m a t i o n to new s a l i n i t y ) . I f the frequency were much longer than the l i f e s p a n , (e.g. c l i m a t i c change and i n s e c t f l i g h t temperature t h r e s h o l d s ) , a d a p t a t i o n s to changes might 6 be much too slow and gradual to be studied in a reasonable length of time. My experimental animals, bark beetles, live among habitats whose important selective characteristics (tree resistance) change every summer, coinciding with the beetle breeding season. These insects also exhibit drastic changes in numbers. Populations in different l o c a l i t i e s are at different points in their cycles at any given time and offer useful cases for comparison. In this study, I compared several characteristics of bark beetles and their host trees which were sampled from two types of areas: endemic (low insect numbers and low tree mortality) and epidemic (high insect numbers and high tree mortality). In particular, for each of the two population states, I investigated the patterning in time and space of suitable tree hosts, beetle reproductive success, beetle dispersal, and several other characters related to dispersal. I wanted to find out whether there were any in t r i n s i c differences between trees or beetles in an endemic area and those in an epidemic area. In addition, I attempted to answer the following questions: How much variability i s there in dispersal characteristics within a population? How closely do offspring resemble parents in these characteristics, and can the degree of resemblance change as the population climbs and drops in numbers? Are observed adaptations which are related to dispersal maintained by selection only when insect numbers are 7 low, or are these a d a p t a t i o n s necessary even i n times of epidemics? F i n a l l y , i s there any s i g n i f i c a n c e to the t r e e age at which i n s e c t outbreaks normally occur i n host t r e e stands? 8 I I . THE BARK BEETLES A. General The bark b e e t l e s (order C o l e o p t e r a , f a m i l y S c o l y t i d a e ) are i n s e c t s which provide a wealth of comparative i n f o r m a t i o n f o r answering some of the q u e s t i o n s posed i n S e c t i o n I. Furthermore, the a s s o c i a t i o n s of these i n s e c t s with t h e i r host t r e e s are f a i r l y w e l l understood and are amenable to obse r v a t i o n and experimental manipulation. Bark b e e t l e l i f e h i s t o r i e s can g e n e r a l l y be d e s c r i b e d as f o l l o w s . Female a d u l t s normally a t t a c k l o g s or t r e e s i n summer or l a t e s p r i n g . They bore g a l l e r i e s underneath the bark and l a y eggs p e r i o d i c a l l y along the g a l l e r y w a l l s as they d i g . The eggs hatch, the l a r v a e extend p e r i p h e r a l g a l l e r i e s some d i s t a n c e , and then they pupate. In many bark b e e t l e s p e c i e s , l a r v a e o r pupae are the main o v e r w i n t e r i n g stage and new a d u l t s do not emerge from the t r e e to a t t a c k new ho s t s u n t i l the f o l l o w i n g s p r i n g or summer. In some ambrosia b e e t l e s , a wood-boring group w i t h i n the s c o l y t i d s , a d u l t s emerge the summer of p a r e n t a l a t t a c k and f l y t o o v e r w i n t e r i n g s i t e s i n the f o r e s t l i t t e r . Bark b e e t l e s i n g e n e r a l a t t a c k hosts v a r y i n g i n s t a t e s of he a l t h from t r e e s f a l l e n months p r e v i o u s l y , e i t h e r by wind or 9 man, to standing and apparently healthy t r e e s . Each s p e c i e s c o n c e n t r a t e s on t r e e s i n a p a r t i c u l a r p h y s i o l o g i c a l s t a t e of h e a l t h , which i s u s u a l l y subnormal (Graham, 1963; Hudinsky, 1962), because a l l t r e e s of a given s p e c i e s are not s u i t a b l e for b e e t l e r e p r o d u c t i o n or growth of t h e i r s y m b i o t i c f u n g i (Graham, 1967; Budinsky, 1962). P r e f e r e n c e of bark b e e t l e s for t r e e s with c e r t a i n p h y s i o l o g i c a l s t a t e s l e a d s to an i n t e r p r e t a t i o n of the r o l e of these i n s e c t s i n f o r e s t communities as n a t u r a l pruners or t h i n n e r s (Graham, 1963; Smithers, 1962). T h i s i s e s p e c i a l l y t r u e of those s p e c i e s which normally a t t a c k t r e e s t h a t are s t i l l s t a n ding and are competing f o r l i g h t and n u t r i e n t s , but which are i n some way p h y s i o l o g i c a l l y weakened. when present i n a t r e e stand i n low numbers, bark b e e t l e s p r e f e r e n t i a l l y remove such t r e e s , decreasing the c o m p e t i t i o n among remaining t r e e s . S u c c e s s f u l l y a t t a c k e d t r e e s (ones t h a t succumb and produce b e e t l e o f f s p r i n g ) are g e n e r a l l y not re-used i n subsequent years by the same bark b e e t l e s p e c i e s , so new t r e e s must be sought each year. A l s o , t r e e s of the p r e f e r r e d s t a t e of h e a l t h are not u s u a l l y s p a t i a l l y d i s t r i b u t e d i n a uniform manner, so many bark b e e t l e s have evolved two types of complex b e h a v i o r a l mechanisms f o r l o c a t i n g t r e e s . The f i r s t mechanism depends upon d i f f e r e n c e s i n t r e e chemicals g e n e r a l l y r e f e r r e d to as primary a t t r a c t a n t s ( F r a n c i a and Graham, 1966; Graham 1959; Graham and Werner, 1956; Benwick and V i t e , 1970). Once s u i t a b l e t r e e s are found, other i n d i v i d u a l s i n the p o p u l a t i o n 10 are a t t r a c t e d to these s i t e s by another s e t of complex a d a p t a t i o n s , u s u a l l y i n v o l v i n g pheromones and g e n e r a l l y c l a s s e d as secondary a t t r a c t a n t s (Renwick and V i t e , 1970; many o t h e r s ) . S i n c e a t t a c k s on s i n g l e t r e e s u s u a l l y occur almost si m u l t a n e o u s l y by many members of a p o p u l a t i o n , owing to secondary a t t r a c t a n t s , even he a l t h y t r e e s can be k i l l e d . Such mass i n s e c t a t t a c k s appear to be a d a p t a t i o n s to overcoming n a t u r a l defense mechanisms of t r e e s such as o l e o r e s i n exudation (Cobb et a l . , 1968; Reid 1962a,1963; V i t e and Wood, 1961). L a t e r , I w i l l d i s c u s s the t r a d e o f f s i n v o l v e d i n mass at t a c k s between i n c r e a s i n g p r o b a b i l i t y of overcoming a t r e e ' s r e s i s t a n c e and d e c r e a s i n g success o f b e e t l e r e p r o d u c t i o n . Bark b e e t l e s are a s s o c i a t e d with symbiotic b l u e - s t a i n i n g f u n g i , which are hypothesized to f u n c t i o n i n h e l p i n g overcome t r e e defense mechanisms, thereby i n s u r i n g s u c c e s s f u l b e e t l e r e p r o d u c t i o n (Graham, 1967; Reid et a l . , 1967; S a f r a n y i k e t a l . , 1974a). The i n s e c t s have s p e c i a l m orphological a d a p t a t i o n s f o r c a r r y i n g f u n g a l spores and mycelia with which to i n f e c t newly a t t a c k e d t r e e s ( F a r r i s , 1963; Graham, 1967; Whitney and F a r r i s , 1969).\ Bark b e e t l e s are e s p e c i a l l y w e l l - s u i t e d f o r d i s p e r s a l s t u d i e s because t h e i r d i s p e r s a l - a n d - a t t a c k phase i s u s u a l l y contained within a s h o r t p e r i o d o f the year, the r e s t of the time being spent i n s i d e the t r e e s o r i n the f o r e s t l i t t e r . A l s o , the s c o l y t i d s e x h i b i t a wide range of d i s p e r s a l s t r a t e g i e s and t a c t i c s , probably owing to the v a r i o u s s t a t e s 11 of h e a l t h of t r e e s a t t a c k e d . I t i s even p o s s i b l e tha t the wi le range of s t a t e s o f t r e e d e t e r i o r a t i o n u t l i l i z e d by d i f f e r e n t s c o l y t i d s i s a c h a r a c t e r d i v e r g e n c e r e s u l t i n g from c o m p e t i t i o n among a n c e s t r a l members of the group . A l s o , the d i f f e r e n c e i n d i s p e r s a l b e h a v i o r between i n s e c t s i n outbreak c o n d i t i o n s might be d i f f e r e n t f rom the b e h a v i o r o f i n s e c t s at a low p o p u l a t i o n l e v e l . T h i s l a s t t o p i c seemed t o be a more f r u i t f u l compar ison because of the problems of comparing d i f f e r e n t s p e c i e s , and i t i s t h e r e f o r e the main l i n e I have f o l l o w e d throughout the s t u d y . B. The Mountain Pine B e e t l e The mountain p ine b e e t l e , Dendroctonus £ o n d e r o s a e Hopkins (= D. roonticolae Hopkins) was chosen f o r i n t e n s i v e s t u d y . T h i s bark b e e t l e i s a n a t i v e of North Amer ica and has been a r e p o r t e d pest s i n c e the e a r l y 1 9 0 0 » s (Powe l l , 1961). It p r e f e r s l o d g e p o l e p ine (Pinus c o n t o r t a Douglas v a r . l a t i f o l i a E n g l . ) as a host but a l s o i n f e s t s ponderosa p i n e (Pinus j3onderosa L a w s ) , western whi te p ine (P inus m o n t i c o l a Dougl . ) and Engelmann s p r u c e , (P icea engelmanni P a r r y ) , ( P o w e l l , 1961; E e i d , 1962a,1962b; Cobb et a l . , 1968). Desp i te i t s s m a l l s i z e (3 to 4 mm i n l e n g t h ) , t h i s i n s e c t a c c o u n t s f o r l a r g e volume l o s s e s of l o d g e p o l e p i n e every y e a r . In western Canada a l o n e , 1.3 m i l l i o n cu f t o f t imber were l o s t per year from 1950 to 1970, a p p r o x i m a t e l y 3$ o f the average annua l cu t (Sa f rany ik e t a l . , 1974a) . 12 The l i f e h i s t o r y of the mountain pine b e e t l e i s s i m i l a r to t h a t of most bark b e e t l e s . a l l a d u l t s c a r r y s y m b i o t i c b l u e - s t a i n i n g f u n g i ( C e r a t o c y s t i s montia (Rumb.) Hunt and an u n i d e n t i f i e d Euroj)hium species) (Robinson, 1962) . a d u l t s and l a r v a e feed on wood as w e l l as on phloem as they d i g g a l l e r i e s i n the outer sap wood-phloem r e g i o n of the host t r e e . Females do the i n i t i a l s e a r c h i n g f o r host t r e e s and a l l of the g a l l e r y d i g g i n g . D i s p e r s a l and a t t a c k on p o t e n t i a l h o s t s occurs i n midsummer (July and August) on days when ambient temperature exceeds about 60« F (16° C) (Reid, 1962a; Shepherd, 1966). B e e t l e s emerging from o v e r w i n t e r i n g s i t e s i n t r e e s are p h o t o p o s i t i v e (Reid, 1962a; Shepherd, 1966) and they f l y to p r e f e r e n t i a l l y a t t a c k l a r g e , r e l a t i v e l y healthy t r e e s (Amman, 1969; Cole and Amman, 1969; Reid, 1963; Roe and Amman, 1970). Preference f o r what appear t o be the most v i g o r o u s t r e e s seems to be r e l a t e d t o moisture content and t h i c k n e s s of the phloem, the r e g i o n where most g a l l e r y c o n s t r u c t i o n occurs (Amman, 1969, 1972; Reid, 1962b, 1963, 1969; Roe and Amman, 1970; Sa f r a n y i k and V i t h a y a s a i , 1971). Trees weakened by drought or dis e a s e are a l s o attacked, but they are normally avoided (Cobb et a l . , 1968; V i t e and wood, 1961). Highest c o n c e n t r a t i o n s of mountain pine b e e t l e a t t a c k s are at the bases of t r e e s , and decrease with i n c r e a s i n g h e i g h t (Reid, 1963; S a f r a n y i k and V i t h a y a s a i , 1971). L i k e other bark b e e t l e s a t t a c k i n g s t a n d i n g t r e e s , D. ponderosae has to fa c e the defense mechanisms of the t r e e s . 1 3 These tr e e r e s i s t a n c e responses, to be d e s c r i b e d i n d e t a i l i n the s e c t i o n on lodgepole n a t u r a l defenses, c o n s i s t mainly of p r o d u c t i o n of r e s i n s which can cause m o r t a l i t y of a t t a c k i n g bark b e e t l e s . The e f f e c t i v e n e s s of r e s i s t a n c e responses i s reduced by mass a t t a c k s . A l a r g e b e e t l e p o p u l a t i o n i s a t t r a c t e d to a t r e e through a secondary a t t r a c t a n t system d e s c r i b e d by Pitman and V i t e (1969), Pitman et a l . (1969), Renwick and V i t e (1970), and V i t e and Pitman (1968). Once a female begins to d i g i n a s u i t a b l e t r e e , a pheromone (trans-verbenol) i s r e l e a s e d which, in c o n j u n c t i o n with a v o l a t i l e t r e e terpene, alpha-pinene, a t t r a c t s more males and females t o the s i t e . The sex r a t i o of b e e t l e s a t t r a c t e d depends on the r e l a t i v e c o n c e n t r a t i o n s of the pheromone and the t r e e terpene. As the female-produced pheromone becomes more predominant, the p r o p o r t i o n of males a r r i v i n g i n c r e a s e s . Both males and females w i l l respond to the pheromone o n l y i n the presence of alpha-pinene (Renwick and V i t e , 1970). T h e r e f o r e , when r e s i n exudation and copious terpene r e l e a s e stop, there are no more new a t t a c k s . Once g a l l e r i e s are s u c c e s s f u l l y e s t a b l i s h e d , mating occurs i n them and the male f r e q u e n t l y goes to another g a l l e r y and mates again (Reid, 1958b). T h i s polygamous behavior r e s u l t s i n sex r a t i o s of about one male f o r every two females i n each t r e e (Reid, 1958b). Between f o r t y and seventy eggs are normally l a i d by a female i n one g a l l e r y system, and, depending on sapwood moisture and ambient weather c o n d i t i o n s , 14 she may emerge and a t t a c k another t r e e before the summer i s over (Reid, 1958b f 1962b). During each egg l a y i n g p e r i o d , f l i g h t muscles degenerate, as i n other s c o l y t i d s (Reid, 1958a). T h i s may be due to r e c h a n n e l l i n g of energy t o the re p r o d u c t i v e system. Toward the end of egg l a y i n g , f l i g h t muscles regenerate to enable a d u l t s to emerge and r e a t t a c k new t r e e s . Adult g a l l e r i e s are as long as 18 inc h e s and are more or l e s s v e r t i c a l l y o r i e n t e d . Larvae d i g t h e i r g a l l e r i e s h o r i z o n t a l l y , as much as 6 i n c h e s away from the parent g a l l e r y ( F i g . 1). Larvae go through f o u r i n s t a r s and c o n s t i t u t e the main o v e r w i n t e r i n g stage (underneath the b a r k ) , although pupae o c c a s i o n a l l y go i n t o the winter (Reid, 1962a, 1962b). T h i s i s a c r i t i c a l stage f o r s e l e c t i o n i n t h i s b e e t l e because the e a r l i e r i n the summer a brood i s e s t a b l i s h e d , the l a r g e r i s the p r o p o r t i o n of o f f s p r i n g r e a c h i n g pupal and t e n e r a l a d u l t stages during the winter. These are the l e a s t c o l d r e s i s t a n t stages, so grea t e r b e e t l e m o r t a l i t y occurs i n broods e s t a b l i s h e d e a r l y i n long, warm summers (Reid, 1963). T h i s phenomenon has important i m p l i c a t i o n s f o r the theory that many mountain pine b e e t l e outbreaks occur during or immediately f o l l o w i n g s e v e r a l years of drought (Smithers, 1962; o t h e r s ) . Harm, dry summers reduce the e f f e c t i v e n e s s of t r e e r e s i s t a n c e and enhance brood development (Reid, 1963). A l s o , when t h e r e i s an u n u s u a l l y warm and dry s p r i n g and summer, the p r o b a b i l i t y of each female*s s u c c e s s f u l l y s t a r t i n g two broods 15 FIGURE 1 A t y p i c a l mountain pine b e e t l e g a l l e r y system with the a d u l t , or egg, g a l l e r y v e r t i c a l l y o r i e n t e d and the l a r v a l g a l l e r i e s extending h o r i z o n t a l l y away from the p a r e n t a l g a l l e r y . 1 6 1 7 i n c r e a s e s because of unusually e a r l y emergence. However, winter s u r v i v a l of the f i r s t brood may be very low i f summer temperatures are high f o r a l o n g p e r i o d and i f the winter i s as c o l d as normal. T h e r e f o r e , depending on the f e c u n d i t y of each female i n her second g a l l e r y , t h e r e may a c t u a l l y be a decrease i n D. fionderosae p o p u l a t i o n s a f t e r very l o n g , warm summers. Another important f a c t o r f o r brood s u r v i v a l i s i n i t i a l a t t a c k d e n s i t y . Reid (1963) and Cole (1962) have found t h a t a d u l t g a l l e r y l e n g t h , egg p r o d u c t i o n per female, and l a r v a l s u r v i v a l per a d u l t become reduced with i n c r e a s i n g a t t a c k d e n s i t y owing to i n t r a s p e c i f i c c o m p e t i t i o n . T h i s e f f e c t of a t t a c k d e n s i t y i s s i m i l a r to the f i n d i n g of flcHullen and A t k i n s (1961) f o r the D o u g l a s - f i r b e e t l e . 18 I I I . THE HOST TREE SPECIES, LODGEPOLE PINE A. Systematics And D i s t r i b u t i o n The p r e f e r r e d host s p e c i e s of mountain pine b e e t l e i s lodgepole pine (Pinus c o n t o r t a Douglas var. l a t i f o l i a E n g l . ) , a n a t i v e North American c o n i f e r . . T h i s v a r i e t y of lodgepole pine i n h a b i t s i n t e r i o r and mountain r e g i o n s of B r i t i s h Columbia, A l b e r t a and some of the western United S t a t e s . There i s great v a r i a t i o n i n the c h a r a c t e r i s t i c s o f lodgepole pine throughout i t s range ( C r i t c h f i e l d , 1957; Smithers, 1956,1962; Roche, 1962); the i n t e r i o r form d i f f e r s remarkably from both the c o a s t a l (or "shore") and S i e r r a Nevada forms. Since the mountain pine b e e t l e i s normally a s s o c i a t e d only with the i n t e r i o r form, a l l f u r t h e r d i s c u s s i o n s of lodgepole pine w i l l p e r t a i n o n l y to t h i s i n t e r i o r v a r i e t y . In p a r t i c u l a r , I w i l l d i s c u s s c h a r a c t e r i s t i c s of lodgepole found i n the Okanagan V a l l e y area and the Rocky Hountain r e g i o n of B. C. and A l b e r t a . B. L i f e H i s t o r y I n t e r i o r l odgepole u s u a l l y i n h a b i t s r e g i o n s t h a t have warm, dry summers and high f i r e hazards due to l i g h t n i n g storms (Armit, 1966). As might be expected f o r t r e e s e v o l v i n g i n such areas, l o d g e p o l e i s w e l l adapted to r e g e n e r a t i n g 19 s u c c e s s f u l l y a f t e r f i r e s (Armit, 1966); Smithers, 1962). I t produces s e r o t i n o u s , or c l o s e d , cones which remain on the t r e e and r e t a i n seeds u n t i l high temperatures (113-122° F or 45-50° C) and low humidity are achieved (Bates, 1930). These c o n d i t i o n s normally are met on l y during a f o r e s t f i r e , although some s e r o t i n o u s cones do open from heating by the sun (Cro s s l e y , 1956b; T a c k l e , 1954). V i a b l e seeds have been found i n cones as much as 75 years o l d (Mason, 1915). Host seeds r e l e a s e d from s e r o t i n o u s cones germinate i n the s p r i n g f o l l o w i n g the summer i n which the f i r e o c c u r s (Eoe, 1956). Clements(1910) has a l s o noted other b e n e f i t s of f i r e : s e ed-eating rodents such as s q u i r r e l s and chipmunks are t e m p o r a r i l y e l i m i n a t e d , the o v e r s t o r y i s opened up to g i v e more s u n l i g h t to the s h a d e - i n t o l e r a n t s a p l i n g s , other competing ground-cover s p e c i e s are destroyed, and ground l i t t e r accumulations are burned and removed, l e a v i n g a more s u i t a b l e bed f o r seed germination and growth. Lodgepole pine i s r e l a t i v e l y s h a d e - i n t o l e r a n t , that i s , young t r e e s do not grow w e l l when underneath an o v e r s t o r y of other t r e e s , even of i t s own s p e c i e s (Clements, 19 10; Fowells, 1965). However, i n open areas, young lodgepole have higher growth r a t e s and can outcompete other t r e e s p e c i e s such as aspen, white spruce or hemlock (Horton, 1956; o t h e r s ) . As a r e s u l t of f i r e - r e l e a s e d seeds, shade i n t o l e r a n c e , and f a s t s a p l i n g growth, l o d g e p o l e tends to produce pure, even-aged stands appearing remarkably uniform over wide areas (Smithers, 20 1962). L a t e r , I w i l l d i s c u s s the i m p l i c a t i o n s of t h i s even-age phenomenon f o r mountain pine b e e t l e p o p u l a t i o n s . Lodgepole pine i s a l s o noted f o r i t s tendency to produce densely stocked stands which r e s u l t i n smaller t r e e s (Smithers, 1962) • ^  Under normal s t o c k i n g d e n s i t i e s (about 250-500 t r e e s per a c r e ) , 90-year-old t r e e s have an average d.b.h. (diameter at breast height) of 10.5 i n c h e s , and 65-year-old t r e e s average 8.5 inches (Smithers, 1962). For overstocked stands, Trappe and H a r r i s (1958) found a 65-year-old stand with 10,000 t r e e s per acre and an average diameter of 2 i n c h e s , and Smithers(1962) found a 9 0 - y e a r - o l d stand with 3010 t r e e s / a c r e and an average d.b.h. of 3.2 i n . Other stands have been seen to range to an extreme of 500,000 t r e e s per acre (Smithers, 1962). Such overstocked stands tend to stagnate, or stop t h e i r growth, at an e a r l y age, and lodgepole i s the most l i k e l y t o stagnate of any North American tree (Fowells, 1965). Seed p r o d u c t i o n by lodgepole i s very s e n s i t i v e to crowding so t h i s c h a r a c t e r i s t i c of o v e r s t o c k i n g i s important when c o n s i d e r i n g the r e p r o d u c t i v e p o t e n t i a l of stands of d i f f e r e n t ages k i l l e d by mountain pine b e e t l e . The e v o l u t i o n a r y s i g n i f i c a n c e of t h i s t o p i c w i l l be considered along with a d e t a i l e d d e s c r i p t i o n of lodgepole r e p r o d u c t i o n i n S e c t i o n X. C, N a t u r a l Enemies Hountain pine b e e t l e i s the major pest s p e c i e s of 21 lodgepole pine but other i n s e c t s a l s o a t t a c k t h i s t r e e , only r a r e l y i n d u c i n g m o r t a l i t y . Some of the more common s p e c i e s are the lodgepole needle miner ( C o l e q t e c h n i t e s s t a r k i ) , j a c k - p i n e budworm (Choristoneura p i n u s ) , l o d g e p o l e sawfly (Naodi^rion b u r k e i ) , t e r m i n a l w e e v i l (Pissodes t e r m i n a l i s ) , and pine l o o p e r s (Cari£eta spp.) {Armit, 1966; Powells, 1965). Dwarf m i s t l e t o e (A££euthobium americanum) p r e f e r s v i g o r o u s h o s t s (Powells, 1965) and redheart s t a i n ( s e v e r a l f u n g i of the genera Stereum and Fomes ) i s most common i n o l d e r t r e e s (Armit, 1966) . Mountain pine b e e t l e s normally k i l l host t r e e s through d e s t r u c t i o n of phloem and outer sapwood (Reid, 1962a). A d u l t and l a r v a l g a l l e r i e s sever c e l l s i n these r e g i o n s , r e d u c i n g moisture t r a n s p o r t w i t h i n t h e t r e e . Although t h i s g i r d l i n g a c t i o n i s probably the most s i g n i f i c a n t cause of t r e e death, the bark b e e t l e ' s s y m b i o t i c b l u e - s t a i n i n g f u n g i are known to be pathogenic to l o d g e p o l e , even i n the absence of mountain pine b e e t l e (Reid et a l . , 1967 and many r e f e r e n c e s t h e r e i n ) . D. N a t u r a l Defenses 1. Types of r e s i s t a n c e Lodgepole has two l e v e l s of defense mechanisms a g a i n s t the mountain pine b e e t l e and i t s a s s o c i a t e d b l u e - s t a i n i n g f u n g i . The f i r s t mechanism i n v o l v e s a f a s t - a c t i n g but passive 22 system which i s present even without b e e t l e a t t a c k while the second i s a delayed but a c t i v e l y produced response to wounding. The p a s s i v e mechanism i s known as primary r e s i n o s i s (Berryman, 1972; Reid et a l . , 1967). When a bark b e e t l e c u t s through the bark, v e r t i c a l and h o r i z o n t a l r e s i n ducts are severed. The p i t c h which flows out of these c a n a l s by f o r c e of g r a v i t y pours over the i n s e c t and, i f copious enough, can e i t h e r k i l l or e x p e l i t . T h i s i n i t i a l r e s i n flow begins almost immediately a f t e r a duct i s c u t . The success of primary r e s i n o s i s i s a f f e c t e d by s e v e r a l r e s i n c h a r a c t e r i s t i c s : 1) exudation p r e s s u r e , 2) r a t e and q u a n t i t y of flow, 3) chemical composition, and 4) r a t e of c r y s t a l l i z a t i o n (Reid, 1963). Resin produced i n response to an a t t a c k o f t e n flows out the b e e t l e ' s entrance hole and hardens i n t o a plug which stands out at r i g h t angles to the bark and can r e a d i l y be seen a t a d i s t a n c e . The presence of such a p i t c h tube does not n e c e s s a r i l y i n d i c a t e t hat the a t t a c k i n g bark b e e t l e was k i l l e d or e x p e l l e d , although b e e t l e s can sometimes be found embedded i n them. I f the bark b e e t l e s u r v i v e s the i n i t i a l onslaught of r e s i n , the t r e e responds f u r t h e r with a delayed system known as secondary r e s i n o s i s (Reid e t a l . , 1967) or h y p e r s e n s i t i v e r e a c t i o n (Berryman, 1972)., As the female bark b e e t l e moves along (at about 1.3 to 2 cm per day; Reid, 1962b) d i g g i n g i t s g a l l e r y and l a y i n g eggs, the t r e e c e l l s surrounding the g a l l e r y begin to change p h y s i o l o g i c a l l y . Bark and sapwood 23 parenchyma c e l l s become swollen with r e s i n (not t h e i r normal f u n c t i o n ) and many of these c e l l s b u r s t and r e l e a s e t h e i r r e s i n i n t o the a d u l t i n s e c t ' s g a l l e r y (Reid et a l . , 1967). T h i s r e s i n i s t o x i c t o both mountain pine b e e t l e eggs (Reid and Gates, 1970) and to the s y m b i o t i c f u n g i (shrimpton and Whitney, 1968; Reid et a l . , 1967). I f the secondary r e s i n o s i s response i s too weak, the bark b e e t l e c o n t i n u e s g a l l e r y c o n s t r u c t i o n and the pathogenic f u n g i continue to spread i n the phloem of the t r e e . S u c c e s s f u l primary r e s i n o s i s e i t h e r k i l l s the mountain pine b e e t l e or causes the l i v e female to back out of the g a l l e r y and seek another t r e e . In e i t h e r case, the pathogenic b l u e - s t a i n i n g f u n g i are c o n t a i n e d (Reid et a l . , 1967). However, when a t r e e e xpels an a d u l t female, t h e r e i s a f i n i t e p r o b a b i l i t y that the i n s e c t w i l l go t o a nearby t r e e , s u c c e s s f u l l y reproduce, and thereby provide new i n s e c t s to a t t a c k the o r i g i n a l e x p e l l i n g t r e e the f o l l o w i n g year. I t would t h e r e f o r e be b e t t e r f o r t r e e s to k i l l a l l a d u l t bark b e e t l e s as they enter the bark. However, there i s a disadvantage i n having a very strong and r a p i d primary r e s i n o s i s response because a bark b e e t l e may have a g r e a t e r chance of escaping unharmed. I t w i l l have dug a s h o r t e r g a l l e r y and w i l l have a lower p r o b a b i l i t y of being trapped by r e s i n . A more delayed t r e e response would seem more p r o p i t i o u s . T h i s i s where secondary r e s i n o s i s might become important. 24 T h i s r e a c t i o n i s delayed u n t i l s e v e r a l days a f t e r g a l l e r y d i g g i n g i s begun (Reid et a l . , 1967). a d u l t b e e t l e s do not f l e e the g a l l e r y even i f t h i s secondary t r e e response i s s u c c e s s f u l i n k i l l i n g eggs and l a r v a e . The bark b e e t l e s behave as though r e p r o d u c t i o n were proceeding normally, which i s s i m i l a r to the e f f e c t of the s t e r i l e male technique of c o n t r o l l i n g screw-worm f l y (Bushland et a l . , 1955). Strong secondary r e s i n o s i s , when combined with weak primary r e s i n o s i s , thus appears to be the most p r o f i t a b l e t r e e r e s i s t a n c e mechanism because a d u l t s are trapped and do not reproduce. One must remember, however, t h a t the primary system i s l a r g e l y a passive one and i s not e a s i l y c o n t r o l l a b l e , whereas an a c t i v e c h a n n e l l i n g of energy i s r e q u i r e d t o produce the secondary response. a l s o , by r e l y i n g on the delayed response, t h e r e may be too great a r i s k i n l e t t i n g the bark b e e t l e and i t s a s s o c i a t e d pathogenic b l u e - s t a i n i n g f u n g i get a f o o t h o l d ; u n p r e d i c t a b l e events which a f f e c t t r e e p h y s i o l o g y r a p i d l y , such as weather, may f r e q u e n t l y prevent t r e e s i n t h i s tenuous s t a t e of balance from c o n t a i n i n g the in v a d i n g s p e c i e s . R a t i o n a l arguments can t h e r e f o r e be made f o r the predominance of e i t h e r t r e e r e s i s t a n c e mechanism i n nature, and r e l e v a n t data w i l l be presented l a t e r . Both primary and secondary r e s i n o s i s can be i n i t i a t e d by i n o c u l a t i n g b l u e - s t a i n i n g f u n g i i n the absence of bark b e e t l e s (Reid and Shrimpton, 1971; Reid e t a l . , 1967). T h i s 25 phenomenon has l e d to the development of a system f o r r a t i n g the p o t e n t i a l of l odgepole pine t o r e s i s t mountain pine b e e t l e (Raid e t a l . , 1967 ; Shrimpton and Reid, 1973). I attempted to use and e v a l u a t e t h i s r a t i n g system and the t o p i c w i l l be considered more f u l l y i n the s e c t i o n e n t i t l e d "Fungal Ratin g System". 2. F a c t o r s a f f e c t i n g r e s i s t a n c e mechanisms S e v e r a l f a c t o r s i n f l u e n c e the e f f e c t i v e n e s s of t r e e r e s i s t a n c e mechanisms. P h y s i o l o g i c a l s t a t e of the t r e e a t the time of a t t a c k i s the major determinant of t h i s e f f e c t i v e n e s s . Tree h e a l t h i s i n t u r n a f f e c t e d by 1) calendar and " p h y s i o l o g i c a l " age ( H o r r i s , 1948) , 2) l o c a l s i t e f a c t o r s such as s o i l type, s l o p e , and d e n s i t y of surrounding and competing t r e e s , 3) r e c e n t moisture a v a i l a b i l i t y , 4) l i g h t n i n g and f i r e h i s t o r y , and 5) h i s t o r i c a l presence of d e f o l i a t o r s or f u n g i i n nearby a r e a s . Reid and shrimpton (1971) note that r e s i s t a n c e a l s o changes through the summer and i n t h e i r study areas, bark b e e t l e s have normally been observed to a t t a c k j u s t a f t e r the peak i n r e s i s t a n c e i n J u l y . The behaviors of D. £onderosae and i t s a s s o c i a t e d f u n g i tend t o decrease the e f f e c t i v e n e s s of both l e v e l s of t r e e r e s i s t a n c e . Primary r e s i n o s i s may be a f f e c t e d by the shape of the p a r e n t a l b e e t l e g a l l e r y . The bark b e e t l e begins g a l l e r y formation by c u t t i n g a J-shaped hook at i t s base (Fig. 1). Berryman (1972) and Reid e t a l . (1967) have suggested that the 26 purpose of t h i s hook i s to d r a i n the v e r t i c a l and h o r i z o n t a l r e s i n ducts which are sev e r e d , causing an i n i t i a l f l u s h of r e s i n but reducing the amount of r e s i n t o be faced l a t e r by the a d u l t . B l u e - s t a i n i n g f u n g i c a r r i e d by bark b e e t l e s grow i n sapwood (outer xylem) c e l l s , r e n d e r i n g them n o n - f u n c t i o n a l as water conductors (Eeid e t a l . , 1967 and r e f e r e n c e s t h e r e i n ) . In a d d i t i o n , phloem c e l l s can no longer d i s t r i b u t e v i t a l products throughout a t r e e once they are severed by a d u l t and l a r v a l b e e t l e g a l l e r i e s . A l l of these e f f e c t s become c r i t i c a l t o the t r e e when l a r g e numbers of bark b e e t l e s a t t a c k . Seduced numbers of f u n c t i o n a l phloem and sapwood c e l l s r e s u l t i n lowered a b i l i t y to produce the a c t i v e l y generated secondary r e s i n o s i s , b e l i e v e d by some to be the more important of the two t r e e r e s i s t a n c e mechanisms (Keid et a l . , 1967). A l s o , a d u l t b e e t l e s u r v i v a l i s enhanced by i n c r e a s i n g a t t a c k d e n s i t y because the more c u t s there are i n r e s i n c a n a l s , the s m a l l e r i s the amount of r e s i n s p i l l i n g i n t o each g a l l e r y . These obvious b e n e f i t s t o bark b e e t l e s of ensuring l a r g e numbers of a t t a c k i n g i n s e c t s on i n d i v i d u a l t r e e s have l e d some i n v e s t i g a t o r s t o conclude t h a t aggregation behavior i s e s s e n t i a l t o k i l l r e l a t i v e l y h e a l t h y t r e e s . However, before c o n c l u d i n g t h a t t h i s i s the primary advantage of aggrega t i o n , we must look at other bark b e e t l e s p e c i e s . For i n s t a n c e , the ambrosia b e e t l e Tryp_odendron lineatum a l s o a t t a c k s en masse through pheromone a c t i o n but i t i s known to p r e f e r f a l l e n 27 t r e e s t h a t have been on the ground f o r three to seven months (Kinghorn and Chapman, 1957). These t r e e s have no v i a b l e r e s i s t a n c e mechanisms, so aggregating behavior cannot f u n c t i o n f o r T. lineatujS 1 D reducing the e f f e c t i v e n e s s of host r e s i s t a n c e . The l i t e r a t u r e o f f e r s no reasonable e x p l a n a t i o n s f o r mass at t a c k s i n T. lineatum or other bark b e e t l e s with s i m i l a r l i f e h i s t o r i e s . There are s e v e r a l r a t h e r s p e c u l a t i v e p o s s i b i l i t i e s , however. T. lineatum w i l l o c c a s i o n a l l y a t t a c k standing t r e e s , but u s u a l l y only those t r e e s which are already extremely unhealthy due to some other cause (K. Graham, pers. comm.). I t c o u l d be t h a t some of these a l t e r n a t i v e hosts are v i g o r o u s enough t h a t t h e r e c o u l d be s t r o n g s e l e c t i o n i n such cases f o r maintaining an aggregating pheromone system to help overcome the h o s t , s r e s i s t a n c e . Such an e x p l a n a t i o n depends on the frequency of these s i t u a t i o n s and the p r o p o r t i o n of the b e e t l e p o p u l a t i o n s which are a f f e c t e d , both of which are unknown. Another p o s s i b l e e x p l a n a t i o n f o r aggregating pheromones i n bark b e e t l e s t h a t normally a t t a c k f a l l e n t r e e s i s t h a t these b e e t l e s may s t i l l maintain a b e h a v i o r a l system, d e r i v e d from t h e i r a n c e s t o r s , which was not l o s t because i t was never s e l e c t e d a g a i n s t ( A t k i n s , 1966a). l e t another p o s s i b i l i t y i s t h a t the p r o b a b i l i t y of each i n s e c t * s being preyed upon or p a r a s i t i z e d may decrease with i n c r e a s i n g numbers of other bark b e e t l e s present (e.g. Tinbergen, 1960). However, t h i s e x p l a n a t i o n depends on 28 the r a p i d i t y of the numerical response of the predators and p a r a s i t e s and on the shapes of t h e i r f u n c t i o n a l response curves. Another e q u a l l y s p e c u l a t i v e i d e a d e a l i n g with e f f i c i e n c y of host u t i l i z a t i o n i n v o l v e s k i n s e l e c t i o n (Hamilton, 1964a, 1964b, 1972). T h i s idea depends upon f u t u r e data on the a t t r a c t i v e d i s t a n c e of pheromones i n the f i e l d , the amount of " f l o c k i n g " during a d u l t d i s p e r s a l , and the degree of g e n e t i c r e l a t e d n e s s of i n d i v i d u a l s t h a t a t t a c k the same t r e e . The s e l e c t i v e importance of tree r e s i n s on bark b e e t l e behavior can be d i s c e r n e d by comparing the a d u l t g a l l e r i e s c o n s t r u c t e d by d i f f e r e n t s p e c i e s of bark b e e t l e s . Berryman (1972) has p o i n t e d out that most s p e c i e s of bark b e e t l e s which at t a c k h e a l t h y , r e s i n o u s s p e c i e s of t r e e s , such as those among the Pinus or P i c e a genera, normally d i g v e r t i c a l a d u l t g a l l e r i e s i n order to sever as few v e r t i c a l r e s i n ducts as p o s s i b l e . In c o n t r a s t , those bark b e e t l e s p e c i e s which a t t a c k t r e e s such as f i r s t h a t do not have well-developed r e s i n duct systems u s u a l l y bore h o r i z o n t a l g a l l e r i e s . I t could be i m p l i e d from these o b s e r v a t i o n s t h a t h o r i z o n t a l a d u l t g a l l e r i e s are the o p t i m a l s i t u a t i o n s f o r bark b e e t l e s because more t r e e t r a n s p o r t c e l l s are severed than when v e r t i c a l g a l l e r i e s are dug. There are thus fewer b e e t l e s r e q u i r e d i n order to k i l l each t r e e by g i r d l i n g a c t i o n . The only reason we f i n d s p e c i e s d i g g i n g v e r t i c a l g a l l e r i e s i s because of the c o n s t r a i n t s imposed by the well-developed r e s i n duct systems 29 i n some t r e e s p e c i e s . i n these t r e e s p e c i e s , there i s s t r o n g s e l e c t i o n a g a i n s t b e e t l e s t h a t d i g h o r i z o n t a l g a l l e r i e s owing to the degree of t o l e r a n c e t o r e s i n that would be r e q u i r e d by the bark b e e t l e s . I t a l s o i s o f i n t e r e s t to know whether t h i s o r i e n t a t i o n of a d u l t g a l l e r i e s i s constant throughout a b e e t l e s p e c i e s or whether, as Berryman (1972) suggests, i t i s a b e h a v i o r a l l y p l a s t i c c h a r a c t e r i s t i c , s h i f t i n g with d i f f e r e n t r e s i n flow c o n d i t i o n s encountered. T h i s i d e a can be t e s t e d by l o o k i n g at bark b e e t l e s p e c i e s which a t t a c k s e v e r a l d i f f e r e n t t r e e s p e c i e s with d i f f e r e n t r e s i n c h a r a c t e r i s t i c s . Within a b e e t l e s p e c i e s , one would p r e d i c t t h a t i f g a l l e r y o r i e n t a t i o n were a v a r i a b l e c h a r a c t e r i s t i c , and i f h o r i z o n t a l g a l l e r i e s were the e v o l u t i o n a r y optimum, a d u l t g a l l e r i e s would tend to be more h o r i z o n t a l i n t r e e s p e c i e s with l e s s developed r e s i n systems than i n t r e e s of equal v i g o r with b e t t e r r e s i n systems. I f g a l l e r y o r i e n t a t i o n were a f i x e d c h a r a c t e r i s t i c of the b e e t l e s p e c i e s , i t should be the same r e g a r d l e s s of host t r e e s p e c i e s . Mountain pine b e e t l e does not o f f e r evidence here because a l l of i t s host s p e c i e s are q u i t e r e s i n o u s . However, evidence may be a v a i l a b l e f o r other bark b e e t l e s p e c i e s . E. V a r i a b i l i t y In Host a t t r a c t i v e n e s s Because c o n d i t i o n s of the t r e e s and the bark b e e t l e s at the time of encounter are v a r i a b l e , not a l l t r e e s are e q u a l l y a t t r a c t i v e t o d i s p e r s i n g bark b e e t l e s t h a t are l o o k i n g f o r a 30 new host. I n t e r n a l f a c t o r s inducing v a r i a b i l i t y i n b e e t l e response to t r e e s w i l l be d i s c u s s e d i n the s e c t i o n on d i s p e r s a l d a t a ; here, o n l y agents a f f e c t i n g t r e e a t t r a c t i v e n e s s are c o n s i d e r e d . There i s p r e s e n t l y a c o n t r o v e r s y surrounding the concept of primary a t t r a c t a n t s (those t r e e chemicals thought to a i d bark b e e t l e s i n choosing h o s t s ) . . Some bark b e e t l e i n v e s t i g a t o r s , most notably those i n D.L. Wood's group at Berkeley, c l a i m t h a t d i s p e r s i n g bark b e e t l e s i n i t i a l l y a t t a c k t r e e s randomly i n space. Other r e s e a r c h e r s b e l i e v e t h a t bark b e a t l e s are ab l e , before a t t a c k , to detect which t r e e s are going t o be most s u i t a b l e f o r r e p r o d u c t i o n . I t appears t h a t the cause of the di s c r e p a n c y i s the d i f f e r e n t s p e c i e s with which the r e s e a r c h e r s work. F i r s t , we must co n s i d e r whether there are d i f f e r e n c e s between s p e c i e s o f t r e e s which can p o t e n t i a l l y be detected by i n s e c t s and second we must decide whether bark b e e t l e s are able to d i s c r i m i n a t e between these d i f f e r e n c e s . Data r e l e v a n t to these q u e s t i o n s f a l l i n t o two c a t e g o r i e s : 1) w i t h i n t r e e s p e c i e s , and 2) between t r e e s p e c i e s . Some bark b e e t l e s such as D o u g l a s - f i r b e e t l e , mountain pine b e e t l e , and western pine b e e t l e are r e l a t i v e l y h o s t - s p e c i f i c i n th a t they are found only i n a few c o n i f e r o u s s p e c i e s (Rudinsky, 1962). The ex i s t e n c e of host s p e c i f i c i t y i n d i c a t e s t h a t at l e a s t some bark b e e t l e s are able t o re c o g n i z e something unique about t h a i r h o s t s , presumably t r e e chemicals. 3 1 The evidence concerning d i f f e r e n c e s between t r e e s w i t h i n s p e c i e s i s not so s t r a i g h t f o r w a r d . P o t e n t i a l v i s u a l cues such as s i z e and l o c a t i o n oust be separated from o l f a c t o r y ones. Trees of any given s p e c i e s t h a t appear t o be the same s i z e may d i f f e r i n chemical c o n s t i t u e n t s . Hoe and Amman (1970) present evidence t h a t the mountain pine b e e t l e p r e f e r e n t i a l l y a t t a c k s and k i l l s ponderosa pine t r e e s which have the t h i c k e s t phloem, even when t r e e s used f o r comparison are a l l the same diameter and i n the same l o c a l i t y . Hoeck(1970b) found t h a t T. lineatum i s a t t r a c t e d by e t h a n o l and Graham (1968) noted that t h i s s p e c i e s responds d i f f e r e n t i a l l y to the v a r i o u s e t h a n o l c o n c e n t r a t i o n s produced by c u t hemlock l o g s o f d i f f e r e n t p h y s i o l o g i c a l ages. Less vi g o r o u s or more senescent t r e e s may produce more ethanol (Graham, 1968). On the other hand, 2» b r e v i c o m i s i t h e western pi n e b e e t l e , appears to a t t a c k ponderosa pine t r e e s a t random (Rood, 1973) and brood success i s h i g h l y v a r i a b l e between t r e e s with equal attack d e n s i t y (Vite and Wood, 1961). Let us c o n s i d e r now the s p e c i f i c case of mountain pine b e e t l e and i t s host t r e e s . Numerous f o r e s t entomologists f e e l t h a t t r e e age i s a very crude p r e d i c t o r of l i k e l i h o o d of a t t a c k by D. fionderosae (e.g., S a f r a n y i k et a l . , 1974b). Only r e c e n t l y have i n v e s t i g a t i o n s begun to r e v e a l the proximal f a c t o r s a s s o c i a t e d with t r e e age which are used by mountain pine b e e t l e i n d e c i d i n g which t r e e s to a t t a c k . 32 Tree aging induces q u a n t i t a t i v e but not q u a l i t a t i v e changes i n chemical compositions of lodgepole and some other pines (Syed, 1972). Syed has shown t h a t there i s more limonene and l e s s 3-carene i n mature ponderosa pine t r e e s than i n younger t r e e s . S i m i l a r l y , Syed (pers. comm.) has found that mature lodgepole pines have more limonene and alpha-pinene than young t r e e s , and that young lo d g e p o l e have more 3-carene than o l d e r t r e e s . F a c t o r s such as prolonged competition and drought which a f f e c t t r e e p h y s i o l o g i c a l age may a l s o cause these chemical changes which are normally a s s o c i a t e d with c a l e n d a r aging. Syed (1972) showed t h a t mountain pine b e e t l e responded p o s i t i v e l y to t o t a l bark e x t r a c t s c o n t a i n i n g these and other chemicals. He a l s o found a p o s i t i v e response to e t h a n o l i n v a r i o u s c o n c e n t r a t i o n s . D. £onderosae are known to respond to the female aggregating pheromone only i n the presence of host t r e e chemicals (Pitman and V i t e , 1969) , p a r t i c u l a r l y alpha-pinene (Pitman, 1969; Renwick and V i t e , 1970). In c o n c l u s i o n , the evidence f o r mountain pine b e e t l e i s t h a t there i s some use of t r e e chemicals i n host s e l e c t i o n , though other bark b e e t l e s such as the western pine b e e t l e do indeed appear to a t t a c k t r e e s randomly. I f t h e r e were no d e t e c t a b l e d i f f e r e n c e s between t r e e s attacked by the western pine b e e t l e , or i f there were no advantage i n d i s c r i m i n a t i n g between t r e e types, then random a t t a c k would be b i o l o g i c a l l y e f f e c t i v e , e s p e c i a l l y when combined with the observed f a c t 33 th a t t h i s b e e t l e has one of the h i g h e s t r e p r o d u c t i v e r a t e s of any bark b e e t l e and can have up to 3 or 4 broods per summer ( M i l l e r and Keen, 1960; Stark and Dahlsten, 1970). On the other hand, D. fionderosae chooses i t s host t r e e s more c a r e f u l l y and has the r e p r o d u c t i v e behavior common among t h i s type of bark b e e t l e : only one or two broods per year and a r e l a t i v e l y s m a l l number of eggs per brood. Bark b e e t l e e c o l o g i s t s use some terms which need to be c l e a r l y d e f i n e d . The p r o b a b i l i t y t h a t an i n d i v i d u a l t r e e w i l l be a t t a c k e d by one or more bark b e e t l e s i s g e n e r a l l y termed the t r e e ' s a t t r a c t i v e n e s s . The p r o b a b i l i t y t h a t a t r e e i n a given v i g o r c l a s s w i l l d i e a f t e r being a t t a c k e d by a given number of b e e t l e s i s termed i t s s u s c e p t i b i l i t y or r e s i s t a n c e a b i l i t y . T h i s r e s i s t a n c e a b i l i t y i s a l s o i n v e r s e l y r e l a t e d t o b e e t l e r e p r o d u c t i v e success. i n a d d i t i o n , l a r g e r numbers of bark b e e t l e s a t t a c k i n g a s i n g l e t r e e of given v i g o r c l a s s w i l l i n c r e a s e the p r o b a b i l i t y of death. 34 IV. STUDY AREAS To answer the q u e s t i o n s posed i n S e c t i o n I concerning d i f f e r e n c e s between epidemic and endemic b e e t l e s and t r e e s , i t was necessary to f i n d two r e p l i c a t e study s i t e s i n each of these two types of a r e a s . I f o n l y one area r e p r e s e n t i n g each type were used, then any d i f f e r e n c e s found c o u l d have been a t t r i b u t e d to d i f f e r e n c e s i n g e o g r a p h i c a l l o c a t i o n r a t h e r than popu l a t i o n s t a t e . The terms epidemic and endemic are somewhat a r b i t r a r y but I d e f i n e them as f o l l o w s . Endemic areas are those i n which o n l y a few dozen t r e e s are k i l l e d every year by mountain pine b e e t l e w i t h i n an area o f , say, s e v e r a l square miles. Epidemic areas, on the other hand, g e n e r a l l y have s e v e r a l hundred a c r e s of t r e e s k i l l e d per annum. Data on t r e e c h a r a c t e r i s t i c s were c o l l e c t e d from May through September 1973 i n f o u r study s i t e s shown i n Pig. 2 and d e s c r i b e d i n Table 1. These areas c o n t a i n mostly lodgepole pine, although the two endemic areas have some l a r g e aspen and white spruce. The mountain pine b e e t l e outbreak a t E l k Creek (henceforth c a l l e d E l k ) , near Canal F l a t s , B.C., has destroyed a high percentage of t r e e s over l a r g e acreage s i n c e about 1967. To date, more than 200,000 a c r e s of l o d g e p o l e pine have been destroyed. Parson, B.C., approximately 75 a i r miles to the northwest i n the Columbia V a l l e y , supports a s m a l l U» ,£onderosae p o p u l a t i o n . Tree l o s s e s numbered about 35 over a square mile i n 1972. There has not been a l a r g e mountain 35 FIGURE 2 A map of southern B r i t i s h Columbia showing the l o c a t i o n of the four 1973 study areas (black squares). TABLE 1 C h a r a c t e r i s t i c s o f the four study areas. P o p u l a t i o n type L o c a t i o n E l e v a t i o n ( f t . ) Average t r e e age (years Number t r e e s age-sampled S.D. t r e e age Average number t r e e s / a c r e I-— Epidemic E l k 3900 97.7 15 10.2 220 T e r r a c e 3550 90.6 5.0 261 Endemic Parson +— 2700 62.0 10 4.5 209 Lean 3600 78. 2 10 6.9 322 i 38 pine b e e t l e p o p u l a t i o n i n the v i c i n i t y of Parson f o r at l e a s t 10 y e a r s . T e r r a c e Creek was the other epidemic area s t u d i e d and i s l o c a t e d 10 m i l e s northwest of Kelowna, B.C. B e e t l e s i n t h i s area have begun to do c o n s i d e r a b l e damage only during the past 3-4 years and dead t r e e s cover s e v e r a l hundred a c r e s . The other endemic a r e a . Lean-to Creek (henceforth c a l l e d Lean), i s l o c a t e d o n l y 1.25 miles to the northeast of the f a r t h e s t extent o f the Terrace Creek dead t r e e s . Owing to the p r o x i m i t y of these two areas, b e e t l e s from Lean might p o s s i b l y have come from Terrace Creek. However, Lean has d e f i n i t e l y supported a very low b e e t l e p o p u l a t i o n from 1969 to 1972 (C. C o t t r e l l , p e r s . comm.). In 1973 the number of t r e e s k i l l e d from the previous year's a t t a c k rose to about 30 per acre from 15 i n a two acre patch, g i v i n g s i g n s t h a t the p o p u l a t i o n may be i n c r e a s i n g i n numbers. Because of these i n d i c a t i o n s t h a t Lean may be i n t r a n s i t i o n t o an outbreak s t a t e , some of my c o n c l u s i o n s concerning d i f f e r e n c e s between epidemic and endemic area t r e e s and b e e t l e s are not as f i r m as they might otherwise be. Throughout the r e s t of t h i s paper, my data on endemic areas w i l l r e f e r s p e c i f i c a l l y to Lean and Parson and epidemic w i l l r e f e r to Elk and T e r r a c e Creek. 39 V. FIELD EVALUATIONS OF TBEES A. Age The a b i l i t y of lodgepole to r e s i s t a t t a c k s by mountain pine b e e t l e i s thought to decrease with age a f t e r a t r e e reaches about 50 years (Shrimpton, 1973).. Increase i n p r o b a b i l i t y of succumbing a f t e r a given number of a t t a c k s i s a l s o a s s o c i a t e d with i n c r e a s e d a t t r a c t i v e n e s s to b e e t l e s , as d i s c u s s e d i n the s e c t i o n on host a t t r a c t i v e n e s s . S a f r a n y i k e t a l . (1974a) note t h a t most D. ponderpsae outbreaks occur i n stands 80-90 years o l d and only a few occur i n younger stands. Tree ages were measured i n the study areas with a standard 3 mm increment borer and r i n g s were counted i n the f i e l d without the a i d of a hand l e n s . R e s u l t s are shown i n Table 1. Trees i n the two epidemic areas are o l d e r (95 vs. 70) than those i n endemic areas ( t=8.50, 41 d.f., P<.001). T h i s s u b s t a n t i a t e s the f i n d i n g s of other workers d i s c u s s e d above. B. Determination Of Tree Status In the past, bark b e e t l e e c o l o g i s t s have concentrated on t r y i n g t o p r e d i c t i n d i v i d u a l t r e e m o r t a l i t y by l o o k i n g at t r e e r e s i s t a n c e a b i l i t y p r i o r t o a t t a c k (e.g. V i t e , 1961; Reid 40 e t a l . # 1967) . I too used such a method (to be d i s c u s s e d i n the s e c t i o n e n t i t l e d "Fungal Rating System"), but I was a l s o i n t e r e s t e d i n t r y i n g to p r e d i c t b e e t l e m o r t a l i t y and re p r o d u c t i v e success. Thus, f o r t e s t i n g the other i d e a s concerning t r e e s , a system was r e q u i r e d f o r determining the success of bark b e e t l e s a t t a c k i n g i n d i v i d u a l t r e e s . T h i s success measure needed t o be d e f i n e d i n terms of bark b e e t l e r e p r o d u c t i o n r a t h e r than t r e e m o r t a l i t y f o r two reasons. F i r s t , although s u c c e s s f u l b e e t l e r e p r o d u c t i o n and t r e e m o r t a l i t y are normally c o r r e l a t e d , some D. £onderosae females do succeed i n reproducing even though the host t r e e i s not k i l l e d . second, i t i s d i f f i c u l t to t e l l i n the s p r i n g (when data were c o l l e c t e d ) whether a tree i s going to d i e from bark b e e t l e a t t a c k s made the previous summer. The f o l l o w i n g system was devised f o r determining the s t a t u s of i n d i v i d u a l lodgepole t r e e s . At most, three randomly chosen b e e t l e g a l l e r i e s l o c a t e d a t br e a s t height were i n v e s t i g a t e d by p e e l i n g o f f the bark with a c h i s e l . I f any l i v e l a r v a e or pupae were found, the t r e e was c l a s s e d " s u c c e s s f u l l y a t t a c k e d " . I f no l i v e l a r v a e or pupae were r e v e a l e d a f t e r three a d u l t g a l l e r y systems were exposed, the tree was c l a s s i f i e d " u n s u c c e s s f u l l y a t t a c k e d " . The reason f o r l a c k of bark b e e t l e r e p r o d u c t i o n was determined by l o o k i n g at len g t h s of a d u l t g a l l e r i e s and presence of r e s i n s . R e s i s t a n c e due t o primary r e s i n o s i s was diagnosed i f , a) the a d u l t g a l l e r y d i d not extend past the entry hole, b) a d u l t s i n the 41 g a l l e r y were dead and covered i n r e s i n , or c) the a d u l t g a l l e r y was present but i t was at l e a s t h a l f - f i l l e d with p i t c h . Secondary r e s i n o s i s (the delayed response) was considered r e s p o n s i b l e i f a d u l t g a l l e r i e s were present and a) no l a r v a l g a l l e r i e s were found, or b) l a r v a l g a l l e r i e s were present but l a r v a e were dead or not v i s i b l e . Trees which showed s i g n s of e x t e n s i v e woodpecker f e e d i n g were not analyzed. On the b a s i s of these c r i t e r i a of success of bark b e e t l e r e p r o d u c t i o n , a t t a c k e d lodgepole t r e e s f e l l i n t o one of three c a t e g o r i e s : 1) s u c c e s s f u l , 2) u n s u c c e s s f u l due to primary r e s i n o s i s , or 3) u n s u c c e s s f u l due to secondary r e s i n o s i s . T h i s a l l - o r - n o n e measure of mountain pine b e e t l e r e p r o d u c t i o n was not as p r e c i s e as a more q u a n t i t a t i v e f i g u r e , but q u a l i t a t i v e data from many d i f f e r e n t t r e e s were deemed more a p p r o p r i a t e f o r answering my comparative types of gues t i o n s than d e t a i l e d q u a n t i t a t i v e data from j u s t a few t r e e s . Data on b e e t l e success were gathered e i t h e r i n nay and e a r l y June, 1973 (before new attacks) or i n l a t e September (two months a f t e r new a t t a c k s ) . Numerous s t a n d i n g t r e e s were sampled i n a d d i t i o n t o those which were cut and brought back to Vancouver. C. S p a t i a l D i s t r i b u t i o n S u c c e s s f u l l o c a t i o n of hosts by bark b e e t l e s depends upon 42 d i s t a n c e over which a p p r o p r i a t e host chemicals are sensed, d e n s i t y of hosts per u n i t area, and s p a t i a l d i s t r i b u t i o n of hosts r e l a t i v e t o d i s p e r s a l a b i l i t y of the s e a r c h i n g s p e c i e s . Numbers of t r e e s per acre were estimated from counts taken i n two 100 f t by 100 f t p l o t s i n each study s i t e . Data at the bottom of Table 1 show t h a t the d e n s i t y of t r e e s i s not n e c e s s a r i l y higher i n epidemic or endemic areas. I hypothesized, however, t h a t s u s c e p t i b l e t r e e s i n epidemic areas might be more u n i f o r m l y d i s t r i b u t e d i n space than t r e e s i n endemic p l o t s . D i s p e r s i n g bark b e e t l e s might then s u f f e r g r e a t e r m o r t a l i t y i n endemic areas because s u s c e p t i b l e t r e e s are clumped, clumps are randomly d i s t r i b u t e d i n space and they are more d i f f i c u l t t o f i n d . To t e s t t h i s h ypothesis, s p a t i a l d i s t r i b u t i o n s of s u c c e s s f u l l y a t t a c k e d t r e e s (defined i n terms of b e e t l e r e p r o d u c t i o n as d e s c r i b e d above) were measured by the " P o i n t - t o - p l a n t d i s t a n c e r a t i o " t e s t of Holgate (1965) . T h i s t e s t uses d i s t a n c e s to p l a n t s from a s e r i e s of randomly chosen p o i n t s . These p o i n t s i n my study s i t e s , determined from random number t a b l e s , a l l f e l l w i t h i n 600 f t of one another. Data i n Table 2 show t h a t f o r the 20 t r e e s sampled i n each area, there was no s i g n i f i c a n t d e v i a t i o n from a random d i s t r i b u t i o n . On the b a s i s of these data, I cannot conclude that s u i t a b l e t r e e s i n endemic areas are more clumped than i n epidemic a r e a s . However, the hypothesis c o u l d be more r i g o r o u s l y t e s t e d i f data from a wider s p a t i a l s c a l e were 43 TABLE 2 S p a t i a l d i s t r i b u t i o n s of s u c c e s s f u l l y attacked lodgepole pine t r e e s u sing Holgate*s t e s t . I f c o e f f i c i e n t of aggregation. A, equals 0.5, d i s t r i b u t i o n i s random; i f l e s s than 0.5, d i s t r i b u t i o n tends toward uniform, and i f gre a t e r than 0.5, clumping i s i n d i c a t e d . Data are based on samples of 20 t r e e s from each area. P o p u l a t i o n type r — r Epidemic Endemic | L o c a t i o n | E l k I T e r r a c e _ i _ Parson I Lean | A ! | 0.482 I 0. 565 | 0.393 | Z s t a t i s t i c ! | 0.286 1.01 | 1.65 | P r o b a b i l i t y that d i s t r i b u t i o n i s random ! I >0.1 „,,, J i >0. 1 I >0.1 I 44 a v a i l a b l e . T h i s s c a l e should be d e f i n e d i n r e l a t i o n to d i s p e r s a l c a p a b i l i t i e s of bark b e e t l e s . Erroneous c o n c l u s i o n s may be drawn i f a r b i t r a r y s p a t i a l s c a l e s are u t i l i z e d , such as the one I used. For example, i f one chose a s c a l e about the same s i z e as the area covered by an extant outbreak, s u c c e s s f u l l y a t t a c k e d t r e e s would be much more evenly d i s t r i b u t e d i n epidemic s i t e s than i n endemic areas when measured with that same s c a l e . I f we determine t h a t d i s p e r s i n g mountain pine b e e t l e s normally cover s e v e r a l miles, than a s c a l e of t h i s magnitude i s needed t o t e s t adequately for h e t e r o g e n e i t y i n hosts with r e s p e c t to the c a p a b i l i t i e s of b e e t l e s f i n d i n g them. S i n c e the mountain pine b e e t l e , being i n p a r t p a s s i v e l y d i s p e r s e d , can t r a v e l at l e a s t a mile at a time (see data i n d i s p e r s a l s e c t i o n ) , the measurements of s p a t i a l d i s t r i b u t i o n made a c r o s s only a few hundred f e e t i n t h i s study are o b v i o u s l y inadequate f o r m e a n i n g f u l l y t e s t i n g the h y p o t h e s i s . I t i s p o s s i b l e that l a r g e - s c a l e a e r i a l photographs of o l d b e e t l e - k i l l e d areas can be more u s e f u l f o r t e s t i n g t h i s i d e a . D. Attack D e n s i t i e s As d i s c u s s e d above, t r e e s with high a t t a c k d e n s i t i e s should be more l i k e l y t o host s u c c e s s f u l l y reproducing bark b e e t l e s than t r e e s with lower a t t a c k d e n s i t i e s . I d e a l l y , t h i s hypothesis should be t e s t e d with t r e e s i n the same s t a t e of h e a l t h p r i o r t o bark b e e t l e a t t a c k . T h i s was not p o s s i b l e , so 45 data were c o l l e c t e d from t r e e s independent o f t h e i r p r e v i o u s p h y s i o l o g i c a l h e a l t h . Attack d e n s i t i e s were measured with a square-gridded w i r e - a n d - s t r i n g o v e r l a y , 1 f t by 1 f t , which was l a i d t i g h t l y over the t r e e trunk. A l l entrance holes ( d i s t i n g u i s h e d from e x i t holes by the presence of p i t c h tubes) i n s i d e t h i s square were counted. Three c o n s e c u t i v e samples were taken per t r e e s t a r t i n g at the base and working upward on the s i d e of the tree having the most a t t a c k s . Reproductive success of b e e t l e s was determined with the g a l l e r y i n s p e c t i o n method d e s c r i b e d i n S e c t i o n V-B. Data were c o l l e c t e d i n each study area by l o o k i n g at t r e e s i n two separate 100 f t by 100 f t p l o t s and on a 1000 f t s t r a i g h t - l i n e t r a n s e c t . Table 3 shows r e s u l t s comparing average a t t a c k d e n s i t i e s on s u c c e s s f u l l y a t t a c k e d t r e e s with those on u n s u c c e s s f u l ones. When a l l f o u r sample areas were lumped, s u c c e s s f u l l y attacked t r e e s d i d have s i g n i f i c a n t l y higher a t t a c k d e n s i t i e s than u n s u c c e s s f u l l y a t t a c k e d ones, i n d i c a t i n g t h a t attack by a l a r g e number of b e e t l e s can r e s u l t i n a g r e a t e r p r o b a b i l i t y of overcoming a t r e e ' s r e s i s t a n c e attempts or t h a t more b e e t l e s are a t t r a c t e d to weaker t r e e s . However, when data are broken down by study area (bottom of Table 3) we f i n d s l i g h t l y d i f f e r e n t r e s u l t s : although t r e e s which produced b e e t l e o f f s p r i n g tended to have higher a t t a c k d e n s i t i e s , on average, than u n s u c c e s s f u l l y a t t a c k e d t r e e s , t h e r e was no s t a t i s t i c a l s i g n i f i c a n c e i n the d i f f e r e n c e s . T h i s was due to the 46 TABLE 3 Comparison of average a t t a c k d e n s i t i e s (from 3 samples per t r e e ) , by a r e a , and by t r e e type d e f i n e d i n terms of bark b e e t l e r e p r o d u c t i v e s u c c e s s . Under t r e e type, S=successful b e e t l e r e p r o d u c t i o n , US=unsuccessful (see t e x t ) . A l l areas lumped: Average a t t a c k d e n s i t y Standard e r r o r N S u c c e s s f u l 8.95 +-U n s u c c e s s f u l | 0.47 140 t - s t a t i s t i c = 2.75, P<.01 7.36 0.34 75 L Broken down by area: P o p u l a t i o n type Loc a t i on Tree type Average a t t a c k d e n s i t y Variance No. t r e e s sampled t - s t a t i s t i c P Epidemic I + Elk I +— I H— 12.1 13.2 17 US 10.3 9.0 9 f « -I L-1.35 I 1.70 >0.1 I >0.09 Terrac e +-S 7.8 15.4 46 US j. 6. 1 11.1 15 Endemic Parson 8.1 12.2 25 US 6.5 16.3 28 1.53 >0. 1 Lean 9.3 15.5 52 US 8.3 18. 1 23 1.00 >0.2 J 47 r e l a t i v e l y l a r g e v a r i a n c e s , p o s s i b l y a s s o c i a t e d with sampling from t r e e s with a wide v a r i e t y of t r e e v i g o r s p r i o r to a t t a c k . Epidemic areas d i d not tend to have d i f f e r e n t a t t a c k d e n s i t i e s than endemic areas wi t h i n e i t h e r the s u c c e s s f u l l y - or u n s u c c e s s f u l l y - h i t t r e e c a t e g o r i e s ( t - t e s t s , P>.2 and P>.5, r e s p e c t i v e l y ) . The important q u e s t i o n concerning the e f f e c t of higher a t t a c k d e n s i t i e s on the p r o b a b i l i t y of overcoming t r e e r e s i s t a n c e i s how r a p i d l y that p r o b a b i l i t y i n c r e a s e s per u n i t i n c r e a s e i n numbers of b e e t l e s a t t a c k i n g . In other words, i s the " e f f e c t i v e " r e s i s t a n c e ( i n t r i n s i c r e s i s t a n c e minus decrease due to b e e t l e s a t t a c k i n g ) a r a p i d l y or s l o w l y decreasing f u n c t i o n of a t t a c k d e n s i t y (see F i g . 3)? The answer to t h i s q u e s t i o n i s s i g n i f i c a n t f o r the understanding of outbreaks because present t h e o r i e s p r e d i c t t h a t epidemic b e e t l e s overcome normally u n a s s a i l a b l e t r e e s and spread f a s t e r by sheer weight of numbers. F i g u r e 4 shows the frequency d i s t r i b u t i o n , by r e p r o d u c t i v e success c a t e g o r i e s , of the average a t t a c k d e n s i t i e s summarized i n Table 3. Three c o n c l u s i o n s can be drawn from these data. 1) The only two areas showing d i f f e r e n c e s i n frequency d i s t r i b u t i o n s between s u c c e s s f u l l y and u n s u c c e s s f u l l y a t t a c k e d t r e e s are Parson and T e r r a c e Creek (X 2, P<.03). There i s no c o n s i s t e n t p a t t e r n based on p o p u l a t i o n type (epidemic or endemic a r e a s ) . 2) Only when attack d e n s i t i e s f a l l below about 3 per sq f t i s there a l a r g e 48 FIGURE 3 A h y p o t h e t i c a l r e l a t i o n s h i p between the p r o b a b i l i t y of t r e e s i n v a r i o u s v i g o r c l a s s e s dying and a t t a c k d e n s i t y of mountain pine b e e t l e . Steepness of the s l o p e s are c r i t i c a l to the concept of epidemic c e n t r e s (see t e x t ) . 49 ATTACK DENSITY 50 FIGURE 4 D i s t r i b u t i o n of a t t a c k d e n s i t i e s on t r e e s by c l a s s of b e e t l e r e p r o d u c t i v e success (see t e x t ) . Attack d e n s i t i e s are the average of 3 samples per t r e e . SUCC=successful and UNSUCC=unsuccessful. E l k and T e r r are the epidemic areas and Pars and Lean are t h e endemics. Chi-square values were found u s i n g frequency data. S i s the number of t r e e s sampled. m r o c n m o ID O CO T3 m CO o PER CENT OF TRFFS WITH THIS ATTACK DENSITY r o t c o t c o t 8 r o l rot POl air ai r o o ro co c r r> o m XI A o ro cn ro x ro D ro oo ZD < m c n m o 33 n CO -a m 70 CO o CD oo PER CENT OF TREES WITH THIS RTTflCK DENSITY c n o ro c n CO c r n o II r o c r co 2 : co . , CO -^ 1 • ' C O ) o D Tl X V P N 03 < rn I D c n m ZD ZD C O CO r n 70 c o o r o t c o f PER CENT OF TREES WITH THIS ATTACK DENSITY o r o o r o cot o r o r o cn f CO c rn ZD II «J cr cn s; co cr rn rn co co TJ X v il o -00 r o < r n 73 :D c n m o ~n :D cn co m X ) co o r o r o co co PER CENT OF TREES WITH THIS ATTACK DENSITY cn c n r o o r o c n - c o c r o o 3 CO CO CO c r r n o co "D X b y ST* 52 p r o b a b i l i t y of t r e e r e s i s t a n c e overcoming b e e t l e s . 3) There does not appear to be any d i f f e r e n c e i n t h i s minimum d e n s i t y f o r success between epidemic and endemic areas. E. P r e d i c t i n g Tree Status Since attack d e n s i t i e s are not s u f f i c i e n t to p r e d i c t t r e e s t a t u s , I attempted to use t r e e age measures, i n c o n j u n c t i o n with a t t a c k d e n s i t y , to d i s c r i m i n a t e between p o p u l a t i o n s of t r e e s with d i f f e r e n t b e e t l e p r o d u c t i v i t y . Tree age and b e e t l e success were determined as before and t r e e circumference was measured at 50 i n c h e s height. I f t r e e age were a good p r e d i c t o r of t r e e r e s i s t a n c e a b i l i t y , one would expect t h a t f o r a given a t t a c k d e n s i t y , s u c c e s s f u l l y reproducing b e e t l e s would be on o l d e r (and presumably weaker) t r e e s . F i g u r e 5 shows t h a t t h i s i s not the case, a t l e a s t f o r the 29 t r e e s sampled, u n s u c c e s s f u l l y a t t a c k e d t r e e s seem t o be d i s t r i b u t e d randomly with r e s p e c t to age, r e g a r d l e s s of a t t a c k d e n s i t y . Tree s i z e may be a more u s e f u l b a s i s f o r d i s c r i m i n a t i n g between t r e e s t a t u s types (based on b e e t l e reproduction) f o r two reasons. One, diameter or circumference r e f l e c t s phloem t h i c k n e s s , which has been r e l a t e d to success of b e e t l e r e p r o d u c t i o n by Amman (1972). Two, s i z e a t any given age r e f l e c t s the degree of competition that a t r e e has been faced with i n the past or t h a t i t i s s t i l l e ncountering. I t i s t h e r e f o r e a measure of p h y s i o l o g i c a l age as determined by the number of years s i n c e i n i t i a l r e l e a s e from c o m p e t i t i o n 53 FIGURE 5 The use of t r e e age and mountain pine b e e t l e a t t a c k d e n s i t y f o r p r e d i c t i n g b e e t l e r e p r o d u c t i v e success on t r e e s . Data come from a l l study areas. As before, SUCC = s u c c e s s f u l l y a t t a c k e d t r e e s and UNSUCC = u n s u c c e s s f u l l y a t t a c k e d t r e e s . Sample s i z e i s 29. 1 2 5 T 100 T 7 5 -5 0 -2 5 -R L L AREAS 1973 + .= S U C C . . < = UNSUCC < . • + '• + + + << + < ++ + fl ' 1 — l 1 —+— 1 1 - t" 1 1 U 0 • 5 10 15 20 NO. A T T A C K S PER S Q . F T . 55 (Morris, 1948) . T h i s may be a b e t t e r i n d i c a t o r of t r e e h e a l t h at any p o i n t i n time than calendar age. When t r e e s i z e i s used as the b a s i s f o r a d i s c r i m i n a n t f u n c t i o n a n a l y s i s along with a t t a c k d e n s i t y , the data p l o t as i n d i c a t e d i n F i g . 6. These two independent v a r i a b l e s do not pr o v i d e a good s e p a r a t i o n of the two t r e e types. However, as before, b e e t l e s a t t a c k i n g with a d e n s i t y of l e s s than about 3 per sq f t do not reproduce s u c c e s s f u l l y , independent of host t r e e s i z e . T h i s suggests, f o r the s i z e s of t r e e s sampled, t h a t t h i s minimum at t a c k d e n s i t y i s r e q u i r e d to overcome t r e e r e s i s t a n c e . When these same data are disaggregated i n t o epidemic and endemic types ( F i g . 7 ) , another i n t e r e s t i n g p o i n t appears. The s i z e d i s t r i b u t i o n s of t r e e s sampled i n both area types were the same (X 2, P>. 1) but t r e e s with s u c c e s s f u l l y reproducing b e e t l e s were, w i t h i n a given attack d e n s i t y c l a s s , s m a l l e r s i z e d i n epidemic than i n endemic areas. T h i s i s seen by comparing the p o s i t i o n on the circumference a x i s of the ••cloud" of +'s ( s u c c e s s f u l l y a t t a c k e d t r e e s ) i n the top and bottom graphs of F i g . 7. These data i n d i c a t e t h a t t r e e s with equal s i z e and equal mountain pine b e e t l e a t t a c k d e n s i t i e s are more l i k e l y t o be overcome by the b e e t l e s i n epidemic a r e a s than i n endemic ones. T h e r e f o r e , higher b e e t l e a t t a c k d e n s i t i e s and/or l a r g e r t r e e s are not the causes of bark b e e t l e s b u i l d i n g up so r a p i d l y i n outbreak r e g i o n s ; there must be something e l s e i n t r i n s i c a l l y d i f f e r e n t about t r e e s or be e t l e s between epidemic and endemic areas. 56 FIGURE 6 Ths use of t r e e circumference and mountain pine b e e t l e a t t a c k d e n s i t y f o r p r e d i c t i n g b e e t l e r e p r o d u c t i v e success on t r e e s . Data come from a l l study areas. SUCC = s u c c e s s f u l l y a t t a c k e d t r e e s and UHSUCC = u n s u c c e s s f u l l y a t t a c k e d t r e e s . Sample s i z e i s 142 t r e e s . 57 CJ -CJ ZD CO CO r- ZZL V + CD ZD *—• 11 V 1 CO 11 V 4. + • + • +v + -H-CX + UJ cc cr V CJ + _ J CJ . i ZD ex CO + 44^ LO 4V + V +++ + $* 9 y V o CD LO O CO LO LO C\J O C M LO O LO O CD a co L U Qu CO CJ X CX CNi ) 33N3y3Jwnoyrj 3 3 y i 58 FIGURE 7 Same data as F i g u r e 6 except t h a t data are broken down i n t o epidemic and endemic areas. SUCC = s u c c e s s f u l l y a t t a c k e d t r e e s and UNSUCC = u n s u c c e s s f u l l y a t t a c k e d t r e e s . Sample s i z e f o r epidemic areas i s 74 t r e e s and f o r endemic i s 78. UJ L J ~Z. LU CH UJ ZD CJ c r i—i CJ UJ UJ cn 75 60 45-30 4 15 ENDEMIC RREflS 1973 +•= SUCC.. < = UNSUCC. 59 0 + + 4 <r 0 + + < < 10 15 20 25 NO. RTTRCKS PER SQ. FT. UJ CJ 21 LU c r LU ZD CJ c r i—i CJ LU UJ cr 75 T 60-45 30 15 0. EPIDEMIC RRERS 1973 .+ = SUCC. . < =' UNSUCC < 4 . 4 . < l < t < 1 T + j . ± + < <+ ++ + < • + < H 1-10 15 20 25 NO. RTTRCKS PER SQ. FT. 60 F. P r o p o r t i o n S u c c e s s f u l l y Attacked I f the p r e v a i l i n g dogma i s c o r r e c t that a t any one time there are more s u s c e p t i b l e t r e e s per u n i t area i n epidemic than endemic areas, then one would p r e d i c t t h a t a s m a l l e r p r o p o r t i o n of bark b e e t l e s i n epidemic areas would be r e s i s t e d by t h e i r host t r e e s . T h i s would be t r u e i f b e e t l e s a t t a c k e d t r e e s randomly or i f t h e i r system f o r d i s c r i m i n a t i n g between host a t t r a c t a n t s were l e s s than p e r f e c t . I f t h i s b e e t l e d i s c r i m i n a t i n g system were a b s o l u t e l y p r e c i s e , there should be no u n s u c c e s s f u l l y a t t a c k e d t r e e s i n e i t h e r epidemic or endemic areas because b e e t l e s should have found only s u s c e p t i b l e t r e e s . These p r e d i c t i o n s assume t h a t , 1) the pheromone system used by i n i t i a l a t t a c k e r s i s very e f f i c i e n t a t a t t r a c t i n g the remaining p o p u l a t i o n to a t t a c k e d t r e e s and 2) there i s no change i n the e f f i c i e n c y of d i s c r i m i n a t i n g systems or pheromone systems with b e e t l e p o p u l a t i o n d e n s i t y . Data sampled from two 100 f t by 100 f t p l o t s and a 1000 f t s t r a i g h t - l i n e t r a n s e c t i n each of the f o u r study areas are summarized i n Table 4. F i r s t , note t h a t when a l l study areas are lumped, more than one t h i r d of the t r e e s s u c c e s s f u l l y r e s i s t the a t t a c k i n g b e e t l e s . T h e r e f o r e , the b e e t l e mechanisms f o r determining which t r e e s are the best to attack are anything but p r e c i s e ; b e e t l e s m i s c l a s s i f i e d about 36% of the 555 t r e e s sampled. When these data are broken down by study area (bottom of 61 TABLE 4 Status of attacked trees defined in terms of production of beetles. Numbers of attacked trees are in each category, proportions of trees in each area are in parentheses. US#1 = primary resinosis, US#2 = secondary resinosis. Succ. Unsucc. #1 Unsucc. #2 N % Unsucc. A l l areas lumped 358 (.64) 99 (.18) 98 (.18) 555 35.5 Broken down by study area: Succ. Unsucc. #1 Unsucc. #2 N % Unsucc. % beetles : on US trees Elk (Epidemic) 168 (.65) 37 (.14) ; 53 (.21) 258 34.9 . 31 Terrace (Epidemic) 80 (.71) 17 (.15) 16 (.14) 113 29.2 20 Lean (Endemic) 84 (.68) 24 (.20) 15 (.12) 123 31.7 28 Parson (Endemic) 26 (.43) 21 (.34). 14 (.23) 61 57.4 47 62 Table 4) two p o i n t s are r a i s e d . F i r s t , t here are no more r e s i s t a n t t r e e s i n endemic than epidemic areas, unless Lean i s cons i d e r e d more epidemic than endemic because of i t s apparent s t a t e of t r a n s i t i o n . Perhaps most of the s u s c e p t i b l e t r e e s i n epidemic areas have a l r e a d y been removed from the stand and i n s e c t s are a t t a c k i n g s u c c e s s f u l l y only because of t h e i r l a r g e p o p u l a t i o n s . Second, the p r o p o r t i o n of female b e e t l e s (determined from my 3 samples per t r e e ) t h a t a t t a c k e d t r e e s on which they could not s u c c e s s f u l l y reproduce ranged from .20 to .47. Such l a r g e p r o p o r t i o n s i n d i c a t e t h a t there should be st r o n g s e l e c t i o n f o r a b e t t e r d i s c r i m i n a t i n g system. However, there may be a hidden advantage i n t h i s apparent i n e f f i c i e n c y : those i n d i v i d u a l s t h a t a t t a c k what are under normal c o n d i t i o n s u n s u i t a b l e hosts, may reproduce q u i t e w e l l should those hosts suddenly become more s u s c e p t i b l e (for example, due to r a p i d l y changing weather c o n d i t i o n s ) . T h i s p o s s i b l e advantage r e s t s on two assumptions; 1) r e s i s t a n t t r e e s are h e a l t h i e r than n o n - r e s i s t a n t ones (see i n t r o d u c t i o n ) and 2) h e a l t h i e r t r e e s provide a b e t t e r medium f o r b e e t l e r e p r o d u c t i o n i f they should q u i c k l y become unable to r e s i s t adequately. T h i s second assumption i s s u b s t a n t i a t e d by S a f r a n y i k and Jahren (1970), who show t h a t l a r g e r i n s e c t s emerge from l a r g e r and t h i c k e r barked t r e e s , and by Reid (1963) who shows t h a t l a r g e r females produce more o f f s p r i n g . It i s a l s o presumed t h a t t h i s change i n tre e p h y s i o l o g y would occur more r a p i d l y than would be d e t e c t a b l e by s o - c a l l e d 63 primary a t t r a c t a n t s . The magnitude of the s e l e c t i v e advantage of a t t a c k i n g normally u n s u i t a b l e hosts i s dependent on the frequency of r a p i d d e t e r i o r a t i o n i n t r e e c o n d i t i o n s . The u n p r e d i c t a b l e nature of c a u s a l agents i n such changes may r e q u i r e some b u i l t - i n f l e x i b i l i t y i n the bark b e e t l e p o p u l a t i o n f o r coping with changing c o n d i t i o n s . Another hypothesis t h a t can be t e s t e d with these data concerns the r e l a t i v e advantage of secondary r e s i n o s i s over primary r e s i n o s i s . P r e v i o u s l y , the r e l a t i v e advantages to t r a e s of each of these r e s i s t a n c e mechanisms were d i s c u s s e d . Data i n Table 4 i n d i c a t e t h a t primary r e s i n o s i s i s not any more common than secondary r e s i n o s i s as the cause of f a i l u r e of b e e t l e r e p r o d u c t i o n ( t - t e s t , p a i r e d by a r e a , P>.1). There are three p o s s i b l e reasons why weak primary and strong secondary r e a c t i o n s do not predominate. F i r s t , as mentioned p r e v i o u s l y , the primary r e s i n o s i s system may be d i f f i c u l t to r e g u l a t e because of i t s passive mode of a c t i o n . Second, t r e e r e s i n systems probably f u n c t i o n i n g e n e r a l wound h e a l i n g (Berryman, 1972) and as a b a r r i e r to i n f e c t i o n (Whitney and Denyer, 1969). There are many other sources of t r e e wounds besides bark b e e t l e s ; f o r example, other f a l l i n g t r e e s and woodpeckers. Because of the presence of these other s e l e c t i o n p r e s s u r e s , one would not expect r e s i n r e a c t i o n s to have evolved i n such a way as to o p t i m i z e only the i n t e r a c t i o n s with bark b e e t l e s . T h i r d , i f primary r e s i n o s i s i s s u f f i c i e n t to r e s i s t the b e e t l e s and t h e i r f u n g i , no secondary response 64 i s needed. Note that the two endemic areas are the only ones with a preponderance of primary responses, although these d i f f e r e n c e s are not s i g n i f i c a n t with a t - t e s t (P>. 1). G. Fungal Bating System Many workers have attempted i n the past to f i n d measures of p o t e n t i a l of i n d i v i d u a l t r e e s t o r e s i s t bark b e e t l e s . These i n d i c e s have ranged from . primary r e s i n exudation pressure (Vite and Wood, 1961) and chemical composition (Smith, 1963) t o dete r m i n a t i o n of crown c l a s s (Keen, 1943). Most workers have concentrated on some aspect of primary r e s i n o s i s . R e c e n t l y , a method was developed and used by Reid et a l . , (1967) and Shrimpton and Reid (1973). T h i s technique d e r i v e s from the o b s e r v a t i o n t h a t lodgepole pine r e a c t s i n the same way to the b e e t l e s ' s y m b i o t i c b l u e - s t a i n i n g f u n g i alone as i t does to the mountain pine b e e t l e i n the presence of f u n g i (Reid and Shrimpton, 1971; Reid et a l . , 1967). Thus, unattacked t r e e s can be i n o c u l a t e d with f u n g i and, through o b s e r v a t i o n of the r e s i s t a n c e responses, an estimate of s u s c e p t i b i l i t y to mountain pine b e e t l e can be obtained. I attempted to use t h i s f u n g a l assay method to t e s t the hypothesis t h a t there are more s u s c e p t i b l e t r e e s i n epidemic than endemic areas. I c l o s e l y f o l l o w e d the technique of Reid et a l . , (1967). C u l t u r e s of the b l u e - s t a i n i n g f u n g i i s o l a t e d from mountain pine b e e t l e s sampled i n the Canadian Rockies were obtained from Dr. H.S. Whitney, Canadian Forest S e r v i c e 65 Research Lab, V i c t o r i a , B.C. Fungi were grown on s t e r i l i z e d wood c h i p s ( p o p s i c l e s t i c k s c u t 1.5 x 4 x 21 ram) which were soaked i n 2% malt e x t r a c t broth. once the f u n g i were e s t a b l i s h e d , wood c h i p s were i n o c u l a t e d i n t o unattacked t r e e s through an i n c i s i o n made i n the bark on the N-E s i d e of the t r e e with a s t e r i l i z e d c h i s e l . C o n t r o l c h i p s ( a l l the same treatments but without a d d i t i o n of fungi) were a l s o i n o c u l a t e d i n t o t r e e s on the N-E s i d e to c o n t r o l f o r e f f e c t s o f a s e p t i c wounds and response t o p l a i n wood c h i p s . Test t r e e s were numbered with spray p a i n t and, approximately three weeks a f t e r i n o c u l a t i o n , the same t r e e s were checked and t h e i r responses recorded. Another aim of t h i s f u n g a l assay experiment was to determine how good t h i s r a t i n g system was f o r p r e d i c t i n g b e e t l e p r o d u c t i v i t y s u c c e s s . Shrimpton and Reid (1973) have evidence t h a t the method i s f a i r l y good at p r e d i c t i n g which lodgepole pine t r e e s w i l l be k i l l e d . In order to provide a r i g o r o u s t e s t f o r t h i s method, I c a r e f u l l y chose p l o t s of t r e e s to be i n o c u l a t e d t h a t were i n the v i c i n i t y of c u r r e n t mountain pine b e e t l e a c t i v i t y i n each of the four o r i g i n a l study areas. These t e s t t r e e s were u s u a l l y w i t h i n 300 to 800 f t o f the nearest t r e e s with l i v e mountain pine b e e t l e l a r v a e . The temporal l a y o u t of the experiment was as f o l l o w s . Test t r e e s were i n o c u l a t e d i n l a t e June and responses were read and r a t i n g s were assigned i n mid-July. T h i s was before mountain pine b e e t l e d i s p e r s a l f l i g h t i n the areas of study. 66 In l a t e September these same t r e e s were i n s p e c t e d to see which ones had been atta c k e d by D. fionderosae and b e e t l e r e p r o d u c t i v e success was a s c e r t a i n e d . Table 5 summarizes the dates of these v a r i o u s steps. For r a t i n g the response to f u n g a l i n o c u l a t i o n , the bark surrounding the wood c h i p was peeled o f f and t r e e s were c l o s e l y i n s p e c t e d . Rated t r e e s f e l l i n t o one of t h r e e c a t e g o r i e s : s u s c e p t i b l e , r e s i s t a n t or i n t e r m e d i a t e , based upon c r i t e r i a s e t out i n Reid e t a l . (1967) and communicated by D.M. Shrimpton (pers. comm.). Those t r e e s which showed e x t e r n a l r e s i n flow, d i s c o l o r e d phloem and r e s i n - s o a k e d wood i n the v i c i n i t y of the wood c h i p were c l a s s e d r e s i s t a n t . Trees not having any of these c h a r a c t e r i s t i c s except f o r e x t e r n a l r e s i n flow were c a l l e d s u s c e p t i b l e . The i n t e r m e d i a t e category was assumed i f some but not a l l of the other c h a r a c t e r i s t i c s were present. R e s u l t s of the J u l y f u n g a l r a t i n g experiment are shown i n Table 6. Fungal r a t i n g s p r e d i c t e d t h a t i n a l l f o u r areas, r e g a r d l e s s of p o p u l a t i o n s t a t e , most of the t r e e s would be r e s i s t a n t to mountain pine b e e t l e . There were no i n d i c a t i o n s that a g r e a t e r p r o p o r t i o n of the t r e e s i n epidemic areas were s u s c e p t i b l e than i n endemic areas. The r e a l t e s t f o r t h i s method of a n a l y s i s came when t r e e s were i n s p e c t e d i n l a t e September, about two months a f t e r new a t t a c k s were i n i t i a t e d . T h i s l a t e date i n s u r e d t h a t t r e e s had 67 TABLE 5 L o g i s t i c s of f u n g a l assay experiment, 1973. P o p u l a t i o n j type | Epidemic I Endemic | L o c a t i o n | I-— Date of f u n g a l j i n o c u l a t i o n | E l k | 24 June | Terrac e I Parson I Lean | .. i . i 27 June I 25 June 1 i 126-27 June | No. of t r e e s j i n o c u l a t e d | 123 | 114 | 116 | 1 3 1 | Date f u n g a l j readings taken| 20 J u l y | 23 J u l y | 21 J u l y |22-23 J u l y | Approximate | week of | b e e t l e f l i g h t j 25 J u l y | 30 J u l y | 30 J u l y | 30 J u l y | Date b e e t l e | Success | measured | i 25 Sept. I J... 27 Sept. | 26 Sept _ji | 27 Sept. | -I. - - i 68 Resistance r a t i n g s of lodgepole pine t r e e s i n J u l y 1973 based on response to i n o c u l a t i o n with b l u e - s t a i n i n g f u n g i . TABLE 6 r~ : r 1 P o p u l a t i o n I Epidemic | Endemic | type | | | i h + ^ L o c a t i o n j Elk | T e r r a c e | Parson | Lean I I ^ + ,. j Number of | 123 | 114 | 116 | 131 | t r e e s I I I I I P r o p o r t i o n : I I I I I R e s i s t a n t | .70 | .55 | .77 I .72 I Intermediate| .28 | .38 | .21 | .24 | S u s c e p t i b l e | .02 I .07 I .02 | .04 | 69 had p l e n t y of time to respond to the bark b e e t l e s and t h e i r b l u e - s t a i n i n g f u n g i . Since i n o c u l a t e d t r e e s were numbered i n June, the same t r e e s were i n s p e c t e d i n September. Bark b e e t l e g a l l e r i e s were evaluated with the same method as de s c r i b e d under "Determination of Tree S t a t u s " . G a l l e r i e s were exposed by p e e l i n g back the bark and success of t r e e r e s i s t a n c e was determined. T h i s t e s t showed t h a t the r e s i s t a n c e r a t i n g s given to t r e e s by the f u n g a l assay method appear to be completely independent of t h e i r a c t u a l a b i l i t y to r e s i s t b e e t l e a t t a c k s . Most t r e e s t h a t were atta c k e d produced bark b e e t l e o f f s p r i n g , whereas the f u n g a l r a t i n g method p r e d i c t e d most of the t r e e s would be r e s i s t a n t . Data presented i n Ta b l e 7 show the number of t r e e s i n each study p l o t , c r o s s - c l a s s i f i e d by f u n g a l and b e e t l e r a t i n g c a t e g o r i e s . The Elk t r e e s r a t e d p o t e n t i a l l y r e s i s t a n t i n J u l y i l l u s t r a t e the lack of r e l a t i o n between these c a t e g o r i e s . Of these 85 t r e e s , 46 were atta c k e d and produced bark b e e t l e s , 22 were at t a c k e d but d i d not, and 17 were not at t a c k e d . Chi-square t e s t s of independence were done on the c r o s s - c l a s s i f i c a t i o n t a b l e s f o r each area s e p a r a t e l y . These t e s t s showed the b e e t l e bioassay and f u n g a l r a t i n g s were independent ( p r o b a b i l i t i e s i n Table 7 ) . There are s e v e r a l p o s s i b l e reasons why my c o n c l u s i o n s about t h i s assay method d i f f e r from those of Shrimpton and fieid (1973). 1) My sample s i z e f o r b e e t l e - h i t t r e e s (168) was s u b s t a n i a l l y l a r g e r than t h e i r s (44) . 2) My b l u e - s t a i n i n g TABLE 7 Results of beetle bioassay of fungal ratings of tree potential to resist. Trees are cross-classified by fungal rating and beetle rating. 1973 data. Area July fungal rating^ September beetle bioassay ratings S US1 US2 2 Not hit Row sums Sample size No. of trees h i t-X 2 P ELK Susceptible 0 0 0 2 2 11J_ L-Intermediate 19 1 7 9 36 123 95 8.25 = .25 Resistant 46 8 14 17 85 TERRACE Susceptible 1 0 1 6 8 Intermediate 9 0 3 30 42 114 23 7.95 = .25 Resistant 5 2 z 55 64 PARSCN Susceptible 1 0 0 1 2 Intermediate 1 3 2 19 25 116 28 5.39 = .5 Resistant 12 5 4 68 89 LEAN Susceptible 0 0 0 5 5 Intermediate 3 4 0 25 . 32 131 22 5.37 = .5 Resistant 9 4 2 79 94 July fungal rating classes: "September beetle bioassay rating classes: Susc. = Likely to be overcome i f attacked Inter. = Intenrediate resistance Resis. = Not likely to be overcome i f attacked S = Successfully produced broods US1 = Unsuccessful broods due to primary resinosis US2 = Unsuccessful broods due to secondary resinosis Not hit = No bark beetles attacked these trees 71 f u n g i may not have been of normal v i a b i l i t y , though t h i s i s d o u b t f u l . 3) The unusually c o o l and wet weather i n my study areas i n the summer of 1973 may have slowed the growth of the b l u e - s t a i n i n g f u n g i or maintained high tree v i g o r f o r l o n g e r than normal. D.B. Shrimpton (pers. comm.) does not f e e l t h a t these c o o l temperatures were a problem because the f u n g i a c t u a l l y p r e f e r c o o l e r temperatures than they experience i n normal, hot summers. U) My study t r e e s were i n d i f f e r e n t g e o g r a p h i c a l l o c a t i o n s from those o f Shrimpton and Reid (1973) and may have had s l i g h t l y d i f f e r e n t c h a r a c t e r i s t i c s . H. Summary Average age and p r o b a b i l i t y of succumbing to a t t a c k s were the o n l y t r e e c h a r a c t e r i s t i c s which were found to be c o n s i s t e n t l y d i f f e r e n t between epidemic and endemic areas. Trees i n outbreak areas were o l d e r than those i n endemic r e g i o n s , and w i t h i n a given s i z e and b e e t l e a t t a c k d e n s i t y c l a s s , t r e e s were more l i k e l y to be overcome and to s u c c e s s f u l l y produce b e e t l e s i n epidemic areas than endemic ones. T h i s l a t t e r f i n d i n g suggests e i t h e r t h a t epidemic t r e e s have d i f f e r e n t r e s i s t a n c e p o t e n t i a l s from endemic t r e e s or that epidemic b e e t l e s are more "potent" a t t a c k e r s than endemics. However, there were no c o n s i s t e n t p a t t e r n s between areas with r e s p e c t to the s p a t i a l d i s t r i b u t i o n of s u c c e s s f u l l y attacked t r e e s , average a t t a c k d e n s i t i e s , or p r o p o r t i o n s of t r e e s which s u c c e s s f u l l y r e s i s t e d mountain pine b e e t l e . The 72 method of assa y i n g t r e e r e s i s t a n c e p o t e n t i a l by using b l u e - s t a i n i n g f u n g i was found to be i n a c c u r a t e when compared with subsequent b e e t l e b i o a s s a y s of the same t r e e s . The r e s t of my work was conc e n t r a t e d on l o o k i n g f o r d i f f e r e n c e s i n bark b e e t l e s occupying the d i f f e r e n t study areas. 73 VI. SIMPLES OF FIELD BEETLES To study i n t r i n s i c d i f f e r e n c e s i n mountain pine b e e t l e p o p u l a t i o n s between epidemic and endemic areas, i t was necessary t o sample b e e t l e s from t r e e s that were as s i m i l a r as p o s s i b l e . T h i s sampling was done i n May of 1972 and 1973 when most i n s e c t s were i n t h i r d or f o u r t h i n s t a r or pupae. Bark was removed from s e v e r a l attacked t r e e s t o determine whether numerous l i v e l a r v a e and pupae were present. I f so, the t r e e was c u t down and two 30-inch-long s e c t i o n s were removed from the base. Sample t r e e s were s e l e c t e d to be h i g h l y p r o d u c t i v e i n order to o b t a i n a good y i e l d o f b e e t l e s i n the l a b . A f t e r logs were c u t , both ends were sealed with l a t e x p a i n t to r e t a i n moisture. Logs were then p l a c e d i n f i n e l y screened cages t o await emergence of a d u l t b e e t l e s . A l l four l o g s ( f i v e f o r Parson) f o r each area were s t o r e d t o g e t h e r i n the same cage. These cages were i n a room with 24 hour f l u o r e s c e n t l i g h t i n g , r e l a t i v e humidity of 60-70%, and temperatures ranging from 68 to 79° F (20-26° C ) . A d u l t s were c o l l e c t e d the day of emergence with an a s p i r a t o r and were stored i n j a r s with a l i c h e n , A l e c t o r i a spp., f o r f u t u r e s i z e measurement or f o r use l a t e r i n the day i n d i s p e r s a l or breeding experiments. B e e t l e s i z e s were measured f o r two main reasons. Heid (1963) has shown that a female's s i z e i s a determinant of the 74 q u a n t i t y of eggs produced. There are a l s o i n d i c a t i o n s t h a t f a t content, which i s weakly r e l a t e d t o body s i z e , a f f e c t s d i s p e r s a l behavior ( A t k i n s , 1966b, 1967, 1973a). Dead i n s e c t s were measured with an o c u l a r micrometer on e i t h e r 20 or 40 power. I n s e c t s were measured i n random order with r e s p e c t to sex, sampling l o c a t i o n and emergence date. Both pronotum width and e l y t r a length were measured. Three measurements were made on the f i r s t s e v e r a l hundred i n s e c t s to o b t a i n an estimate of e r r o r v a r i a n c e . Within-animal v a r i a n c e amounted to 0.001 mm f o r pronotum width (an i n s i g n i f i c a n t amount compared with the average width of 1.5 to 2.5 mm) and hence i t i s i g n o r e d i n a l l subsequent analyses. Females are l a r g e r than males, so a l l s t a t i s t i c a l a n a l y s es of s i z e data are performed with sexes separated. The f i r s t s t e p was to compare i n s e c t s i z e s between study areas. Adult b e e t l e s emerged i n the temporal p a t t e r n s shown i n F i g , 8, Some of the major d e v i a t i o n s from a normal d i s t r i b u t i o n are accounted f o r by the presence of holes i n the Lean cage d i s c o v e r e d on J u l y 2, 6, 16 and 17. Insect s i z e s v a r i e d with emergence date as shown i n F i g s . 9 and 10. Females predominated among the o f f s p r i n g with the sex r a t i o changing from 4:1 at the s t a r t of emergence to approximately 1:1 at the end. Pronotum width was h i g h l y c o r r e l a t e d with e l y t r a l e n g t h (r=.94, N=1350, P<.001), so i t w i l l be the only s i z e measure d i s c u s s e d . Beetle s i z e was r e g r e s s e d a g a i n s t emergence date, number of b e e t l e s of both sexes emerging the 75 FIGURE 8 Emergence patterns of adult mountain pine beetles from logs sampled from trees i n the f i e l d and stored i n the laboratory. The two areas on the l e f t are the epidemics and the two on the right are the endemics. N=total number of insects energing from logs, by area. AUQ/DNI9cJ3N3 S331339 d38WnN I d l O l 77 FIGURE 9 S i z e s of female mountain pine b e e t l e s i n r e l a t i o n to emergence date. v e r t i c a l bars r e p r e s e n t ± one standard e r r o r i n s i z e f o r b e e t l e s measured on t h a t date. Data are from l o g s sampled i n the f o u r 1973 study areas. U = the number of emerging i n s e c t s t h a t were s i z e d with a microscope. TOTAL NUMBER BEETLES EMERGING/DRY TOTRL NUMBER BEETLES EMERGING/DRY PRONOTUM WIDTH (MM) PRONOTUM WIDTH (MM) TOTAL NUMBER BEETLES EMERGING/DRY cz rn -< ID CD TOTAL NUMBER BEETLES EMERGING/DRY cr -< X) cz CD PRONOTUM WIDTH (MM) PRONOTUM WIDTH (MM) 8L 79 FIGURE 10 S i z e s of male mountain pine b e e t l e s i n r e l a t i o n to emergence date. V e r t i c a l bars r e p r e s e n t ± one standard e r r o r i n s i z e f o r b e e t l e s measured on t h a t date. Data are from l o g s sampled i n the f o u r 1973 study areas. N = the number of emerging i n s e c t s t h a t sere s i z e d with a microscope. TOTAL NUMBER BEETLES EMERGING/DAY TOTAL NUMBER BEETLES EMERGING/DAT CO CO PRONQTUM WIDTH (MM) cz m cz -< zo czz 4^ CO CD PRONQTUM WIDTH (MM) TOTAL NUMBER BEETLES EMERGING/DAY TOTAL NUMBER BEETLES EMERGING/DAY cz ro NE CO OJ CO , . c CO cz r~ 1—--< CO ro CO (VI CO zo ro cz GO - J ro co o ro CD CO CO CD CO ro o ro LO O CD CO X I zz r~ - J m co co ro ro ro PRONQTUM WIDTH (MM) J^, CO CO PRONQTUM WIDTH (MM) 08 81 same day i n that cage, and cumulative number of b e e t l e s emerging to t h a t date i n the cage. R e s u l t s i n Table 8 show data f o r the only s i g n i f i c a n t (P<.05) r e g r e s s i o n , which i s s i z e on emergence date (as measured i n numbers of days from s t a r t of emergence i n each cage). T h i s r e g r e s s i o n e s t a b l i s h e s that i n a l l areas but T e r r a c e Creek, e a r l y emerging male and female b e e t l e s are i n g e n e r a l l a r g e r than b e e t l e s of the same sex coming out l a t e r . These f i r s t - e m e r g i n g b e e t l e s may have a r i s e n from eggs l a i d the e a r l i e s t i n g a l l e r i e s , or from f i r s t a t t a c k i n g females. I t i s not p o s s i b l e t o know which i s the case because the p e r i o d over which any tr e e i s attacked by new females i s not p r e c i s e l y known. My own o b s e r v a t i o n s i n the f i e l d i n d i c a t e t hat l a s t a t t a c k s on any one t r e e occur 4-7 days a f t e r f i r s t a t t a c k s . A n a l y s i s of v a r i a n c e showed no trend f o r endemic b e e t l e s to be l a r g e r than epidemics (P>.1). Comparisons of r e g r e s s i o n s of s i z e on emergence date a l s o showed no t r e n d with r e s p e c t to p o p u l a t i o n type (ANOV A, P>.1). The c o e f f i c i e n t of v a r i a t i o n of s i z e (standard deviation/mean) was c a l c u l a t e d f o r each group of b e e t l e s by emergence date and by sex. Since the exact d i s t r i b u t i o n of the c o e f f i c i e n t of v a r i a t i o n i s unknown (N. G i l b e r t , p ers. comm.), I used non-parametrie t e s t s t o analyze the data. A Wilcoxon matched-pairs, signed-rank t e s t on these c o e f f i c i e n t s f o r corresponding emergence dates showed t h a t females had s i g n i f i c a n t l y h i g h e r v a r i a b i l i t y than males (N=31, 82 TABLE 8 C o r r e l a t i o n and r e g r e s s i o n data f o r b e e t l e s i z e as a f u n c t i o n of emergence date. Standard e r r o r s are i n parentheses. P o p u l a t i o n type | Epidemic | E ndemic L o c a t i o n | E l k | Te r r a c e | Parson | Lean HALESi ! I 1 I N r P r * | 226 | 56 | | .298 | .018 | | <. 01 | >. 1 | | .089 | .0003 | 110 .318 <. 01 .101 | 162 | . 157 I =.05 | .0 25 I n t e r c e p t | 1 .'99 (.024) | 1.88 (.06) | 1.98 (.033) | 1.87 (.027) Slope j-.009 (.002) I-.0OO5 (.004) -.009 (.003) | -.004 (.002) P | <.001 | >.1 | <.001 | =.046 ± FEMALES:. N r . P r * r _ ™ ^ , r , „., — f | 400 | 244 | | .131 | .014 | | <.01 | >. 1 | | .017 | .0002 | 264 .189 <.01 .036 | 456 | . 178 | <.01 | .032 I n t e r c e p t | 2.03 (.021) |2.01 (.03) | 1 .99 (.022) | 2.06 (.021) Slope | -.003 (.001) |-.0004 (.002) -.004 (.001) | -.006 (.001) P | <.01 | >.5 | <.005 | <.001 83 P<,005). The e c o l o g i c a l s i g n i f i c a n c e of g r e a t e r v a r i a b i l i t y i n the female p o p u l a t i o n i s not c l e a r . There could be s e l e c t i o n f o r s i z e v a r i a b i l i t y i n females, or the phenomenon could simply be due to s e l e c t i o n a g a i n s t v a r i a b i l i t y i n males but not females. The p o s s i b l e advantages of t h i s female s i z e v a r i a t i o n might l i e i n m a i n t a i n i n g v a r i a b i l i t y i n d i s p e r s a l p a t t e r n s by pioneer females (to be d i s c u s s e d l a t e r ) , or a b i l i t y to u t i l i z e t r e e s with phloem of d i f f e r e n t t h i c k n e s s e s . Friedman's two-way a n a l y s i s of variance showed that the c o e f f i c i e n t s of v a r i a t i o n were not a f f e c t e d i n e i t h e r males or females by emergence date, p l a c e , or p o p u l a t i o n type from which b e e t l e s were sampled ( a l l P>.1). 84 VII. BREEDING EXPERIMENTS i n t r o d u c t i o n I n t r a s p e c i f i c c o m p e t i t i o n i s an important determinant of r e p r o d u c t i v e success i n s e v e r a l bark b e e t l e s p e c i e s , i n c l u d i n g mountain pine b e e t l e ( M i l l e r and Keen,1960; Knight, 1961; McMullen and A t k i n s , 1961; Cole, 1962; R e i d , 1963; and Berryman and Pienaar, 1973). Competition i s d i r e c t l y r e l a t e d to a t t a c k d e n s i t y (McMullen and A t k i n s , 1961; Cole, 1962; Schmitz and Rudinsky, 1968). These workers found that as a t t a c k d e n s i t y on a t r e e i n c r e a s e s , a d u l t g a l l e r y l e n g t h decreases along with the number of eggs per u n i t g a l l e r y l e n g t h . L a r v a l s u r v i v a l a l s o drops o f f , probably due to l i m i t a t i o n s on the q u a l i t y and q u a n t i t y of food eaten. An i n t e r e s t i n g t r a d e o f f i s brought out here. I t i s advantageous to a t t r a c t numerous other bark b e e t l e s to the same t r e e i n order to i n c r e a s e the p r o b a b i l i t y of overcoming the t r e e ' s r e s i s t a n c e mechanisms; t h i s i s the f u n c t i o n of the pheromone systems. However, too many other bark b e e t l e s a t t a c k i n g the same t r e e r e s u l t s i n decreased r e p r o d u c t i v e success. T h i s i s why some bark b e e t l e s t h a t use pheromone a t t r a c t i n g systems a l s o have mechanisms which i n v o l v e e i t h e r other pheromones or t r e e chemicals t o turn o f f t r e e a t t r a c t i v e n e s s a f t e r a given number of a t t a c k s (Renwick and 85 V i t e , 1970). Because of i t s e f f e c t on crowding and subsequent r e p r o d u c t i v e success, the p r e c i s i o n of t h i s s w i t c h i n g - o f f mechanism may be c r i t i c a l i n determining how low a bark b e e t l e p o p u l a t i o n can be maintained i n endemic stands and how r a p i d l y p o p u l a t i o n s can b u i l d up i n epidemic areas. Attack d e n s i t i e s might a l s o a f f e c t o f f s p r i n g s i z e i n bark b e e t l e s through d e p l e t i o n of l a r v a l food supply at high d e n s i t i e s . Such crowding e f f e c t s are a g e n e r a l phenomenon f o r l e p i d o p t e r a n s p e c i e s (reviewed i n Gruys, 1970) but are not w e l l - s t u d i e d i n other groups. Female parent c h a r a c t e r i s t i c s , i n c l u d i n g s i z e , are a l s o thought to play a r o l e i n v a r i o u s r e p r o d u c t i v e measures i n c l u d i n g q u a n t i t y (Engelmann, 1970; HcGehey, 1971; Reid, 1963; Tantawy, 1961) and q u a l i t y (Wellington, 1957, 1960, 1965) of eggs produced. P h y s i o l o g i c a l r a t h e r than g e n e t i c mechanisms are r e s p o n s i b l e f o r some of these e f f e c t s . F i n a l l y , i n s e c t s i z e has been shown t o be a h e r i t a b l e c h a r a c t e r i s t i c i n Pj£oso£hila (Reeve and Robertson, 19 53; Tantawy, 1961); o f f s p r i n g of l a r g e maternal parents are l a r g e r than o f f s p r i n g of s m a l l females. Methods In t h i s study of D. £ondejcosae, experiments were designed to look a t the e f f e c t s of both a t t a c k d e n s i t y and p a r e n t a l female s i z e on r e p r o d u c t i v e success and o f f s p r i n g s i z e . I 86 a l s o wanted to see whether t h e r e were i n t r i n s i c d i f f e r e n c e s between r e p r o d u c t i v e success or s i z e h e r i t a b i l i t y ( e i t h e r g e n e t i c a l l y or p h y s i o l o g i c a l l y mediated) i n b e e t l e s from the d i f f e r e n t sampling l o c a t i o n s and p o p u l a t i o n types. The mountain pine b e e t l e can be bred i n the l a b o r a t o r y i n cut s e c t i o n s of lodgepole pine and o f f s p r i n g w i l l begin to emerge approximately 7-8 weeks l a t e r . The experimental design i s shown i n F i g . 11. T e s t s on p a r e n t a l female s i z e e f f e c t s were made only with b e e t l e s emerging from the p o p u l a t i o n sampled at Lean Creek. S i z e d females were s o r t e d i n t o l a r g e and s m a l l types (exact s i z e d i s t r i b u t i o n s are given i n Tab l e 9 ) . Females from each s i z e group were then bred at both high (18 p a i r s / s q f t ) and low (2 p a i r s / s g f t ) d e n s i t i e s on d i f f e r e n t l o g s . There were two r e p l i c a t e logs f o r each experimental group. S i m i l a r experiments were done with b e e t l e s emerging from logs sampled from each of the other three areas, but i n s t e a d of using l a r g e and s m a l l p a r e n t a l females, i n t e r m e d i a t e s i z e d ones were used (Table 9) . Logs f o r breeding experiments were cut on 5 J u l y 1973 from healthy, unattacked lodgepole pine i n the Smith Creek v a l l e y , southeast of P r i n c e t o n , B.C. Twenty 30-inch-long s e c t i o n s averaging 8.5 inches i n diameter were removed from a t o t a l of f i v e d i f f e r e n t t r e e s . A f t e r two days of drying to prevent molding, l o g ends were p a i n t e d with l a t e x p a i n t to r e t a i n the remaining moisture. P a r e n t a l i n s e c t s were c o l l e c t e d on the evening cf 9 J u l y 87 FIGOBE 11 The experimental design f o r 1973 breeding experiments. These experiments were designed to t e s t the e f f e c t s of a t t a c k d e n s i t y and parent female s i z e on r e p r o d u c t i v e success and o f f s p r i n g s i z e . l o g numbers given at bottom are the r e p l i c a t e l o g numbers which are r e f e r r e d t o i n the t e x t . A t o t a l o f 200 parent females were used i n t h i s experiment. High a t t a c k d e n s i t y = 18 p a i r s / s g f t Low a t t a c k d e n s i t y = 2 p a i r s / s q f t Area LEAN Emerging Population Large 0_ Small 0_ high low high lew attack attack attack attack density density density density N=2 . N=2 N=2 N=2 #'s: 1,7 2,8 3,5 4,6 For each of the 3 other study areas Emerging Population Average 0_ Average 0_ high low high low attack attack attack attack density density density density N=2 N=2 N=2 N=2 9,11,13 10,12,15 14,17,18 16,19,20 TABLE 9 Size distributions of female parents used i n breeding experiments. Pronotum widths are i n mm. Average sized females: Attack Density = 2/sq f t Attack Density = 18/sq f t Number Size Number Size Mean Size 2 1.93 1 2.27 3 2.13 6 2.00 1.95. 4 1.87 • 4 1.73 2 sized females: Attack Density = 2/sq f t Attack Density = 18/sq f t Number Size Number Size Mean Size 2 2.13 2 2.40 (Log #1) 3 2.27 2.18 13 2.13 3 2.27 (Log # 7) 12 2.13 2.13 3 2.00 Small sized.females: Attack Density = 2/sq ft Attack Density = 18/sq f t Number Size Number Size .Mean Size 2 1.73 14 1.87 3 1.73 1.83 1 1.60 90 (middle of the emergence period) and s i z e d immediately with a b i n o c u l a r d i s s e c t i n g microscope equipped with an o c u l a r micrometer. Sexes were separated by h o l d i n g i n s e c t s l o o s e l y between the f i n g e r s and l i s t e n i n g f o r male s t r i d u l a t i o n , a method used f o r another s c o l y t i d by Chapman (1955). I t e r a t i v e sampling experiments with s e v e r a l s m a l l groups of b e e t l e s and subsequent m i c r o s c o p i c examination f o r the male s t r i d u l a t i n g mechanism (Lyon, 1958) confirmed that t h i s a u d i t o r y method was a r e l i a b l e way of sexing mountain pine b e e t l e . A c t u a l breedings were begun 10 J u l y a f t e r i n s e c t s had been s t o r e d o v e r n i g h t i n j a r s with some l i c h e n , A l e c t o r i a spp., to prevent b e e t l e s from walking over one another and l o s i n g t h e i r antennae and l e g s . A t k i n s ' (1973b) technique was used f o r s t a r t i n g breedings. A 6 mm hole was d r i l l e d i n the host l o g through the outer bark and to the depth of the phloem. k s i z e d female was then put i n t o the hole and i t was plugged with an empty h a l f of a #1 g e l a t i n c a p s u l e . Twenty-four hours l a t e r , each hole was checked f o r s i g n s of bor i n g dust and i f any was present, the capsule was removed, a randomly s i z e d male from the corresponding geographic l o c a t i o n was added, and the cap s u l e was r e p l a c e d . Some females d i d not immediately s t a r t d i g g i n g g a l l e r i e s , e i t h e r because the wood was not s u i t a b l e i n t h e i r p a r t i c u l a r l o c a t i o n , or f o r some p h y s i o l o g i c a l reason. Those which had not dug any g a l l e r y a f t e r one day were r e p l a c e d by a female of the sane s i z e , emergence date and sampling l o c a t i o n . Of the 91 200 females s t a r t e a on 10 J u l y , 31 did not s t a r t d i g g i n g t h a t day. The replacement procedure was f o l l o w e d and 9 females s t i l l had not taken by the 12th. However, a l l females had males added t o t h e i r g a l l e r i e s by the 14th save one, which f i n a l l y took on the 15th (on Log 10). Only one square f o o t was used on each l o g f o r l a y i n g out the entrance h o l e s . The s p a t i a l arrangement of a t t a c k h o l e s was as shown i n F i g . 12. The extreme values of 2 and 18 entrances per square f o o t were chosen because they were a t the extremes of observed f i e l d a t t a c k d e n s i t y d i s t r i b u t i o n s ( F i g . 4 ) , and they were thought to be most l i k e l y t c produce d i f f e r e n c e s i n o f f s p r i n g s i z e or r e p r o d u c t i v e success. A f t e r i n i t i a t i o n of g a l l e r y c o n s t r u c t i o n and a d d i t i o n of males, a l l l o g s were put i n s i d e 3 0 - i n c h - t a l l p l a s t i c b a r r e l s with screened tops and with a g l a s s Mason j a r mounted i n a hole cut i n the b a r r e l s i d e . These b a r r e l s were s t o r e d i n a room with 24 hour f l u o r e s c e n t l i g h t i n g from above, 60-70% r e l a t i v e humidity and a constant temperature of 70° F (21° C). When a b a r r e l top was l o o s e l y covered with black p l a s t i c , p h o t o p o s i t i v e emergent b e e t l e s moved i n t o the j a r , s i n c e t h i s was the o n l y source of l i g h t . Emergent i n s e c t s were c o l l e c t e d d a i l y from these j a r s , counted, sexed, and s t o r e d i n the same way as b e e t l e s from f i e l d p o p u l a t i o n s . 92 FIGURE 12 S p a t i a l arrangement of a t t a c k h o l e s which were d r i l l e d f o r females used i n the breeding experiments. A l l of these h o l e s were used f o r the a t t a c k d e n s i t y of 18/sg f t and the two c i r c l e d holes were used f o r the a t t a c k d e n s i t y of 2/sg f t . The r e l a t i v e p o s i t i o n s of some a d u l t g a l l e r i e s are a l s o shown. 93 7 • { . ; : ; ! •; • J •• .1 11 a, 4 ' I I 1 i ' I • < 1 • < i 1 « .' > I , I i , 1 , ' « •' . ' ' . " Li ! • •' •' '' • * o « i • 6' !« v> -12 I I !. ,i ° 0 ; < i t < • • < ! < . ' : i 1 1 i • ' • :\ ,'/ 0 0 0 0_ 0 o 0 0 1 1 1 1 t 3" I \ f 1 1 1 1   1 1 1 1 I I< 23s" ->| I 12 94 R e s u l t s A., Attack Density E f f e c t on Reproductive Success The e f f e c t s of a t t a c k d e n s i t y on r e p r o d u c t i v e success are shown i n Table 10. Hhen a l l twenty experimental l o g s are c o n s i d e r e d , there are s i g n i f i c a n t d i f f e r e n c e s i n v a r i o u s r e p r o d u c t i v e measures between a t t a c k d e n s i t y treatments. The number of b e e t l e s emerging per female parent was s i g n i f i c a n t l y higher at low a t t a c k d e n s i t y than a t high a t t a c k d e n s i t y (44 vs. 12). More t o t a l o f f s p r i n g emerged per l o g a t high a t t a c k d e n s i t i e s , but i t was only 2.46 times as many as at the low a t t a c k d e n s i t i e s , not 9 times as many as expected i f there were no a t t a c k d e n s i t y e f f e c t . Hhen experimental l o g s with l a r g e and s m a l l female parents are removed from t h i s a n a l y s i s l e a v i n g only logs with average s i z e d parents (bottom h a l f Table 10), the same r e s u l t s are obtained except t h a t the number of b e e t l e s emerging per entrance i s not s i g n i f i c a n t l y d i f f e r e n t between d e n s i t y treatments. The same t r e n d i s present as before, but between-log v a r i a n c e i s too g r e a t f o r the d i f f e r e n c e s to be s i g n i f i c a n t . These a t t a c k d e n s i t y e f f e c t s agree with the f i n d i n g s of Cole (1962) and Reid (1963) i n showing t h a t more crowded c o n d i t i o n s r e s u l t i n fewer o f f s p r i n g per parent. However, my 95 TABLE 10 Attack d e n s i t y e f f e c t s on r e p r o d u c t i v e success of mountain pine b e e t l e . N g i v e s the number of r e p l i c a t e logs i n the treatment c l a s s . Means are given and standard e r r o r s are i n parentheses. Data from a l l 20 l o g s : | P a r e n t a l | Number e x i t | Number b e e t l e s | T o t a l number | I attack | holes per | emerging per | o f f s p r i n g I I d e n s i t y j entrance | entrance |emerging | I High | 10. 09 (0.86)~f 12.17 (1. 4) f 220.8 (24. U)] I »=10 I I I I \ Low ~ | 50.65 (10.8) | 44.15 (12.5) j~89.6 (24.8) \ I N=10 | | | | | ^ | j | T - s t a t i s t i c | 3.76 | 2.53 | 3.78 | I P I <,01 | =.02 | <.01 | « L 1. L I Data f o r l o g s #9-20 only (no l a r g e or s m a l l female parents) : i T 1 ~t 1 | P a r e n t a l | Number e x i t | Number b e e t l e s j T o t a l number j | a t t a c k | holes per I emerging per ( o f f s p r i n g I | d e n s i t y \ entrance | entrance |emerging I 1 +- +— H 1 I High | 10.96 (1.07) | 13.23 (1.7) | 238.2 (31 .3) | I N=6 | | | | | Low | 56.92 (16.1) J 48.92 (19.9) | 100.0 (39. 1)1 I N=6 | | | | |~ T - s t a t i s t i c } 2.85 { 1.78 f 2.76 t I P | <.02 | >. 1 | <.02 | 96 o f f s p r i n g per parent r a t i o s are much l a r g e r than Cole's {at an attack d e n s i t y of 18/sq f t he got 0.3 and I got 6.08), probably because Cole used o n l y one-foot-long l o g s f o r breeding s u b s t r a t e s . I used l o g s two and one h a l f f e e t l o n g and t h i s reduced the crowding of g a l l e r i e s ; Reid (1962b) has noted t h a t a d u l t females i n the l a b o r a t o r y w i l l t u rn around and s t a r t d i g g i n g downward upon r e a c h i n g the end of the l o g . T h e r e f o r e , Cole was probably working with much higher e f f e c t i v e d e n s i t i e s than i n d i c a t e d by h i s number of entrances per sg f t . T - t e s t s w i t h i n a t t a c k d e n s i t y treatments showed that there was no e f f e c t on r e p r o d u c t i v e success of l o c a t i o n or p o p u l a t i o n type from which parents were sampled (P>.1). Number of e x i t h oles per entrance i s a u s e f u l measure f o r f i e l d d e t e r m i n a t i o n of emergent b e e t l e p o p u l a t i o n s (r=.89, P<.01). However, a c o r r e c t i o n f u n c t i o n i s needed because, as Reid(1963) and my own data i n F i g . 13 have shown, s e v e r a l b e e t l e s tend to come from each emergence hole under high a t t a c k d e n s i t y c o n d i t i o n s . a l s o , a t low d e n s i t i e s , t h e r e o c c a s i o n a l l y are fewer emergent b e e t l e s than e x i t h o l e s ( F i g u r e 13). T h i s i s probably a t t r i b u t a b l e to the observed behavior of some b e e t l e s c u t t i n g emergence holes and then not using them (Reid, 1963). Other data which are r e l e v a n t here but which come from t r e e s sampled i n the f i e l d i n 1972 and 1973, r a t h e r than breeding experiments, are i l l u s t r a t e d i n F i g . 14. Trees sampled i n d i f f e r e n t study areas as sources of b e e t l e 97 FIGURE 13 The r e l a t i o n between numbers of b e e t l e s emerging per sq f t and numbers of e x i t holes counted per sq f t i n the same l o g . A 1:1 r a t i o i s i n d i c a t e d by the dashed l i n e ; p o i n t s f a l l i n g above t h i s l i n e i n d i c a t e that t h e r e were more b e e t l e s emerging than the number of e x i t h o l e s . Each p o i n t r e p r e s e n t s one l o g i n the 1973 breeding experiments. 75 Q. FT 6 0 -cn C D ^ 4 5 -C D Crl L U 2 1 3 0 -L U C O L U i i I— L U 1 5 -L U C Q # 0 -1973 BREEDING EXPERIMENTS LOGS CLASSED BY ATTACK DENSITY < = HIGH D E N S I T Y , + = LOW DENSITY •k 0 10 20 30 40 # •EX IT H O L E S / S Q . F T . 50 CO 99 FIGURE 14 Data from l o g s sampled i n the f i e l d showing the r e l a t i o n between a t t a c k d e n s i t y and r e s u l t i n g numbers of emergence hol e s per a t t a c k . Data are s p l i t i n t o those t r e e s sampled i n 1972 and those i n 1973. 100 # RTTnCKS/ SQ. F T . # ATTACKS/ SQ. F T . 101 p o p u l a t i o n s show the e f f e c t of i n c r e a s i n g a t t a c k d e n s i t y on the number of emergence ho l e s per entrance. Both r e g r e s s i o n s and c o r r e l a t i o n s are s i g n i f i c a n t with P <.01 (Table 11). These f i e l d data are c o n s i s t e n t with the r e s u l t s of my breeding experiments i n showing t h a t higher a t t a c k d e n s i t i e s r e s u l t i n fewer o f f s p r i n g per a d u l t . No d i f f e r e n c e s were found between epidemic and endemic samples i n the numbers of emergence ho l e s per a t t a c k ( t - t e s t r P>.1). B. Attack Density E f f e c t On O f f s p r i n g S i z e A t w o - l e v e l nested a n a l y s i s of variance on the data from a l l 20 l o g s showed t h a t a t t a c k d e n s i t y had a s i g n i f i c a n t e f f e c t on o f f s p r i n g s i z e (P<.001) but the between-log e f f e c t , though always s m a l l e r , was a l s o s i g n i f i c a n t (p<.001). T h i s was t r u e f o r both males and fe males (Table 12) • In other words, the e f f e c t of d i f f e r e n c e s between l o g s on o f f s p r i n g s i z e i s about the same as the e f f e c t of a t t a c k d e n s i t y . Looking at the s i z e d i s t r i b u t i o n s (Figs. 15 and 16), the a t t a c k d e n s i t y e f f e c t s are present (higher a t t a c k d e n s i t i e s producing s m a l l e r o f f s p r i n g ) and s t a t i s t i c a l l y s i g n i f i c a n t f o r females o n l y (X 2, P<.03). F i g . 15 i l l u s t r a t e s o f f s p r i n g s i z e s for a l l 20 experimental l o g s while F i g . 16 shows onl y o f f s p r i n g from l o g s 9-20 (average s i z e d parent females). C. P a r e n t a l Female S i z e E f f e c t On Reproductive Success Table 13 shows that l a r g e females tend to produce more TABLE 11 The effect of attack density on reproductive success as measured by number of emergence holes per attack. Data come from trees which were sampled in 1972 and 1973 as sources of beetle populations for laboratory studies. Regressions are illustrated in Figure.14. Standard errors are in parentheses. 1972 data: N = 21 Correlation: r = 0.618 P < .01 that r = 0.0 r 2 = 0.38 Regression: Y-intercept a = 14.95(2.05) Slope b = -1.449(0.38) P < .005 that b = 0.0 1973 data: N = 17 Correlation: r = 0.763 P < .01 that r = 0.0 r 2 = 0.58 Regression: Y-intercept Slope a = 13.12(1.54) b = -0.827(0.16) P < .001 that b = 0.0 TABLE 12 Results of a two--level nested analysis of variance on data from a l l 20 experimental breeding logs. The dependent variable i s beetle pronotum width and the independent variables are attack density and replicate log number. Males: Term Attack Density Log number Remainder D.F. Mean Square 1 0.01729 18 0.00987 494 0.00131 13.17 COOl 7.51 <.001 Comparison of Attack Density mean square with Log # mean square F = 1.753, P>.2 Females: Term D.F. Mean Square F P Attack Density 1 0.020 12.92 <.001 Log number 18 0.00929 5.98 <.001 Remainder 817 0.00155 Comparison of Attack Density mean square with Log # mean squar F = 2.16, P>.1 104 FIGURE 15 Size d i s t r i b u t i o n s of o f f s p r i n g c l a s s e d by sex and a t t a c k d e n s i t y a t which parents were bred. Chi-square values were found using frequency data. Data i n t h i s f i g u r e d i f f e r from those i n F i g u r e 16 i n that o f f s p r i n g from a l l 20 breeding l o g s are i l l u s t r a t e d , i n c l u d i n g those from lo g s with l a r g e and s m a l l female parents. Sample s i z e s are as f o l l o w s : Females from high a t t a c k d e n s i t y = 582 Females from low a t t a c k d e n s i t y = 255 Males from high a t t a c k d e n s i t y = 317 Males from low a t t a c k d e n s i t y = 197 SIZES OF OFFSPRING BY DENSITY TYPE HIGH . LOW N=837 FEMRLES X 2 =31.3 P <.OQ5 1.7 1 .8 1 .9 2 2 .1 PRONOTUM WIDTH (MM) 2 . 2 n r h 2 . 3 SIZES OF OFFSPRING BY DENSITY TYPE HIGH . LOW N=514 MALES —\->-X =25.6 P <.Q3 1-7 1.8 1 . 9 2 2 .1 PRONOTUM WIDTH (MM) •'H m 2 . 2 2 . 3 106 FIGURE 16 S i z e d i s t r i b u t i o n s of o f f s p r i n g c l a s s e d by sex and attack d e n s i t y a t which parents were bred. Chi-square values were found using frequency data. Data i n t h i s f i g u r e d i f f e r from those i n F i g u r e 15 i n t h a t only o f f s p r i n g from the 12 lo g s with average s i z e d parent females are used. Sample s i z e s are as f o l l o w s : Females from high a t t a c k d e n s i t y = 362 Females from low a t t a c k d e n s i t y = 163 Males from high a t t a c k d e n s i t y = 214 Males from low a t t a c k d e n s i t y = 134 107 30-24 • SIZES OF OFFSPRING BY DENSITY TYPE HIGH — . LOW N=525 FEMALES 1 8 -r=26.05 P =026 1 2 -r i 1 -5 1.6 1 .7 1.8 1 . 9 2 2 .1 PRONOTUM WIDTH (MM) 11 11 11 2 . 2 2.3 2 . 4 CE I ZD D_ O LU C_) ce LU Q_ 30-24 18-12 - P -SIZES OF OFFSPRING BY DENSITY TYPE HIGH . LOW 1 .5 1 .6 1 .7 N=348 MALES X 2 = 20.9 P =0.11 1.0 1 . 9 2 2 .1 2 . 2 PRONOTUM WIDTH (MM) 2 . 3 2 . 4 108 TABLE 13 E f f e c t s of female parent s i z e on r e p r o d u c t i v e success. N gi v e s the number of r e p l i c a t e l o g s i n the treatment c l a s s . Means are given and standard e r r o r s are i n parentheses. Both a t t a c k d e n s i t i e s lumped: i r T 1 1 I P a r e n t a l J Number e x i t | Number b e e t l e s | T o t a l number | | female I h o l e s per | emerging per | o f f s p r i n g | I s i z e | entrance | entrance I emerging I \ h + -I ^ I Large | 33.4 (16.2) | 32.2 (14.1) | 171.3 (52.4) | | N=4 | | | | | Small 1 16.6 (6.2) |~ 15.4 (5.8) |~97.5 (32. 4) { I N=4 | | | | 1 j j h 1 | T - s t a t i s t i c | 0.97 I 1.10 | 1.20 | I P | >0.2 | >0.2 | >0.2 | I . L L L J 109 o f f s p r i n g than s m a l l ones. However, the d i f f e r e n c e s cannot be shown s t a t i s t i c a l l y because of the between-log v a r i a n c e . T h i s i s s t i l l true even when these data are broken down by a t t a c k d e n s i t y c l a s s e s (Table 14). Reid (1962b) found that l a r g e r female mountain pine b e e t l e s l a i d more eggs per day than s m a l l e r females. U n f o r t u n a t e l y , h i s data do not extend through to emerging a d u l t s . The presence of l a r g e r n u t r i t i o n a l s t o r e s f o r egg production i s probably the reason why l a r g e r females produce more o f f s p r i n g . D. P a r e n t a l Female S i z e E f f e c t s On O f f s p r i n g S i z e My experiments showed t h a t there was a s i g n i f i c a n t r e l a t i o n between o f f s p r i n g s i z e and p a r e n t a l female s i z e , but only f o r female o f f s p r i n g of parents bred at high a t t a c k d e n s i t y (X 2, P<,01). Large females tended to produce l a r g e r female o f f s p r i n g than s m a l l females. Other comparisons of o f f s p r i n g d i s t r i b u t i o n s were not s i g n i f i c a n t (X 2,P>.1). A l s o , o f f s p r i n g - o n - p a r e n t r e g r e s s i o n s , a standard method of measuring h e r i t a b i l i t y (Falconer, 1960) , d i d not r e v e a l s i g n i f i c a n t r e l a t i o n s h i p s except perhaps f o r female o f f s p r i n g (Table 15). Lack of a s i g n i f i c a n t r e g r e s s i o n i s not s u r p r i s i n g i n view of the experimental d e s i g n : two non-overlapping d i s t r i b u t i o n s of s i z e s of female parents had to be used (Table 9) and f o r any one experimental l o g i t was im p o s s i b l e t o know which o f f s p r i n g came from which females w i t h i n the d i s t r i b u t i o n s . 110 TABLE 14 Table 13 broken down by a t t a c k d e n s i t i e s . E f f e c t of female parent s i z e on r e p r o d u c t i v e success. Means are given and standard e r r o r s are i n parentheses. High a t t a c k d e n s i t y o n l y : P a r e n t a l female s i z e Large N=2 Small N=2 T - s t a t i s t i c P Humber e x i t h oles per entrance 10.8 (1.3) Number b e e t l e s emerging per entrance —+ 13.1 (4.2) 6.74 (0.15) + 3.01 >.2 8.1 (1.0) 1. 15 >.4 T o t a l number o f f s p r i n g emerging 240.0 (72.0) *~149.5 (22. 5)^ + 1.20 >.2 i Low attack d e n s i t y o n l y : r 1 1 : T : 1 | P a r e n t a l | Number e x i t | Number b e e t l e s | T o t a l number | | female I holes per | emerging per | o f f s p r i n g I | s i z e | entrance | entrance |emerging | |~ Large | 56.0 ( 2 3 . 5 ) ~ t 51.2 (21.2) j 102.5 ( 4 2 . 5 ) t | N=2 | | | | | Small ~ | 2 6 . 5 (6.0) |"~ 22.8 (9.7) J 4 5 . 5 ( 1 9 . 5 ) ~ | I N=2 I I I I |~ T - s t a t i s t i c f 1.22 ~| 1.22 f 1.22 { I P | >.2 I >.2 | >.2 | .j TABLE 15 Relation between offspring size and female parent size using offspring on parent regressions. Standard errors are in parentheses. Female offspring: N = 312 r = 0.103 P = .06 that r = 0.0 r 2 = 0.01 Regression: Y-intercept a. = 0.733 (0.475) slope b = 0.432 (0.237) P = .072 that b = 0.0 Male offspring: N = 166 No significant correlation or regression (P > .1) 112 However, when we look a t the s i z e d i s t r i b u t i o n s of o f f s p r i n g , an i n t e r e s t i n g phenomenon appears. F i g . 17 shows the data f o r a l l e i g h t l o g s i n v o l v e d i n the p a r e n t a l s i z e experiments, r e g a r d l e s s of a t t a c k d e n s i t y . O f f s p r i n g o f s m a l l female parents appear to have a bimodal s i z e d i s t r i b u t i o n whereas o f f s p r i n g of l a r g e females tend to have the expected normal d i s t r i b u t i o n . T h i s b i m o d a l i t y seems to be much weaker in male o f f s p r i n g than females. I t i s very d i f f i c u l t to show s t a t i s t i c a l l y t h a t a d i s t r i b u t i o n i s bimodal; the bes t one can do i s to show that a d i s t r i b u t i o n i s more p l a t y k u r t i c ( f l a t t e n e d ) than a normal d i s t r i b u t i o n , and such i n f o r m a t i o n i s not very u s e f u l i n the present c o n t e x t . When the data i n F i g . 17 are broken down i n t o the two att a c k d e n s i t y treatments, another r e l a t i o n s h i p appears. Female o f f s p r i n g ( F i g . 18) of s m a l l maternal parents s t i l l show a s t r o n g e r b i m o d a l i t y than o f f s p r i n g of l a r g e females, r e g a r d l e s s of a t t a c k d e n s i t y , but male o f f s p r i n g (Fig. 19) show weak b i m o d a l i t y r e g a r d l e s s of p a r e n t a l female s i z e . Chi-square values shown i n F i g u r e s 17-19 are the r e s u l t s of comparing the s i z e d i s t r i b u t i o n s of o f f s p r i n g of l a r g e females with those of s m a l l females. There are s e v e r a l p o s s i b l e causes of t h i s b i m o d a l i t y i n o f f s p r i n g s i z e . F i r s t , there may be an emergence date e f f e c t on s i z e , as with f i e l d p o p u l a t i o n s , and o f f s p r i n g emergence dates may be l i n k e d t o parent female s i z e . F i g s . 20-23 show 113 FIGURE 17 Siz e d i s t r i b u t i o n s of o f f s p r i n g c l a s s e d by sex and s i z e of female parents. Both a t t a c k d e n s i t i e s are lumped. Chi-sguare values were found u s i n g frequency data. Sample s i z e s are as f o l l o w s : Females from l a r g e parents = 187 Females from s m a l l parents = 125 Males from l a r g e parents = 108 Males from s m a l l parents = 58 114 40 3 2 -24 OFFSPRING SIZES CLASSED BY PARENT FEMALE SIZE BOTH ATTACK DENSITIES LUMPED LARGE . SMALL N=312 FEMALES X 2 ~28.0 P <.oi 16 -41 1 .5 1 .6 1 .7 1 .8 1 .9 2 2 .1 2 . 2 OFFSPRING PRONOTUM WIDTH (MM) •~1 rP 'T> 2 . 3 2 . 4 4 0 -3 2 -24 16 -OFFSPRING SIZES CLASSED BY PARENT FEMALE SIZE BOTH ATTACK DENSITIES LUMPED LARGE . SMALL N=166 MALES a 1 .5 1 .6 1 .7 X2=11.9 P > o . i 1.8 1 .9 2 2 .1 2 . 2 OFFSPRING PRONOTUM WIDTH (MM) 2 . 3 2 . 4 115 FIGURE 18 Size d i s t r i b u t i o n s of female o f f s p r i n g c l a s s e d by s i z e of female parent and a t t a c k d e n s i t y at which parents were bred. Top graph i s high a t t a c k d e n s i t y and bottom graph i s low a t t a c k d e n s i t y . Chi-square values were found using frequency data. Sample s i z e s were as f o l l o w s : Females from high a t t a c k d e n s i t y and l a r g e parents = 131 Females from high a t t a c k d e n s i t y and small parents = 89 Females from low a t t a c k d e n s i t y and l a r g e parents = 56 Females from low a t t a c k d e n s i t y and smal l parents = 36 O F F S P R I N G S I Z E S C L A S S E D BY PARENT F E M A L E S I Z E H I G H ATTACK D E N S I T Y L A R G E •. SMALL ' N=220 F E M A L E S X 2 =27.6 P <.oi 1 ' i - i i • • 5 1 .6 1.7 1.8 1.9 2 2 .1 2 . 2 O F F S P R I N G PRONOTUM WIDTH (MM) i p P 2 . 3 2 . 4 O F F S P R I N G S I Z E S C L A S S E D BY PARENT F E M A L E S I Z E LOW ATTACK D E N S I T Y L A R G E , SMALL N=92 F E M A L E S 1 . 5 1 .6 -14-1 .7 1 . 8 1 . 9 2.1 X =17.1 P >o.i JLH 2 . 2 P i n 2 . 3 2 . 4 O F F S P R I N G PRONOTUM WIDTH (MM) 117 FIGURE 19 Size d i s t r i b u t i o n s of male o f f s p r i n g c l a s s e d by s i z e of female parent and a t t a c k d e n s i t y a t which parents were bred. Top graph i s high attack d e n s i t y and bottom graph i s low at t a c k d e n s i t y . Chi-square values were found u s i n g frequency data. Sample s i z e s were as f o l l o w s : Males from high a t t a c k d e n s i t y and l a r g e parents = 7 1 Males from high a t t a c k d e n s i t y and smal l parents = 32 Males from low a t t a c k d e n s i t y and l a r g e parents = 37 Males from low a t t a c k d e n s i t y and smal l parents = 26 OFFSPRING SIZES CLASSED BY PARENT FEMALE SIZE HIGH ATTACK DENSITY LARGE . SMALL N=103 MALES X2=14.1 P >o.i SL JL 1 -6 1 .7 1 .8 1.9 2 2 .1 2 . 2 OFFSPRING PRONOTUM WIDTH (MM) 2.3 OFFSPRING SIZES CLASSED BY PARENT FEMALE SIZE LOW ATTACK DENSITY LARGE-' . SMALL N=63 MALES 1.6 1 .7 XL P >o.i 1.8 1 .9 2 2 .1 2 . 2 OFFSPRING PRONOTUM WIDTH (MM) 2 . 3 119 FIGURE 20 S i z e s of female mountain pine b e e t l e s i n r e l a t i o n to emergence date. v e r t i c a l bars r e p r e s e n t ± one standard e r r o r i n s i z e f o r b e e t l e s measured on t h a t date. Data are from breeding l o g s 1-4. PRONOTUM WIDTH (MM) PRONOTUM WIDTH (MM) OZT 121 FIGURE 21 S i z e s of female mountain pine b e e t l e s i n r e l a t i o n to emergence date. V e r t i c a l bars r e p r e s e n t ± one standard e r r o r i n s i z e f o r b e e t l e s measured on t h a t date. Data are from breeding l o g s 5-8. ZZT 123 FIGURE 22 S i z e s of male mountain pine b e e t l e s i n r e l a t i o n t o emergence date, v e r t i c a l bars r e p r e s e n t ± one standard e r r o r i n s i z e f o r b e e t l e s measured on t h a t date. Data are from breeding l o g s 1-4. PRONOTUM WIDTH (MM) PRONOTUM WIDTH (MM) cn ' •— PO f\) (V) — J co >-» co cn co m ~a — t m zz CO m o —I o aa m TO cn CD ro ro cn cn cn ro ro cn CO II cn ro tt ro ZD r~ •-. m co co co co m T3 — i m CD m o o —I Q cn m 70 m co cn co C O m CD m TO Q o — t o CD m TO PRONOTUM WIDTH (MM) — _ PO ro ro cn --j co »-» co cn PRONOTUM WIDTH (MM) CO ro ro CD CD co ro t—> ro co co z a II cn ro CO tt -< ZD r~ >~-m co co co co m TP — i m r s co m TO Q o —I Q CD m 70 — — — ro cn — i co » 125 FIGURE 23 S i z e s of female mountain pine b e e t l e s i n r e l a t i o n to emergence date. V e r t i c a l bars r e p r e s e n t ± one standard e r r o r i n s i z e f o r b e e t l e s measured on t h a t date. Data a r e from breeding l o g s 5-8. PRONOTUM WIDTH (MM) PRONOTUM WIDTH (MM) cn — — ro ro ro —i co * co cn o cn co m co C D —I C O CO m "U —I m is CD m TO Q o —I Q C O m ZO >— >— ro ro ro cn —j co i—» co cn cn CD ro ro co co co ro ro co co M ui o cn cn rn CO co CO PRONQTUM WIDTH (MM) PRONOTUM WIDTH (MM) •— >-» ro ro ro cn —j co »—» co cn > — > — > - ' ro ro ro cn - J co >-* co cn z o II co ro Ul ZZ C O ZD r~ *—* m co co -j C O CO m C O m 73 O o —i o C O m ZO co CO ro ro co CO ro ro co co II C O I—• to 3d v~ >-• P I C O C O - ~ J C O 93T 127 o f f s p r i n g s i z e s by emergence date. Lack of s i g n i f i c a n t c o r r e l a t i o n s or r e g r e s s i o n s f o r these data (P>.1) i n d i c a t e s that t h i s e f f e c t i s not the cause of b i m o d a l i t y . Second, the b i m o d a l i t y may have been produced by d i f f e r e n c e s i n food q u a l i t y between r e p l i c a t e l e g s . T h i s appears to be the case f o r both male and female o f f s p r i n g of s m a l l parentage bred at low d e n s i t y (logs 4 & 6). Table 16 shows t h a t by comparing o f f s p r i n g s i z e d i s t r i b u t i o n s between r e p l i c a t e l o g s , these two are the only cases i n which o f f s p r i n g from r e p l i c a t e l o g s show d i f f e r e n t d i s t r i b u t i o n s . Thus, n u t r i t i o n a l d i f f e r e n c e s between logs might account f o r o f f s p r i n g b i m o d a l i t y a t low a t t a c k d e n s i t y , but they do not account f o r the h i g h a t t a c k d e n s i t y s i t u a t i o n where there a r e , i n c i d e n t a l l y , many more data p o i n t s . The t h i r d p o s s i b l e reason f o r b i m o d a l i t y i s suggested by the high a t t a c k d e n s i t y cases. O f f s p r i n g on the l a r g e end of the bimodal d i s t r i b u t i o n may have a r i s e n from eggs l a i d cn the s i d e s of p a r e n t a l g a l l e r i e s where l a r v a e were ab l e to move away from competing g a l l e r i e s . I t i s c l e a r ( F i g . 12) t h a t a t high a t t a c k d e n s i t i e s , only l a r v a e on the o u t s i d e edges of the perimeter a d u l t g a l l e r i e s can extend t h e i r g a l l e r i e s u n i n h i b i t e d . G a l l e r y o v e r l a p data were i m p o s s i b l e to c o l l e c t i n my high a t t a c k d e n s i t y l o g s because, a f t e r bark removal, there was only a maze of b o r i n g dust and c r i s s - c r o s s i n g g a l l e r i e s . However, i f t h i s " p o s i t i o n " e f f e c t were an important determinant of a d u l t s i z e , we would expect most 128 TABLE 16 Comparison of size distributions of offspring between replicat^ logs in the various treatment classes. A significant X value indicates that the two replicate logs differ in the sizes of offspring that they produced. Female offspring: Treatment classes X2 d.f. P Log #'s Attack Density Parent Size High Small 10.90 10 >.l 3 & 5 High Large 9.35 12 >.5 1 & 7 Low Small 24.77 11 <01 4 & 6 Low Large 9.75 9 >.l 2 & 8 Male offspring: High Small 5.28 8 >.5 3 & 5 High Large 7.35 11 >.5 1 & 7 Low Small 19.47 5 <.005 4 & 6 Low Large 11.14 9 >.l 2 & 8 129 a d u l t s i n the bimodal d i s t r i b u t i o n to be on the s m a l l end. F i g . 18 shows t h a t j u s t the opposite i s the case; most are on the l a r g e end. Furthe r experimentation i s needed to c l e a r l y demonstrate st r o n g e r b i m o d a l i t y i n s i z e s of o f f s p r i n g of small females. If my o b s e r v a t i o n s are supported, one i n t e r e s t i n g e x p l a n a t i o n may be that s m a l l female parents (for example, from crowded c o n d i t i o n s ) may attempt t o maintain some p o p u l a t i o n v a r i a b i l i t y i n s i z e , which i n t u r n may be r e l a t e d to d i s p e r s a l behavior. some females may produce a l l the s m a l l o f f s p r i n g , or there may be a bimodal s i z e d i s t r i b u t i o n among o f f s p r i n g of any one female. E. Summary The breeding experiments have shown t h a t : a) the number of o f f s p r i n g per female emerging from a l o g decreases with i n c r e a s i n g a t t a c k d e n s i t y , b) s m a l l females tend to produce o f f s p r i n g with s i z e d i s t r i b u t i o n s t h a t are more s t r o n g l y bimodal than o f f s p r i n g of l a r g e females, and c) female o f f s p r i n g of l a r g e females bred at high attack d e n s i t y tend to be l a r g e r than those of smal l females bred at the same d e n s i t y . Trends which are suggested from the data but which are not c o n s i s t e n t l y s t a t i s t i c a l l y s i g n i f i c a n t a r e : a) s m a l l e r o f f s p r i n g may r e s u l t from higher a t t a c k d e n s i t i e s and b) lower r e p r o d u c t i v e output may be a c h a r a c t e r i s t i c of s m a l l e r parent females. However, q u a l i t a t i v e d i f f e r e n c e s between 130 experimental l o g s make the experiments d i f f i c u l t to i n t e r p r e t . 131 V I I I . DISPERSAL A. I n t r o d u c t i o n Previous s e c t i o n s d e a l t with d i f f e r e n c e s i n t r e e c h a r a c t e r i s t i c s and b e e t l e r e p r o d u c t i v e and s i z e measures between epidemic and endemic areas. T h i s s e c t i o n compares d i s p e r s a l behavior of b e e t l e s from each of the four sample areas and from the two experimental attack d e n s i t i e s . I a l s o compare v a r i a t i o n s i n d i s p e r s a l within each of these p o p u l a t i o n s . A t k i n s ' s t u d i e s with the D o u g l a s - f i r bark b e e t l e (1966b, 1967, 1973a,b) show that an i n s e c t ' s f a t content, which i s weakly r e l a t e d to body s i z e , i s an important determinant of d i s p e r s a l b e h a v i o r . S t u d i e s on other i n s e c t s a l s o i n d i c a t e that body s i z e a f f e c t s d i s p e r s a l a b i l i t y (Rose, 1972). My d i s p e r s a l s t u d i e s were designed, i n p a r t , to see whether the observed v a r i a b i l i t y i n s i z e s of D. £onderosae (see e a r l i e r s e c t i o n s ) c o u l d be r e l a t e d to v a r i a b i l i t y i n d i s p e r s a l c a p a b i l i t y . I asked the f o l l o w i n g q u e s t i o n s concerning mountain pine b e e t l e d i s p e r s a l : 1) What f a c t o r s i n t r i n s i c to the b e e t l e s a f f e c t t h e i r d i s p e r s a l behavior? 2) I s there any v a r i a t i o n i n d i s p e r s a l c h a r a c t e r i s t i c s w i t h i n a group of b e e t l e s ? 3) I f 132 so, are there d i f f e r e n c e s i n the d i s t r i b u t i o n of these c h a r a c t e r i s t i c s between epidemic and endemic b e e t l e s ? 4) Can f a c t o r s a f f e c t i n g the observed d i f f e r e n c e s between i n d i v i d u a l s of the f i e l d p o p u l a t i o n s be determined through breeding experiments? 5) Do "pi o n e e r " b e e t l e s e x i s t , i . e . , are e a r l y emerging b e e t l e s d i f f e r e n t i n d i s p e r s a l c h a r a c t e r i s t i c s from the r e s t of the p o p u l a t i o n ? The d i s p e r s a l c h a r a c t e r s t u d i e d was the response of flown i n d i v i d u a l s t o host t r e e chemicals i n the presence of l i g h t . T h i s measure was chosen f o r two reasons. F i r s t , ample evidence e x i s t s f o r D. £onderosae concerning the use of t r e e chemicals i n host s e l e c t i o n . Pheromones appear to be only a second stage mechanism t h a t i s used by the bulk of d i s p e r s i n g i n d i v i d u a l s f o r f i n d i n g the s u i t a b l e h osts a l r e a d y detected by other d i s p e r s e r s . T herefore, response to host chemicals was f e l t t o be a b e t t e r measure of how p o p u l a t i o n s would d i s t r i b u t e themselves i n space than responses to pheromones. second, measurement of the f l i g h t p o t e n t i a l of i n d i v i d u a l s (based on f a t r e s e r v e s or f l i g h t m i l l t e s t s ) may not be r e l a t e d to how f a r bark b e e t l e s w i l l a c t u a l l y f l y when i n the presence of host s t i m u l i . Thus, behaviors of pre-flown b e e t l e s were observed i n the presence of t r e e chemicals. The sequence of events i n bark b e e t l e d i s p e r s a l are e x e m p l i f i e d by the w e l l - s t u d i e d ambrosia b e e t l e , Trygodendron lineatum, which has many d i s p e r s a l c h a r a c t e r i s t i c s of the mountain pine b e e t l e . D e t a i l s of these behaviors are 133 d e s c r i b e d by Chapman (1962), F r a n c i a and Graham (1966), Graham (1959, 1961, 1968), and Werner and Graham (1957). D i s p e r s i n g !• iilieatum l e a v e o v e r w i n t e r i n g s i t e s sometime i n l a t e s p r i n g or e a r l y summer, and are i n i t i a l l y p h o t o p o s i t i v e (Daterman, fiudinsky and S a g e l , 1965). They f l y up toward the f o r e s t canopy but probably do not get above i t . E a r l y i n the f l i g h t , b e e t l e s are not r e s p o n s i v e to a p p r o p r i a t e host t r e e chemicals even i f the chemicals are present. However, a f t e r f l y i n g f o r some time, i n s e c t s respond to such chemicals by becoming p o s i t i v e l y anemotactic and by f l y i n g upwind u n t i l the source i s found. T h i s t h r e s h o l d f l i g h t time r e q u i r e d before the b e e t l e changes to p o s i t i v e anemotaxis v a r i e s between i n d i v i d u a l s . Chemical c o n c e n t r a t i o n has a l s o been found to a f f e c t b e e t l e response (Graham, 1968). The mechanism which r e l e a s e s the b e e t l e s from p h o t o p o s i t i v e domination of t h i s anemotaxis appears to be i n c r e a s i n g pressure from the formation of a gas bubble i n the v e n t r i c u l u s during f l i g h t (Graham, 1961)., T h i s gas bubble mechanism, which was f i r s t noted by Chapman (1958), i s s i m i l a r to the one d e s c r i b e d by Wellington (1948) i n spruce budworm l a r v a e . Mountain pine b e e t l e appears to have the same d i s p e r s a l b e h a v i o r s , but the mechanism f o r i n i t i a t i n g response t o t r e e chemicals a f t e r some f l i g h t i s unknown. A t k i n s (1961, 1966b) and B i j h o l t (1965, 1967, 1969) suggest f o r other bark b e e t l e s p e c i e s t h a t e n e r g e t i c l i m i t a t i o n s may be r e s p o n s i b l e . T h e r e f o r e , i t i s important to know not o n l y how f a r 1 3 4 i n d i v i d u a l s w i l l f l y given the chance, but a l s o how f a r they need to f l y before responding t o t r e e chemicals. The f l i g h t t h r e s h o l d phenomenon i s important i n D. £onderosae even f o r the segment of the p o p u l a t i o n that r e l i e s mainly upon pheromones f o r t r e e l o c a t i o n , because i t has been shown f o r t h i s b e e t l e t h a t pheromones are a t t r a c t i v e o n l y i n the presence of a t r e e chemical, alpha-pinene (see i n t r o d u c t i o n ) . The f l i g h t t h r e s h o l d phenomenon and i t s v a r i a t i o n between i n d i v i d u a l s a l l o w s c o n s i d e r a b l e f l e x i b i l i t y i n the d i s t a n c e i n s e c t s w i l l f l y before i n i t i a l l y a t t a c k i n g t r e e s . V a r i a b i l i t y i n f l i g h t d i s t a n c e w i t h i n a p o p u l a t i o n i s s i g n i f i c a n t , because v a r i o u s s p a t i a l d i s t r i b u t i o n s of s u s c e p t i b l e t r e e s w i l l n e c e s s i t a t e d i f f e r e n t f l i g h t p a t t e r n s i n order to make abundant i n s e c t c o l o n i z a t i o n p o s s i b l e . S e v e r a l f a c t o r s can cause v a r i a t i o n i n d i s t a n c e flown between i n d i v i d u a l s . F i r s t , the temperature t h r e s h o l d f o r f l i g h t may vary, r e l e a s i n g b e e t l e s sooner or l a t e r i n the season than normal, when t r e e s would be i n d i f f e r e n t stages of a t t r a c t i v e n e s s and s u s c e p t i b i l i t y . T h i s d i f f e r e n c e i n s p a t i a l d i s t r i b u t i o n of s u i t a b l e h o s t s would cause a change i n the d i s t a n c e s flown by b e e t l e s before a t t a c k i n g . Second, the gas bubble pressure or f l i g h t time that i s r e g u i r e d to r e l e a s e p h o t o p o s i t i v e domination of anemotaxis may vary. T h i r d , the c o n c e n t r a t i o n of host chemicals needed f o r response, once the b e e t l e i s no l o n g e r p h o t o p o s i t i v e , may vary. 135 One more j u s t i f y i n g statement needs to be made concerning the a c t i v e d i s p e r s a l behaviors which I s t u d i e d . I t might be argued t h a t p a s s i v e d i s p e r s a l by wind (as opposed to a c t i v e d i s p e r s a l by f l i g h t ) , i s the most important d i s p e r s a l stage. However, I would argue t h a t s i n c e the b e e t l e s p r e s e n t l y have complex a c t i v e o r i e n t a t i o n mechanisms, the a c t i v e d i s p e r s a l stage i s of e v o l u t i o n a r y s i g n i f i c a n c e , even though only a sma l l p r o p o r t i o n of a b e e t l e ' s f l i g h t time may be spent i n a c t i v e movement. The b e e t l e s probably f l y a c t i v e l y out of the wind c u r r e n t when a p p r o p r i a t e s t i m u l i are sensed, much l i k e aphids do (Kennedy and Booth, 1963). Seven f a c t o r s were s t u d i e d to see what e f f e c t s they had on d i s p e r s a l c h a r a c t e r i s t i c s of mountain pine b e e t l e . These were: 1) sex, 2) geographic sampling l o c a t i o n , 3) p o p u l a t i o n type from which they were sampled (epidemic or endemic), 4) body s i z e , 5) emergence date, 6) a t t a c k d e n s i t y of parents, and 7) f l i g h t h i s t o r y . B. Methods Be e t l e responses to t r e e chemicals were observed i n a chamber ( F i g . 24) s i m i l a r t o t h a t d e s c r i b e d by Moeck(1970a) and Syed(1972), i n a darkened c o n t r o l l e d environment room kept at 70° F (21© c) and 60% R.H. The chamber frame had a hinged g l a s s top over a t e s t arena. A narrow beam of h e a t - f i l t e r e d l i g h t was d i r e c t e d a c r o s s the arena by a Nikon microscope l i g h t with a 6V tungsten bulb (#6V5ATB1) connected through a 136 FIGURE 24 Ths t e s t i n g chamber f o r measuring the response of mountain pine b e e t l e to chemical e x t r a c t s from lodgepole pine. See t e x t f o r f u l l d e s c r i p t i o n . Light To vacuum P g r p Foam rubber *?2 / Airstream A Foam rubber X Light beam Chemical injection tube A V Hinged glass top 1 3 8 v o l t a g e r e g u l a t o r (to prevent i n t e n s i t y f l u c t u a t i o n s ) and a 7.5 amp Powerstat transformer s e t on 80 v o l t s . Air was drawn ac r o s s the f i e l d from r i g h t to l e f t by a pump l o c a t e d o u t s i d e the closed-environment room. A i r speed i n the t e s t chamber was approximately 4 f e e t per second. The width of the chemical-bearing a i r s t r e a m i n the chamber was determined by p u t t i n g dry i c e i n hot water i n s i d e the chemical i n j e c t i o n tube. The r e s u l t i n g t h i c k , white c l o u d s permitted easy t r a c i n g of the course of the a i r s t r e a m . Because of l o g i s t i c problems with measuring responses of f l y i n g b e e t l e s , a l l response t e s t s were made using p e d e s t r i a n b e e t l e s which had been pre-flown (to be d e s c r i b e d ) . Other workers have shown t h a t behavior of p e d e s t r i a n b e e t l e s i n the l a b o r a t o r y i s q u a l i t a t i v e l y the same as t h a t of f l y i n g b e e t l e s i n the f i e l d (e.g. Moeck, 1970b). Experimental i n s e c t s were put i n t o the t e s t chamber at p o i n t A i n F i g . 24 and were allowed to wander f r e e l y . Most i n s e c t s moved toward the l i g h t and encountered the a i r stream. At t h i s p o i n t , they had to make a c h o i c e between the l i g h t or the a i r s t r e a m . T h e i r r e s u l t i n g b e h a v i o r s were recorded as one of f i v e c a t e g o r i e s : 1) no t u r n i n g or h e s i t a t i o n ; kept on moving toward l i g h t , 2) a b r i e f pause i n movement but no t u r n i n g ; ended up at l i g h t , 3) turned s l i g h t l y (<45° from i n i t i a l d i r e c t i o n of movement) toward chemical source but continued on to l i g h t , 4) o r i e n t e d toward chemical source (turned >45°), 5) turned toward chemical source and walked 139 upstream. These responses were assigned the valu e s 1 to 5, r e s p e c t i v e l y . F i g . 25 shows r e p r e s e n t a t i v e responses f o r some of these c a t e g o r i e s . Each i n s e c t was t e s t e d s i x c o n s e c u t i v e times, three times with the c o n t r o l or blank a i r stream (no t r e e chemicals i n i n j e c t i o n tube) and then three times with t r e e chemicals present. A l l t r e e chemicals used i n my 1973 experiments were taken from a 95-year-old lodgepole pine t r e e of 9.75 in c h diameter cut at the epidemic T e r r a c e Creek study area on 1 June, 1973. T h i s t r e e was untouched by mountain pine b e e t l e but was on the edge of a group o f t r e e s which were i n f e s t e d . Thus, i t came as c l o s e as p o s s i b l e t o what I could determine was a s u s c e p t i b l e t r e e . A 30 in c h long s e c t i o n was removed, tr a n s p o r t e d back to Vancouver w i t h i n 28 hours, and s t o r e d i n a c o l d room at 1° C. E x t r a c t s were taken from t h i s l o g f o r ten days s t a r t i n g 11 June with a technique d e s c r i b e d by Syed (1972). A 4 i n c h square p i e c e of bark was s t r i p p e d o f f the source l o g and immediately placed i n a c l o s e d chamber. The only a i r o u t l e t from t h i s chamber ran through a Pyrex U-tube immersed i n l i q u i d n i t r o g e n . The a i r was drawn from the chamber c o n t a i n i n g the bark sample and a l l v o l a t i l e s passed i n t o the U-tube and were s o l i d i f i e d on i t s s i d e s . The low temperature of l i q u i d n i t r o g e n (-196 to -210° C) i n s u r e d that most v o l a t i l e s were trapped. A f t e r two hours of pumping, the s o l i d i f i e d bark e x t r a c t was t r a n s f e r r e d to a f l i n t - g l a s s v i a l and s t o r e d at -10° C u n t i l needed. Tree chemicals were added 140 FIGURE 25 Examples of the paths f o l l o w e d by three t y p i c a l b e e t l e s i n runs through the t e s t chamber i n the presence of lodgepole pine e x t r a c t . Numbers r e p r e s e n t the d i f f e r e n t behavior c l a s s e s which were assigned t o these b e e t l e s (see text) . 141 1 42 to the i n j e c t i o n tube by dropping 0.05 ml of bark e x t r a c t onto pure c e l l u l o s e f i l t e r paper i n s i d e a small t i n f o i l cup. The e x t r a c t was added to the f i l t e r paper o u t s i d e the c o n t r o l l e d environment room to avoid contamination of the a i r i n the room. Both t i n f o i l and dry f i l t e r paper were present during blank a i r s t r e a m t r i a l s . In a l l experiments, the c o n c e n t r a t i o n of tree e x t r a c t s was kept constant. No attempt was made to t e s t the e f f e c t o f changing host chemical c o n c e n t r a t i o n on b e e t l e behavior. Experimental i n s e c t s were d e r i v e d from two sources: f i e l d samples from the fou r study areas and o f f s p r i n g from the breeding experiments. A l l groups were t e s t e d i n the same way. I n s e c t s were c o l l e c t e d the day they emerged from t h e i r host l o g s and were immediately sexed, roughly s i z e d by eye, and s p l i t up i n t o t hree f l i g h t time groups. Each f l i g h t time group had the f u l l range of i n d i v i d u a l s i z e s present t h a t day. Each i n s e c t was then s u b j e c t e d to i t s p r e - s e l e c t e d f l i g h t regime of 0, 15, or 30 minutes, with some v a r i a t i o n (Table 17). I n s e c t s were flown p r i o r t o t e s t s i n the b e h a v i o r a l chamber by t e t h e r i n g them by the d o r s a l thorax s u r f a c e to t h i n copper wire with Lepage's household cement. The other end of the copper wire was looped and t h i s loop was p i e r c e d by a pin stuck i n t o a styrofoam board, p e r m i t t i n g f r e e c i r c u l a r movement of the i n s e c t . F l y i n g i n s e c t s were exposed to f l u o r e s c e n t l i g h t i n g from above and a 100 watt incandescent 143 TABLE 17 Distributions of total f l i g h t times of insects used i n dispersal experiments. Data are in minutes. SHORT FLIGHT: MEDIUM FLIGHT: LONG FLIGHT1: Time Number Tine Number Time Number 0 144 10 4 24 1 1 9 11 1 25 2 2 11 12 1 26 2 3 8 13 3 27 0 4 2 14 3 28 1 5 3 15 152 29 0 6 1 16 9 30 144 7 1 17 4 31 5 8 2 18 2 32 2 9 3 19 1 33 2 20 2 34 1 144 l i g h t from s t r a i g h t ahead. a s l i g h t f l i c k of the copper wire u s u a l l y i n i t i a t e d f l i g h t . Not a l l b e e t l e s flew f o r t h e i r a l l o t t e d time without i n t e r r u p t i o n ; most stopped at l e a s t once and had to be r e s t a r t e d by g i v i n g t h e i r wires a f l i c k . Clocks were stopped during such times o f f l i g h t i n t e r r u p t i o n , a f t e r f l i g h t , i n s e c t s were removed from t h e i r wires and pl a c e d under s m a l l p e t r i d i s h e s u n t i l used some minutes l a t e r i n the t e s t chamber. I n s e c t s chosen f o r zero f l i g h t were t r e a t e d i n e x a c t l y the same way as the o t h e r s , i n c l u d i n g being t e t h e r e d and hung on wire, except they were not allowed to f l y . P r i o r to these d i s p e r s a l experiments, a l l i n s e c t s probably had f l i g h t experiences i n t h e i r cages between emergence and c o l l e c t i o n times. T h e r e f o r e , the experimental f l i g h t times should be i n t e r p r e t e d only as r e l a t i v e , not a b s o l u t e , measures of f l i g h t experience. Motion p i c t u r e s of two t e t h e r e d b e e t l e s showed t h a t the e g u i v a l e n t d i s t a n c e s covered i n 30 minutes of f l i g h t were 0.7 oailes f o r one b e e t l e and 1.3 f o r the other. a f t e r t e t h e r e d f l i g h t , each i n s e c t was run through the t e s t chamber (described above) and i t s responses to blank and chemical a i r streams were recorded. animals which were not p h o t o p o s i t i v e (about 4 2 of those tethered) were not used i n the t e s t s . Each t e s t e d i n s e c t was saved f o r l a t e r exact s i z i n g . Each i n s e c t was t e s t e d with o n l y one f l i g h t time; the same i n s e c t was never reused because of h a b i t u a t i o n t o the experimental apparatus. With t h i s experimental design i t was p o s s i b l e to t e s t f o r 145 d i f f e r e n c e s i n f l i g h t time t h r e s h o l d s between d i f f e r e n t groups of b e e t l e s . Such d i f f e r e n c e s were then r e l a t e d t o sex, pop u l a t i o n type, a t t a c k d e n s i t y , body s i z e and emergence date. Experiments were done i n both 1972 and 1973, but onl y the 1973 data are d i s c u s s e d . The 1972 data are not as trustworthy because I had s t o r e d the b e e t l e s a t 2° C f o r s e v e r a l days between emergence and t e s t i n g . T h i s storage time r e s u l t e d i n decreased v i a b i l i t y and a c t i v i t y . C. R e s u l t s 526 b e e t l e s were t e s t e d , 267 from f i e l d p o p u l a t i o n s and the r e s t from breeding experiments. A l l d i s p e r s a l data were analyzed using s i n g l e and m u l t i p l e r e g r e s s i o n and a n a l y s i s of v a r i a n c e . To meet the assumptions of these s t a t i s t i c a l t e s t s , response data were normalized using the dummy-Y v a r i a t e technique ( G i l b e r t , 1973) and I - r e s i d u a l s were found to be normally d i s t r i b u t e d . Each i n s e c t had th r e e runs i n the c o n t r o l a i r s t r e a m and three i n the chem i c a l - b e a r i n g stream, and the f i r s t n o t i c e a b l e f a c t about the data i s a marked h a b i t u a t i o n of i n s e c t s to the t e s t i n g apparatus. For both the c o n t r o l and e x t r a c t t r i a l s , responses were l e s s i n t e n s e i n the second run than the f i r s t , and a l s o i n the t h i r d than the second (Table 18). For each i n s e c t , v a r i o u s measures of response were c a l c u l a t e d , some of which took t h i s h a b i t u a t i o n i n t o c o n s i d e r a t i o n . These were: 146 TABLE 18 Data on the habituation of insects to the dispersal response apparatus. Data are ireans (and standard errors) of responses which were numbered from one to five (see text). F i r s t , second and third responses refer to the different runs for the same insect. Sample size = 526 insects Responses to Controls (blank air) F i r s t Second Third Mean 1.260 1.219 1.169 S.E. 0.0315 0.0273 0.0252 Responses to Extract F i r s t Second Third Mean 2.640 2.144 1.892 S.E. 0.059 0.056 0.054 147 1) F i r s t response to e x t r a c t 2) F i r s t e x t r a c t response minus f i r s t c o n t r o l response 3) Average of the three e x t r a c t responses, 4) Measure 3) minus average of three c o n t r o l responses, 5) Weighted averages of the t h r e e e x t r a c t responses, where weightings were .6, .3, .1 on the f i r s t , second and t h i r d responses, r e s p e c t i v e l y , and 6) Number 5) minus the same weighted average of c o n t r o l responses. The weightings of the f i f t h measure were chosen because of h a b i t u a t i o n . A l l s t a t i s t i c a l analyses were performed using these s i x response i n d i c e s . Since a l l of them gave the same answers, I w i l l h e n c e f o r t h only r e f e r t o a gen e r a l d i s p e r s a l response measure and data w i l l be presented only f o r the f i f t h mea sure. The f i r s t s tep i n the a n a l y s i s was to determine which b l o c k s of data c o u l d be lumped and which had to be kept separate. The best p r e d i c t i o n of d i s p e r s a l response f o r a l l the data was found i n a m u l t i p l e r e g r e s s i o n with emergence date, sex, s i z e and f l i g h t as the independent v a r i a b l e s ( r e g r e s s i o n data i n Table 19). F i e l d and breeding experiment b e e t l e s were compared using a n a l y s i s of c o v a r i a n c e and the slo p e s and i n t e r c e p t s f o r t h i s m u l t i p l e r e g r e s s i o n were not d i f f e r e n t (P>.1). T h e r e a f t e r , these two data blocks were lumped. Male data were compared with female data and i t was found t h a t the p r o b a b i l i t y of the i n t e r c e p t s of the best 148 TABLE 19 Multiple regression data of dispersal response measure discussed i n text on emergence date, sex, size and f l i g h t time. Standard errors are i n parentheses. N = 526 insects Independent variable Slope Emergence date -0.060(0.009) Sex -0.296(0.118) Size -0.763(0.353) Flight tine 0.281(0.057) Intercept = 4.451 (0.626) Regression Mean square = 23.19 Residual Mean square = 1.11 F = 20.9 P < .001 that slope =0.0 1 49 m u l t i p l e r e g r e s s i o n (emergence date, s i z e and f l i g h t time as X- v a r i a t e s ) being the same was <0.005 (between-block M.S./residual M.S. = 8. 6, 1 6 521 d . f . ) . T h e r e f o r e , males and females were c o n s i d e r e d s e p a r a t e l y i n a l l subsequent d i s p e r s a l data a n a l y s e s . Data were then blocked i n t o three f l i g h t time c l a s s e s , s h o r t , medium and long d e f i n e d as 0-9, 9.1-22, and 23-34 minutes, r e s p e c t i v e l y . These c l a s s boundaries are reasonable c o n s i d e r i n g the d i s t r i b u t i o n s of f l i g h t times i n my experiments (Table 17). For data blocked by f l i g h t times and sex, the r e g r e s s i o n of response on emergence date was the most s i g n i f i c a n t (Tables 20 5 21). Taking b e e t l e s i z e i n t o account did not i n c r e a s e p r e d i c t a b i l i t y , but males d i d show a weak n o n - s i g n i f i c a n t (P > .1) negative slope of response on s i z e , much as A t k i n s (1966b) found. S i z e s squared and cubed were a l s o t r i e d i n order to account f o r volume changes but these did not g i v e any more s i g n i f i c a n t r e s u l t s than the l i n e a r s i z e measure. The probable reason t h a t s i z e d i d not s i g n i f i c a n t l y h e lp to p r e d i c t d i s p e r s a l response i s t h a t i n s e c t s of the same s i z e may not have had the same energy r e s e r v e s . A t k i n s (1967,1973a) found t h a t D o u g l a s - f i r b e e t l e s spent a v a r i a b l e length of a d u l t l i f e underneath the bark before emerging. F a t content of t h i s b e e t l e was r e l a t e d to t h i s l e n g t h of time. Thus, my b e e t l e s which emerged on the same date and were i n the same s i z e c l a s s might not have had the same energy 150 TABLE 20 Data for regressions of female dispersal response #5 (see text) on emer-gence date, by flight class as shown in Fig. 26. Standard errors are in parentheses. Females: Overall regression, a l l flight classes lumped: N = 327 r = .205 P < .01 that r = 0.0 r 2= .04 Y-intercept a = 2.679(0.113) Slope b = -0.0385(0.010) P < .001 that b = 0.0 Short flight: N = 102 r = 0.175 P = .08 that r = 0.0 r 2= .031 Y-intercept a = 2.201(0.177) Slope b = -0.027(0.012) P < .03 that b = 0.0 Medium flight: N = 117 r = 0.167 P = 0.6 that r =0.0 r2= 0.028 Y-intercept a = 2.684(0.191) Slope b = -0.0341(0.0175) P = 0.05 that b = 0.0 Long flight: N = 108 r = 0.272 P < .025 that r = 0.0 r 2= .07 Y-intercept a = 3.119(0.199) Slope b = -0.0545(0.0186) P < .005 that b = 0.0 (males' data on next page) TABLE 21 Same as Table 20 but data are for males only instead of females. Males: Overall regression, a l l flight classes lumped: N = 199 r = 0.408 P < .01 that r = 0.0 r 2= .016 Y-intercept a = 3.63(0.181) Slope b = -0.094(0.015) P < .001 that b = 0.0 Short flight: N = 82 r = .383 P < .01 that r = 0.0 r 2= .147 Y-intercept a = 3.369 (0.278) Slope b = -0.0866(0.024) P < .001 that b = 0.0 Medium flight: N = 65 r = .522 P < .01 that r = 0.0 r 2= .27 Y-intercept a = 3.928(0.293) Slope b = -0.115(0.023) P < .001 that b = 0.0 Long flight: N = 52 r = .248 P = .08 that r = 0.0 r 2= .06 Y-intercept a = 3.52(0.391) Slope b = -0.0655(0.0338) P = .050 that b = 0.0 but additional regression on X 2 is significant, P < .005 1 5 2 r e s e r v e s . I f energy s t o r e s do a f f e c t responses to hosts i n mountain pine b e e t l e as they do i n the D o u g l a s - f i r b e e t l e (Atkins 1966b), then l a c k of c o n t r o l f o r f a t r e s e r v e s added to the v a r i a b i l i t y i n my data. The r e g r e s s i o n s i n F i g . 26 (data i n Tables 20 fi 21) e s t a b l i s h t h a t s o - c a l l e d pioneer b e e t l e s do e x i s t ; the e a r l i e r a b e e t l e emerges, the more l i k e l y i t i s to respond to t r e e chemicals a f t e r a given l e n g t h of f l i g h t time. One e x p l a n a t i o n f o r t h i s decrease i n response with l a t e n e s s of emergence may be t h a t my t r e e chemicals changed during storage. Syed(1972) suggested t h a t a u t o - o x i d a t i o n may occur i n ponderosa pine logs or e x t r a c t s t h a t have been s t o r e d f o r 2 to 6 weeks and he found that a t t r a c t i v e n e s s of these chemicals to D. fionderosae changed. However, h i s r e s u l t s may have been biased by using b e e t l e s from d i f f e r e n t p a r t s of the emergence p e r i o d i n the t e s t s with t r e e chemicals s t o r e d f o r d i f f e r e n t l e n g t h s of time. In any case, my r e s u l t s do not i n d i c a t e a change i n a t t r a c t i v e n e s s over time because, a c c o r d i n g to data presented e a r l i e r , there was no d i f f e r e n c e i n the responses, within emergence date groups, between f i e l d b e e t l e s and breeding experiment o f f s p r i n g . These two groups of b e e t l e s were t e s t e d from u to 18 J u l y and 30 Aug. to 20 Sept., r e s p e c t i v e l y . I f the t r e e chemical samples had decreased i n a t t r a c t i v e n e s s with i n c r e a s i n g time s i n c e the sample t r e e was cut, t h e r e should have been lower responses among breeding o f f s p r i n g than f i e l d p o p u l a t i o n s . T h i s was not the case, so 153 FIGURE 26 Ths r e l a t i o n i n mountain pine b e e t l e between tendency to respond to e x t r a c t s of lodgepole pine and b e e t l e emergence date. Regressions are broken down by sex and f l i g h t h i s t o r y c l a s s . S t a t i s t i c s f o r these r e l a t i o n s h i p s are given i n Tables 20 and 21. FEMALES N= 327 R = - 0 . 2 0 5 P < .001 LONG FLIGHT MEDIUM SHORT FLIGHT H h H 1 1 1 1 1 1 10 15 20 EMERGENCE DATE MALES N= 199 R = - 0 . 4 1 P < .001 ITlEDIUm FLIGHT H 1 1 1 1 1 - 4 — 1_ 5 10 15 20 EMERGENCE DATE H 1 155 the d i f f e r e n c e s i n responses of b e e t l e s from d i f f e r e n t emergence dates (shown i n F i g . 26) were due to d i f f e r e n c e s i n b e e t l e s , not t r e e samples. The r e g r e s s i o n s i n F i g . 26 are s i g n i f i c a n t ; the c o r r e l a t i o n c o e f f i c i e n t s are i n Tables 20 8 21. However, the low p r o p o r t i o n of the v a r i a n c e accounted f o r i s d i s a p p o i n t i n g . R e s i d u a l v a r i a n c e may be due to v a r i a t i o n i n energy s t o r e s (above) or to the h e r i t a b i l i t y of d i s p e r s a l behavior. Data on the e x i s t e n c e of the l a t t e r phenomenon are d i s c u s s e d by Dingle (1968) and Rose (1972) though they do not e s t a b l i s h whether t h e i r i n h e r i t a n c e mechanisms are g e n e t i c or p h y s i o l o g i c a l . I was not a b l e to o b t a i n data on t h i s i n h e r i t a n c e because of the time r e q u i r e d f o r d i s p e r s a l experiments i n r e l a t i o n to the c o n s t r a i n t s imposed by emergence date e f f e c t s on d i s p e r s a l . For females i n F i g . 26, s i g n i f i c a n t d i f f e r e n c e s were found i n Y - i n t e r c e p t s f o r the r e g r e s s i o n l i n e s of each f l i g h t c l a s s but t h e r e was no d i f f e r e n c e between s l o p e s (Table 22). In other words, a l l three l i n e s were p a r a l l e l with d i f f e r e n t i n t e r c e p t s . T h i s means t h a t the longer a female b e e t l e emerging on a given date f l i e s , the g r e a t e r i s the p r o b a b i l i t y t h a t her f l i g h t t h r e s h o l d w i l l have been passed and t h a t she w i l l a t t a c k a t r e e with a p p r o p r i a t e chemicals. T h i s supports the theory d e r i v e d from Graham (196 1). However, my o b s e r v a t i o n s do not p r o v i d e any i n f o r m a t i o n concerning the mechanism f o r o p e r a t i o n of the f l i g h t time t h r e s h o l d ; i t could be gas bubble formation or drainage of energy s t o r e s . The 156 TABLE 22 Testing for the difference in slopes and intercepts between the regression lines shown in Figure 26 for females. Test for difference between intercepts: F = between block mean square/residual mean square where data are blocked by the three flight time classes. F = 12.6058/1.046 = 12.05 P < .001 Therefore, these lines have different intercepts. Test for difference between slopes: Individual Regr. Sum of Squares 2.4656 + 3.9057 + 9.79 = 16.16 3 d.f. Ccmbined S.S. 14.83 = 14.83 1 d.f. Regr. x blocks S.S. 1.33 2 d.f. F = 1.33/2 /residual M.S. = 0.635 P > .5 Therefore slopes are not different for females. 157 l a t e r a female emerges from the host t r e e , the longer she w i l l have to f l y t o reach the same p r o b a b i l i t y of responding. T h i s means t h a t i n s e c t s coming out l a t e r i n the emergence p e r i o d w i l l f l y f a r t h e r out and tend t o spread out more than e a r l i e r b e e t l e s . For males i n F i g . 26, r e g r e s s i o n s and c o r r e l a t i o n s are s i g n i f i c a n t but there are no s i g n i f i c a n t d i f f e r e n c e s between i n t e r c e p t s or s l o p e s of l i n e s f o r the three f l i g h t time c l a s s e s (Table 23). T h i s means that male b e e t l e s respond e q u a l l y w e l l t o host chemicals no matter what t h e i r f l i g h t h i s t o r y up t o the moment of encounter. And, as with females, there i s a decrease i n response with i n c r e a s i n g l a t e n e s s of emergence. Data were then broken down by sex, experimental a t t a c k d e n s i t y , and p o p u l a t i o n type (epidemic or endemic) i n order to t e s t f o r d i f f e r e n c e s i n d i s p e r s a l c h a r a c t e r i s t i c s between these groups. These t e s t s were made by comparing the s l o p e s and i n t e r c e p t s of r e g r e s s i o n s of response on emergence. U n f o r t u n a t e l y , when data were disaggregated i n t o t h i s many groups, the on l y r e g r e s s i o n s t h a t c o u l d be compared were male epidemic and endemic b e e t l e s and there was no s i g n i f i c a n t d i f f e r e n c e (P>.1) between these two r e g r e s s i o n s . For each of the other p a i r s of comparisons, a t l e a s t one of the r e g r e s s i o n s was not s i g n i f i c a n t l y d i f f e r e n t from zero, (P>.2) e l i m i n a t i n g the p o s s i b i l i t y of comparison. T h e r e f o r e , t h e r e i s no evidence f o r i n t r i n s i c d i f f e r e n c e s i n d i s p e r s a l between 158 TABLE 23 Same as Table 22 except that data are for males. Test for difference between intercepts; F = 2.2768/1.1998 = 2.03 P > .1 Therefore, the intercepts are not different. Tests for difference between slopes: Individual Regr. S.S. 15.04 + 29.1425 + 4.3787 = 48.56 3 d.f. Combined S.S. 46.66=46.66 1 d.f. Regr. x blocks S.S. 1.90 2 d.f. F = 1.90/2 /residual M.S. =0.79 P > .5 Therefore slopes are not different for males. 159 epidemic and endemic b e e t l e s , bred at high or low d e n s i t y . D. D i s c u s s i o n or between o f f s p r i n g of parents The d i s p e r s a l t e s t r e s u l t s have i n t e r e s t i n g e c o l o g i c a l i m p l i c a t i o n s . F i r s t , i t i s known th a t mountain pine b e e t l e females are the i n i t i a l s e a r c h e r s f o r hosts and d i g g e r s of g a l l e r i e s ; males a r r i v e a f terwards by homing i n on the female pheromone and a t r e e terpene (Renwick and V i t e , 1970). My observed l a c k of e f f e c t of f l i g h t time on male behavior f i t s these o b s e r v a t i o n s . Males are as r e s p o n s i v e to t r e e chemicals when they f i r s t emerge as they are a f t e r having flown f o r 30 minutes. T h e r e f o r e , males are ready to respond to females as soon as they encounter t h e i r pheromones i n the presence of alpha-pinene. A l s o , males have a higher p o s i t i v e response e a r l y i n the emergence p e r i o d than do females, f o r any f l i g h t time. Near the middle of the emergence p e r i o d {Fig. 8 ) , i n t e n s i t y of male response drops below that of females. In my experiments, i n s e c t s which emerged e a r l i e s t had g r e a t e r responses a f t e r a given f l i g h t time than b e e t l e s emerging l a t e r . T h i s i s a p p a r e n t l y the o p p o s i t e of female D o u g l a s - f i r b e e t l e behavior; A t k i n s (1966b) found l a t e emergers to be more l i k e l y to a t t a c k t r e e s . His apparent use of b e e t l e s without f l i g h t h i s t o r i e s might be i n t e r p r e t e d as the cause of the d i f f e r e n c e , but my s h o r t f l i g h t c l a s s shows a negative s l o p e and most f l i g h t times i n t h i s c l a s s were zero. 160 Be need to c o n s i d e r the e f f e c t s of sequences of a t t a c k s on the r e l a t i o n s h i p between emergence date and tendency to respond to t r e e chemicals i n order to d i s c e r n why these two s p e c i e s have op p o s i t e responses to emergence date. f i r s t , l e t us t r a n s l a t e the axes of F i g . 26 i n t o a d i f f e r e n t measure of d i s p e r s a l . The present measure shows t h a t , f o r a given emergence date, a female w i l l have a c e r t a i n p r o b a b i l i t y of responding to host chemicals a f t e r a s h o r t f l i g h t . I f she f l i e s l o n ger, t h a t p r o b a b i l i t y i n c r e a s e s . The l a t e r her emergence date, the longer she w i l l have to f l y i n order to reach that same p r o b a b i l i t y of responding. T h e r e f o r e , we can t r a n s l a t e the r e l a t i o n s h i p shown i n F i g . 26 to a changing slope r e l a t i o n between average d i s t a n c e flown and p r o p o r t i o n of the p o p u l a t i o n responding to t r e e chemicals ( F i g . 27a). T h i s means t h a t d u r i n g the course of D. ponderosae emergence, i n s e c t s w i l l f i r s t tend to a t t a c k t r e e s c l o s e i n to the source t r e e and l a t e r w i l l tend to f l y out f a r t h e r before responding to t r e e chemicals. T h i s should produce a slowly expanding s e r i e s of c o n c e n t r i c waves of a t t a c k i n g b e e t l e s through the emergence p e r i o d (Fig. 28a). In the D o u g l a s - f i r b e e t l e , the o p p o s i t e appears to be the case. A t k i n s ' (1966b) f i n d i n g t h a t e a r l y emergers tend to be weaker responders i m p l i e s t h a t p i o n e e r i n g b e e t l e s w i l l have to f l y f a r t h e r than l a t e emergers before a t t a c k i n g t r e e s ( F i g . 27b). I f l a t e emergers needed to sense pheromones before a t t a c k i n g , then the bulk of the p o p u l a t i o n would be 161 FIGOEE 27 The r e l a t i o n between average d i s t a n c e flown and the p r o p o r t i o n of b e e t l e s responding to t r e e chemicals and a t t a c k i n g . Top graph i s f o r D. ponderosae and bottom one i s f o r the D o u g l a s - f i r b e e t l e . Proportion of Population Responding to Tree Chemicals Dendroctonus ponderosae Late emergers Average Distance Flown B Proportion of Population Responding to Tree Chemicals Dendroctonus pseudotsugae Early emergers Average Distance Flown FIGURE 28 h y p o t h e t i c a l sequence of a t t a c k s over space by D. £onderosae and the D o u g l a s - f i r b e e t l e . Numbers r e p r e s e n t the order i n which t r e e s at v a r i o u s d i s t a n c e s from the source t r e e (in the center) are a t t a c k e d . 164 165 drawn f a r t h e r out from the source t r e e than i n the mountain pine b e e t l e . I f l a t e emergers d i d not r e q u i r e pheroraones, then the sequence of a t t a c k s would be s t a r t i n g f a r away and working inward, i n s t e a d of v i c e versa, as i n p., ponderosae ( F i g . 28b) . The e c o l o g i c a l s i g n i f i c a n c e of these o p p o s i t e p a t t e r n s of a t t a c k i n D. £onderosae and p. pseudotsugae might be found i n the d i f f e r e n t types of p r e f e r r e d hosts when i n s e c t s are at endemic l e v e l s . D. fionderosae a t t a c k s only s t a n d i n g , r e l a t i v e l y h e a l t h y t r e e s whereas the D o u g l a s - f i r b e e t l e p r e f e r s g r e a t l y weakened or r e c e n t l y f a l l e n t r e e s (McMullen and a t k i n s , 1961; Rudinsky, 1970). The number of i n s e c t s r e q u i r e d to overcome host r e s i s t a n c e i s lower i n the case of the D o u g l a s - f i r b e e t l e because of the weaker t r e e s attacked. As noted above, f i r s t a t t a c k i n g b e e t l e s i n t h i s s p e c i e s , i f we assume random d i r e c t i o n a l i t y of movements, w i l l a t t ack t r e e s f a r t h e r away from the o r i g i n a l host than w i l l D. ponderosae. T h i s means t h a t the i n i t i a l c o n c e n t r a t i o n of b e e t l e s on t r e e s w i l l be lower i n p. £§eudotsucjae than i n p. ponderosae because there w i l l be fewer b e e t l e s per u n i t area a t a given d i s t a n c e p u r e l y due to d i f f u s i o n . Subsequent c o n c e n t r a t i o n s of b e e t l e s would depend on the r e l a t i v e e f f e c t i v e d i s t a n c e s of each s p e c i e s ' r e s p e c t i v e pheromones and how much response behavior i s a l t e r e d by the changing v o l a t i l e host chemical c o n c e n t r a t i o n s caused by i n c r e a s i n g numbers of a t t a c k s . S i n c e f i r s t - a t t a c k i n g mountain pine b e e t l e s i n f e s t t r e e s c l o s e i n . 1 6 6 they would be assured of " f i l l i n g up" these t r e e s f i r s t and then a t t a c k i n g more d i s t a n t ones l a t e r . I n d i v i d u a l D o u g l a s - f i r b e e t l e s , on the other hand, co u l d spread f a i r l y t h i n l y and s t i l l have high r e p r o d u c t i v e success because of the weak t r e e s a t t a c k e d . T h i s t h i n n i n g of the p o p u l a t i o n a l s o reduces the p r o b a b i l i t y o f o f f s p r i n g s u f f e r i n g from too much i n t r a s p e c i f i c c o m p e t i t i o n . a t r a d e o f f i s i m p l i e d i n the above d i s c u s s i o n : average d i s p e r s a l d i s t a n c e s r e g u i r e d before responding to hosts must be s h o r t enough to i n s u r e a l a r g e enough number of a t t a c k i n g b e e t l e s to overcome r e s i s t a n c e of t r e e s , and yet long enough to f i n d s u i t a b l e hosts. The s h o r t e r f l i g h t d i s t a n c e of pioneer mountain pine b e e t l e s , when compared with the D o u g l a s - f i r b e e t l e , r e f l e c t s the r e l a t i v e importance of having gre a t e r numbers of a t t a c k i n g i n s e c t s f o r overcoming the r e l a t i v e l y h e a l t h y hosts. The longer d i s t a n c e r e g u i r e d by p i o n e e r i n g D o u g l a s - f i r b e e t l e s emphasizes the need to search f o r d i s t a n t (and perhaps r a r e r ) hosts and de-emphasizes the reguirement f o r numerous a t t a c k i n g b e e t l e s per t r e e . Nothing i s known about d i r e c t i o n a l i t y of d i s p e r s a l i n mountain pine b e e t l e , but i t i s c l e a r t h a t two d i f f e r e n t types of f a c t o r s , i n t e r n a l and e x t e r n a l , a f f e c t how f a r £ • E2£^®E2SS§ w i l l d i s p e r s e before a t t a c k i n g t r e e s . I have e s t a b l i s h e d that the d i s p e r s a l c h a r a c t e r i s t i c s of male and female D. £onderosae are weakly a f f e c t e d by t h e i r emergence dates. E a r l y emergers are more prone to respond than l a t e r 167 emergers a f t e r the same f l i g h t time. For females, f l i g h t h i s t o r y a f f e c t s tendency t o respond to host chemicals, but f o r males i t does not. Body s i z e was shown t o have no e f f e c t on the d i s p e r s a l c h a r a c t e r i s t i c s measured, but works of other people suggest t h a t f a t content (only weakly r e l a t e d to s i z e ) may be more important. One e x t e r n a l f a c t o r i n d i s p e r s a l i s the t r e e chemical complex, f o r which my t e s t samples c o n s t i t u t e d only one p o i n t i n the range of c o n c e n t r a t i o n s probably encountered by b e e t l e s i n the f i e l d . Other workers have a l s o shown pheromones to be a s i g n i f i c a n t determinant of f l i g h t behavior and i t s t e r m i n a t i o n , but some f u r t h e r q uestions need to be i n v e s t i g a t e d . I s a pheromone a more e f f e c t i v e a t t r a c t a n t i n the presence of h i g h e r c o n c e n t r a t i o n s of a p p r o p r i a t e host chemicals? What are the a t t r a c t i v e d i s t a n c e s over which pheromones operate? And f i n a l l y , do l a t e emerging b e e t l e s have the same response to pheromones i n a given c o n c e n t r a t i o n as do e a r l y emergers, or i s there a changing p a t t e r n as I have found i n response to t r e e chemicals? 168 IX. GENEBAL DISCUSSION The b a s i c l i f e h i s t o r y a d a p t a t i o n s of mountain pine b e e t l e give us some h i n t s about the s e l e c t i o n p r e s s u r e s on t h i s s p e c i e s . The most i n t e r e s t i n g problem d e a l s with energy l i m i t a t i o n s and m o r t a l i t y during d i s p e r s a l . Data on t h i s problem are d i f f i c u l t to o b t a i n but the general f e e l i n g among bark b e e t l e e c o l o g i s t s i s t h a t m o r t a l i t y d u r i n g d i s p e r s a l i s high because of i n a b i l i t y t o f i n d s u i t a b l e hosts (e.g. 30-50%, Amman and Baker, 1972). However, I b e l i e v e that host d i s c o v e r y cannot be a very s e r i o u s problem. Female p. fionderosae do the i n i t i a l searching f o r t r e e s , take the brunt of the i n i t i a l host r e s i s t a n c e , and do a l l the g a l l e r y d i g g i n g . The female a l s o has a g r e a t e r energy demand (for egg production) than the male. I argue t h a t i f energy were very o f t e n l i m i t i n g i n the course of d i s p e r s a l or i f even a s m a l l p r o p o r t i o n of females were l o s t s e a r c h i n g f o r hosts, the p i o n e e r i n g r o l e would be held by the males, as i s the case with most l£s s p e c i e s . In these s p e c i e s , the male does the s e a r c h i n g , i n i t i a l g a l l e r y c o n s t r u c t i o n , and pheromone prod u c t i o n , which a t t r a c t s the r e s t of the p o p u l a t i o n . One p o s s i b l e argument a g a i n s t t h i s reasoning i n p. fionderosae i s that such a l a r g e number of b e e t l e s are needed to k i l l h e a l t h y host t r e e s t h a t no i n d i v i d u a l c o u l d a f f o r d to have an o f f s p r i n g sex r a t i o t h a t would produce 169 mostly pioneer males; t h e r e would be too few females to reproduce the genotype. A t e s t of t h i s argument i s to see whether t h e r e are any Ips s p e c i e s that a t t a c k healthy t r e e s . I am aware of no such case. E a r l y emerging or pioneer females might be at a disadvantage. D i s p e r s a l m o r t a l i t y might be higher i n t h i s group than i n l a t e r emergers because there i s no phercmone to f o l l o w to s u i t a b l e hosts and when t r e e s are i n i t i a l l y a t t a c k e d , t h e i r r e s i s t a n c e mechanisms are more l i k e l y to overcome b e e t l e s . However, there may be an advantage i n being among pi o n e e r i n g b e e t l e s i f , under normal summer c o n d i t i o n s , t h e r e i s a g r e a t e r p r o b a b i l i t y of e a r l y a t t a c k e r s being able to s u c c e s s f u l l y produce a second brood than l a t e r emergers. T h i s i n c r e a s e . i n p r o d u c t i v i t y of pioneer b e e t l e s c o u l d only be s i g n i f i c a n t i f i t can be shown t h a t e a r l i n e s s of emergence i s , i n any sense, h e r i t a b l e . For example, the o f f s p r i n g of e a r l y a t t a c k e r s on f a i r l y weak hosts may develop sooner than l a t e r a r r i v a l s , assuming t h a t e a r l y - l a i d l a r v a e do not stop f e e d i n g and developing a t some p o i n t while the l a t e r l a r v a e " c a t c h up". I f a l l these assumptions are t r u e , then e a r l y emergers are probably r e l a t e d from one g e n e r a t i o n t o the next and the disadvantages of being a pioneer b e e t l e may be outweighed by the i n c r e a s e d p r o b a b i l i t y of s u c c e s s f u l l y producing two broods. Another aspect of the emergence p e r i o d i n mountain pine b e e t l e i s i t s timing r e l a t i v e to the changing s t r e s s e s on 170 lodgepole pine. Since the b e e t l e p r e f e r s r e l a t i v e l y h e a lthy t r e e s , one would expect the timing of a t t a c k s to be a d j u s t e d w i t h i n the season so t h a t the bulk of a t t a c k s would occur when t r e e s were weakest. However, Reid and Shrimpton*s (1971) data show t h a t t r e e r e s i s t a n c e i s j u s t past i t s peak when most mountain pine b e e t l e s normally a t t a c k i n these areas. The b e e t l e s simply may not be a b l e to s h i f t emergence and a t t a c k any l a t e r because of the minimum time needed f o r l a r v a l development to o v e r w i n t e r i n g s i z e . a l e s s l i k e l y but more elegant e x p l a n a t i o n f o r t h i s seemingly premature a t t a c k time i s t h a t the i n c r e a s e d p a y o f f s of o c c a s i o n a l l y being a b l e to produce a second brood outweigh the r i s k s of a t t a c k i n g while t r e e s are more vigorous. a l s o , mass a t t a c k behavior h e l p s reduce these r i s k s . The kind of e v o l u t i o n a r y s h i f t i n emergence and a t t a c k p e r i o d which i s assumed p o s s i b l e i n the above d i s c u s s i o n has been shown to occur i n the f a l l webworm (Morris and F u l t o n , 1970a). These i n v e s t i g a t o r s found t h a t heat requirements f o r development and diapause t e r m i n a t i o n were g e n e t i c a l l y c o n t r o l l e d . S e v e r a l t e s t a b l e hypotheses a r i s e from the p r e v i o u s few paragraphs. 1) O f f s p r i n g of e a r l y emerging b e e t l e s w i l l emerge e a r l i e r than the bulk of the p o p u l a t i o n . 2) Most of the s u c c e s s f u l second broods w i l l be f r u i t s of these e a r l y emergers. 3) Heat requirements f o r l a r v a l and pupal development may be under g e n e t i c c o n t r o l . <4) Males w i l l be the pioneer or s e a r c h i n g sex i n bark b e e t l e s p e c i e s where high 171 m o r t a l i t y i s s u f f e r e d during the d i s p e r s a l stage and where r e l a t i v e l y weak t r e e s are u t i l i z e d . The evidence f o r bark b e e t l e s i n ge n e r a l i s th a t p o p u l a t i o n s are e s s e n t i a l l y o v i p o s i t i o n - s i t e l i m i t e d owing to narrow host t r e e p h y s i o l o g y t o l e r a n c e ranges (Berryman, 1972; Rudinsky, 1962). Since these p o p u l a t i o n s are l i m i t e d by abundance of s u i t a b l e t r e e s , outbreaks of i n s e c t s can a c t u a l l y be construed as normal developments f o l l o w i n g "outbreaks" of s u s c e p t i b l e t r e e s . T h i s i n t e r p r e t a t i o n f i t s the mountain pine b e e t l e - l o d g e p o l e pine system q u i t e well because o f the evan-aged stand c h a r a c t e r i s t i c of lodgepole and the r e l a t i o n of t r e e age t o s u s c e p t i b i l i t y . When l a r g e areas of even-aged lodgepole reach s u s c e p t i b l e ages, there are abundant s i t e s f o r r e p r o d u c t i o n and the b e e t l e s reproduce q u i t e s u c c e s s f u l l y . F o r e s t e r s i n the United S t a t e s have r e c e n t l y become aware of t h i s concept and have proposed that n a t u r a l f i r e s i n lodgepole stands be l e f t to burn out n a t u r a l l y (L.L. Loope and G.D. Amman, pers. comm., 1972). They hope t h a t by breaking up the s p a t i a l d i s t r i b u t i o n o f age c l a s s e s i n t o a mosaic, there w i l l be lower p r o b a b i l i t i e s of mountain pine b e e t l e outbreaks. However, t h i s r e a s o n i n g assumes t h a t a f t e r some number of years, the stands of s u i t a b l e t r e e s w i l l be s m a l l enough to support o n l y s m a l l D. fionderosae p o p u l a t i o n s and w i l l be separated by bands of u n s u i t a b l e t r e e s wide enough t o keep mountain pine b e e t l e d i s p e r s a l between stands very low. However, u n t i l we know more about how f a r these b e e t l e s 172 d i s p e r s e , the proposal seems a slow, r i s k y , and c o s t l y way of t e s t i n g i d e a s . One aspect of the dynamics of i n s e c t p o p u l a t i o n s which has r e c e i v e d l i t t l e a t t e n t i o n i s the r o l e of w i t h i n - p o p u l a t i o n v a r i a t i o n . A l b r e c h t (1962) shows t h a t l o c u s t s which are su b j e c t e d t o changes i n d e n s i t y produce o f f s p r i n g with higher phenotypic v a r i a b i l i t y than the p a r e n t a l g e n e r a t i o n . T h i s has been i n t e r p r e t e d as a means f o r i n c r e a s i n g the s u r v i v a l of genotypes i n the face of environmental u n c e r t a i n t i e s . The apparent b i m o d a l i t y of o f f s p r i n g s i z e t h a t I found i n mountain pine b e e t l e s might a l s o be i n t e r p r e t e d i n t h i s way. Another example of t h i s g e n e r a l nature concerns the western t e n t c a t e r p i l l a r . W e l l i n g t o n (1964, 1965) has shown t h a t w i t h i n - p o p u l a t i o n v a r i a b i l i t y i n v i g o r and d i s p e r s a l c h a r a c t e r i s t i c s enable t h i s s p e c i e s t o c o l o n i z e h a b i t a t s of d i f f e r e n t s e v e r i t i e s . v a r i a b i l i t y among o f f s p r i n g of one female i n c r e a s e s the p r o b a b i l i t y of her genotype s u r v i v i n g . The w i t h i n - p o p u l a t i o n v a r i a b i l i t y i n d i s p e r s a l c h a r a c t e r i s t i c s of my mountain pine b e e t l e s a l s o suggests t h a t an advantage may be gained by maintaining w i t h i n - p o p u l a t i o n h e t e r o g e n e i t y , i f the d i s t r i b u t i o n of s u i t a b l e hosts i s u n p r e d i c t a b l e i n time and space. Many mountain pine b e e t l e c h a r a c t e r i s t i c s seem to be the product of s e l e c t i o n a t low, or endemic, l e v e l s . There are h i g h l y r e f i n e d mechanisms f o r sensing s u i t a b l e hosts and a t t r a c t i n g other members of the p o p u l a t i o n to such hosts. 173 Such c h a r a c t e r i s t i c s are most advantageous when i n s e c t p o p u l a t i o n s are low and there are few s u i t a b l e t r e e s . Thus, the c r i t i c a l q u estions f o r bark b e e t l e e c o l o g i s t s may not be "Why does t h i s s p e c i e s f l u c t u a t e i n numbers so widely?" but "How does t h i s s p e c i e s p e r s i s t i n times of low s u i t a b l e host abundance?" R e l a t e d t o t h i s p o i n t . Watt (1971) noted t h a t many e c o l o g i c a l t h e o r i e s are bia s e d by data from r e l a t i v e l y few, w e l l - s t u d i e d s p e c i e s . A d d i t i o n a l l y , these s p e c i e s are ones t h a t have a t t r a c t e d our a t t e n t i o n by a c h i e v i n g high d e n s i t i e s or by f l u c t u a t i n g widely. 174 X. SPECULATIONS ON LODGEPOLE FITNESS G i l b e r t and Hughes (1971), Hughes and G i l b e r t (1968), and G i l b e r t and G u t i e r r e z (1973) have po i n t e d out the s i g n i f i c a n c e of the adjustment of r e l a t i o n s h i p s between p l a n t and h e r b i v o r e p o p u l a t i o n s . They suggest f o r t h e i r aphid and host p l a n t s i t u a t i o n s t h a t r e p r o d u c t i o n and d i s p e r s a l to new p l a n t s occur i n such a way as to o p t i m i z e c e r t a i n f i t n e s s measures. I t i s i n t e r e s t i n g to look i n a s i m i l a r manner at the i n t e r a c t i o n of mountain pine b e e t l e with lodgepole pine. S a f r a n y i k et a l . (1974a) have noted t h a t most mountain pine b e e t l e outbreaks i n t h i s century i n western Canada have taken p l a c e i n t r e e stands about 80 years o l d . T h i s age i s remarkably c l o s e to f o r e s t e r s * r o t a t i o n age f o r a lodgepole pine stand on an average s i t e (70 f t at 80 y e a r s ) , as i n d i c a t e d by normal y i e l d t a b l e s (Smithers, 1962). The general term r o t a t i o n age r e f e r s to the age a t which i t i s b e s t to harvest a stand and i s d e f i n e d by some (e.g., stanek, 1966; Smithers, 1962) as the age a t which mean annual increment reaches i t s maximum. Although there i s some controversy over how r o t a t i o n age should be determined, the c o r r e c t d e f i n i t i o n i s unimportant here; the concept which i s s i g n i f i c a n t i s t h a t mountain pine b e e t l e outbreaks normally occur i n t r e e stands of about the age at which f o r e s t e r s b e l i e v e they can maximize long term wood p r o d u c t i o n . 1 7 5 The s i m p l e s t e x p l a n a t i o n f o r t h i s p e c u l i a r phenomenon i s that t r e e s keep from being s u s c e p t i b l e as long as i s f e a s i b l e and t h a t bark b e e t l e s a t t a c k these t r e e s as e a r l y as i t i s p o s s i b l e to achieve some degree of success; i . e . , when t r e e s begin to grow old and weak. I f t h i s senescence age i s f a i r l y c o n s i s t e n t between stands, then the observed phenomenon would be e x p l a i n a b l e . However, I hypothesize t h a t there i s a more s u b t l e e v o l u t i o n a r y e x p l a n a t i o n ; lodgepole may have adjusted i t s age of i n c r e a s e d s u s c e p t i b i l i t y t o mountain pine b e e t l e so t h a t t r e e stands are decimated before they produce so many seeds t h a t i n d i v i d u a l t r e e r e p r o d u c t i v e f i t n e s s decreases due to overcrowding i n the next g e n e r a t i o n . T h i s hypothesis suggests t h a t D. £onderosae i s more than a " n a t u r a l t h i n n i n g agent" f o r lodgepole pine; i t i s a removal agent which a c t s i n i t s own i n t e r e s t and t h a t of the t r e e s . The evidence which l e d to t h i s hypothesis d e a l s with the r e p r o d u c t i v e c h a r a c t e r i s t i c s of lodgepole pine. As noted i n i n t r o d u c t o r y s e c t i o n I I I , lodgepole i s a p r o l i f i c seed producer and i s w e l l known f o r i t s tendency to overstock and cause s t a g n a t i o n , or e a r l y t e r m i n a t i o n i n growth. One important c h a r a c t e r i s t i c of these overcrowded stands i s that seed production i s g r e a t l y reduced due to crown s u p p r e s s i o n ; only a few per cent of the t r e e s i n such stands ever produce cones. There i s thus an advantage f o r i n d i v i d u a l t r e e s to become s u s c e p t i b l e t o mountain pine b e e t l e a f t e r they have produced enough seed to i n s u r e some s u r v i v a l of o f f s p r i n g ; 176 stands w i l l have g r e a t e r p r o b a b i l i t i e s of being k i l l e d by mountain pine b e e t l e and the f i r e s which are e s s e n t i a l f o r seed r e l e a s e w i l l come through the stand t h a t much sooner. The reason t h i s i s an i n d i v i d u a l s e l e c t i o n mechanism and not a p o p u l a t i o n one i s that there i s almost no spreading of seeds i n a stand; most seeds f a l l w i t h i n a 200 f o o t r a d i u s of the parent t r e e (Boe f 1956; Dahms, 1963). T h i s means t h a t a t r e e producing a l a r g e number of seeds w i l l cause competition and reduced r e p r o d u c t i v e success mainly among i t s own o f f s p r i n g . Thus, the b a s i c i d e a behind the hypothesis at hand i s that t r e e s welcome " h a r v e s t " a t the p o i n t of d i m i n i s h i n g r e p r o d u c t i v e r e t u r n s . The kind of e v o l u t i o n a r y adjustment i n b e e t l e and t r e e c h a r a c t e r i s t i c s which t h i s hypothesis r e q u i r e s i s e a s i l y imaginable. s e l e c t i o n p r e s s u r e s between these s p e c i e s have been and s t i l l are i n t e n s e , s i n c e mountain pine b e e t l e ' s major host i s lodgepole pine and i n most regions t h i s b e e t l e i s the main source of the t r e e ' s m o r t a l i t y other than senescence. The kinds of questions which are i n t e r e s t i n g t o pursue deal with the consequences f o r l o d g e p o l e of a l t e r i n g the normal age (80) at which t r e e s become s u s c e p t i b l e to and are "harvested" by mountain pine b e e t l e . For i n s t a n c e , what would happen over a number of t r e e g e n e r a t i o n s i f lodgepole evolved much more potent r e s i s t a n c e mechanisms, keeping b e e t l e p o p u l a t i o n s low u n t i l many years l a t e r than usual? Conversely, what would be the consequences of the mountain pine b e e t l e ' s e v o l v i n g 177 g r a a t a r a b i l i t y to withstand t r e e r e s i s t a n c e mechanisms, thereby lowering the normal t r e e age at which outbreaks normally occurred? These q u e s t i o n s can be answered by t a k i n g a c l o s e r look at n a t u r a l lodgepole r e p r o d u c t i o n . Natural r e p r o d u c t i o n i s purposely d i s t i n g u i s h e d here from man-enhanced rese e d i n g p r a c t i c e s because of obvious problems i n d e a l i n g with the e v o l u t i o n a r y q u e s t i o n s at hand. Information i s needed on seed v i a b i l i t y , f a c t o r s a f f e c t i n g seed l o s s , and numbers of seeds a v a i l a b l e i n stands of v a r i o u s ages and d e n s i t i e s . S e v e r a l independent f a c t o r s a f f e c t annual p r o d u c t i o n of se r o t i n o u s cones of i n t e r i o r lodgepole. More middle-aged t r e e s produce cones than o l d e r or younger t r e e s ( F i g . 29), (C r o s s l e y , 1956a; Latham, 1965). The number of cones per tre e ( F i g . 30) i n c r e a s e s e x p o n e n t i a l l y with t r e e diameter, due to the i n c r e a s e i n crown volume as t r e e s grow (Crossley, 1956a; Latham, 1965). F i g . 31 shows t h a t the p r o p o r t i o n of t r e e s i n a stand which bears cones a l s o goes up with diameter (Eates, 1930; C r o s s l e y , 1956a). Tree diameters at given ages are i n turn g r e a t l y a f f e c t e d by stand d e n s i t y (Alexander et a l . , 1967; Dahms, 1967; Smithers, 1962; Trappe and H a r r i s , 1958). Some data on t h i s e f f e c t (from Lee, 1966) are shown i n F i g . 32 f o r t r e e s with a s i t e index of 70 f t a t 80 years. Crown suppression due to crowding a l s o decreases the number of cones on a t r e e (Baker, 1950; Bates, 1930; Clements, 1910; Smithers, 1962) . 178 FIGURE 29 The r e l a t i o n s h i p between t r e e age and p r o p o r t i o n s of t r e e s bearing any cones f o r lo d g e p o l e pine. Data are from C r o s s l e y (1956a) and Latham (1965). PROPORTION TREES BEARING CONES o ro • 4^ co co »-O i 1 1 1 1 1 1 1 1 1 1 cn 1 o 6LT 180 FIGURE 30 The r e l a t i o n between t r e e diameter and the number of cones per t r e e f o r lodgepole pine. Data are from Crossley(1956a) and Bates (1930). 1000 800 LU LU C£ h - 6 0 0 CO LU Mn 4 0 0 C_J # 2 0 0 0 CROSSLEY (1956R) RND LflTHRM (1965) 0 3 6 9 12 15 RVERRGE DIRMETER (IN.) CO 182 FIGURE 31 The r e l a t i o n between t r e e diameter and the p r o p o r t i o n of t r e e s i n a stand of l o d g e p o l e pine t h a t bears any cones, assuming that a l l t r e e s are of the same age. Data are from C r o s s l e y (1956a). I CROSSLEY (1956) AND RVERRGE D.B.H. (IN.) s 184 FIGURE 32 The r e l a t i o n between lodgepole pine stand d e n s i t y and average diameters a t g i v e n ages from 20 to 150 years. Data from ages 20 t o 100 are d i r e c t l y from Lee (1966) and data from ages 110 to 150 are e x t r a p o l a t i o n s from o t h e r curves of Lee (1966) . NORMAL Y I E L D T A B L E . L E E ( 1 9 6 6 ) 15T BY STAND AGE 12-0 1000 2000 3000 4000 5000 I N I T I A L STOCKING DENSITY T R E E S / A C R E CO 186 The p r o p o r t i o n of seeds (at about 25 to 40 seeds per cone) which become s a p l i n g s the next g e n e r a t i o n i s determined by s e v e r a l f a c t o r s . I n i t i a l l y , seeds are r e l e a s e d from cones by f i r e s , as noted e a r l i e r , approximately 50% of these seeds are v i a b l e (Bates, 1930; Clements, 1910; others) but only 1 out of 278 of the v i a b l e ones a c t u a l l y r o o t s s u c c e s s f u l l y on burned ground (Lotan, 1964). S q u i r r e l s are a major source of seed l o s s and most cones do not s u r v i v e t h e i r i n t e n s e h a r v e s t i n g more than 25 years ( C r o s s l e y , 1956a). A l l these agents of cone l o s s are f a i r l y constant, a c t i n g independently of t r e e d e n s i t i e s . However, as can be seen from the data r e f e r r e d to i n the preceding two paragraphs, most f a c t o r s a f f e c t i n g cone production are density-dependent, t y p i c a l l y g i v i n g r i s e t o sequences of events as f o l l o w s . L e t us assume t h a t t h e r e i s only one f i r e i n a stand per generation. Lodgepole i n a f a i r l y open stand, say 250 t r e e s per acre, w i l l produce many seeds. I f a f i r e burns the stand when i t i s 90 years o l d , t h e r e may be enough seed a v a i l a b l e to produce a very dense stand, say 4,000 t r e e s per acre. T h i s new stand, being crowded, w i l l have s m a l l e r t r e e s with s m a l l e r crowns than the previous g e n e r a t i o n . The new number of seeds produced at any age w i l l be much sm a l l e r than i n the p r e v i o u s g e n e r a t i o n as w i l l the number of s a p l i n g s i n the f o l l o w i n g generation i f t h i s stand i s burned before i t becomes too o l d . T h i s w i l l l e a d to an o s c i l l a t i o n i n numbers of t r e e s per acre through s e v e r a l g e n e r a t i o n s . 187 A simple s i m u l a t i o n model was c o n s t r u c t e d u s i n g the f u n c t i o n a l r e l a t i o n s h i p s j u s t d e s c r i b e d . In a d d i t i o n , stands of d i f f e r e n t ages were given v a r i o u s p r o b a b i l i t i e s of having f i r e s a c c o r d i n g to estimates of J. Burraro (pers. comm.). Stands from 10-40 years o l d were somewhat l i k e l y to be burned, 50-90 year stands were f a i r l y immune, and o l d e r stands were very s u s c e p t i b l e , becoming more so with age. The b a s i c s t r u c t u r e of the model was to c a l c u l a t e , f o r every ten years, the number of seeds produced by a stand of t r e e s i n a one a c r e p l o t . These c a l c u l a t i o n s were made f o r every ten year p e r i o d u n t i l the stand was decimated e i t h e r by f i r e or by mountain pine b e e t l e . At t h i s p o i n t the v i a b l e seeds were summed up and s u r v i v i n g s a p l i n g s were c a l c u l a t e d . These new s a p l i n g s became the s t a r t of the next g e n e r a t i o n and the c a l c u l a t i o n s f o r every ten year p e r i o d were c a r r i e d out i n the same way as before. T h i s process was repeated f o r ten g e n e r a t i o n s . Various f i t n e s s measures were c a l c u l a t e d each g e n e r a t i o n , i n c l u d i n g number of seeds s u r v i v i n g to reproduce, p o p u l a t i o n s i z e , o f f s p r i n g per grandparent, e t c . The model had f o u r main assumptions: 1) There are t h r e e causes of t r e e m o r t a l i t y : mountain pine b e e t l e , senescence, and f i r e . 2) There i s only one f i r e i n the stand each g e n e r a t i o n , e i t h e r immediately a f t e r bark b e e t l e s decimate the stand or at some randomly determined time.. Such f i r e s k i l l a l l t r e e s ( i f not a l r e a d y dead from beetles) and r e l e a s e lodgepole seeds. 3) E x t e r n a l p o p u l a t i o n s of bark b e e t l e s and 1 8 8 lodgepole pine do not a f f e c t the simulated p o p u l a t i o n s . 4) Bark b e e t l e s do no damage to lodgepole u n t i l the decade of outbreak, when a l l t r e e s i n the stand are k i l l e d . T h i s s i m u l a t i o n model allowed e x p l o r a t i o n of the p o s s i b i l i t y t h a t some tre e (not beetle) f i t n e s s measures are being optimized i n the mountain pine b e e t l e - l o d g e p o l e pine system. simulated numbers of seeds c l o s e l y approximated f i e l d data. S e v e r a l random sequences of f i r e s were t e s t e d i n the model and ages at which stands became v u l n e r a b l e to b e e t l e s were v a r i e d from 50 to 150 years. The r e s u l t s of t h i s s e r i e s of s i m u l a t i o n s show t h a t there may be something meaningful i n the o b s e r v a t i o n that most outbreaks of mountain pine b e e t l e occur i n stands of t r e e s p e c i e s , i n c l u d i n g l o d g e p o l e , of about 80 years of age. Two t r e e f i t n e s s measures show a peak near the age of b e e t l e h a r v e s t which i s normally observed i n nature. These are numbers of s u r v i v i n g s a p l i n g s per reproducing grandparent (F i g . 33) and numbers of s u r v i v i n g s a p l i n g s per t o t a l t r e e - y e a r s d u r i n g each g e n e r a t i o n ( F i g . 34). Ten other f i t n e s s measures (such as o f f s p r i n g per reproducing parent and o f f s p r i n g per t r e e at time of harvest) a l l e i t h e r i n c r e a s e d or decreased m o n o t o n i c a l l y with age of s u s c e p t i b i l i t y . The most reasonable f i t n e s s measure f o r lodgepole i s o f f s p r i n g per reproducing grandparent because of the time l a g i n v o l v e d i n determination of genotype s u r v i v a l . I f one were to look only at o f f s p r i n g per parent one would f i n d t h a t the l a t e r a stand 189 FIGURE 33 The simulated r e l a t i o n between numbers of s a p l i n g s produced per grandparent (reproducing t r e e s two generations back) and the age at which lodgepole stands become s u s c e p t i b l e to mountain pine b e e t l e a t t a c k . T h i s i s one example of the curves showing t h i s r e l a t i o n s h i p when only one set of random numbers r e p r e s e n t i n g p r o b a b i l i t i e s of f i r e s i s used. S R P L I N G S P E R R E P R O D U C I N G GRANDPARENT 4^ o CD -j-icn m JD ID rn co _ C D rn =R CO ^ro — o^ 0 cn J-o CO — I — CO —I— CD — i — ro — i — cn —i o <c rn <c —i rn rn TO :z: JD GO GO rn rn rn 70 ZD ZZL rn co o co CO 0 6 T 191 FIGDRE 34 The simulated r e l a t i o n between numbers of s a p l i n g s produced per t r e e per year per g e n e r a t i o n and the age a t which lodgepole stands become s u s c e p t i b l e to mountain pine b e e t l e a t t a c k . T h i s i s one example of the curves showing t h i s r e l a t i o n s h i p when onl y one set of random numbers r e p r e s e n t i n g p r o b a b i l i t i e s o f f i r e s i s used. 192 CD CO o CO UJ ZZL CT C£ UJ UJ CD CX 2: Od UJ UJ cr cr UJ 00 C D XT 0 + C\J o LO CO UJ CO ct: CX CX LU o _ x r~ o CO UJ CX 0 L J 0 0 ^ UJ ^ UJ UJ UJ CQ CD, CD CXI C D C D iN3S3dd S33di I b l O l d3d S3NIldUS 193 i s h a rvested, the l a r g e r i s the number of o f f s p r i n g per parent s u r v i v i n g due to the l a r g e r seed crop and low s a p l i n g m o r t a l i t y . However, i n such crowded c o n d i t i o n s , very few of those o f f s p r i n g would reproduce, and, s i n c e f i t n e s s measures p e r p e t u a t i o n of genotype, a f i t n e s s measure i n v o l v i n g a ona-generation time l a g would be more a p p r o p r i a t e . The number of s a p l i n g s s u r v i v i n g per t o t a l t r e e - y e a r s per g e n e r a t i o n has a l o c a l maximum at 90 years, drops and then r i s e s a g a i n ( F i g . 34). T h i s drop i s caused by the s h a r p l y i n c r e a s e d p r o b a b i l i t y of f i r e s i n stands aged 90 years and o l d e r . Changes i n the p r o b a b i l i t i e s of f i r e s o c c u r i n g changed q u a n t i t a t i v e values of these v a r i o u s f i t n e s s measures but only s l i g h t l y a l t e r e d the shapes of the curves ( F i g . 35). Two other f i t n e s s measures, numbers of seeds produced per generation and average p o p u l a t i o n s i z e per g e n e r a t i o n a l s o showed peaks at 80-90 years. However, present e c o l o g i c a l theory does not support the n o t i o n that these are reasonable f i t n e s s measures and thus we must r e l y on the two more l i k e l y measures of numbers of s u r v i v i n g s a p l i n g s per r e p r o d u c i n g grandparent and per t r e e - y e a r . Since these are reasonable f i t n e s s measures and s i n c e they f i t the o b s e r v a t i o n of ages at which lodgepole stands are normally k i l l e d by D. ponderosag, i t may be t h a t by a d j u s t i n g i t s age of s u s c e p t i b i l i t y , lodgepole i s maximizing these r e p r o d u c t i v e f i t n e s s measures. T h i s c o n c l u s i o n assumes t h a t s u s c e p t i b l e age can be a t l e a s t p a r t i a l l y i n h e r i t e d , e i t h e r 194 FIGURE 35 An example of how one of the f i t n e s s measures changed when d i f f e r e n t p r o b a b i l i t i e s of f i r e s as a f u n c t i o n of stand age were used. S R P L I N G S P E R R E P R O D U C I N G G R A N D P A R E N T o OJ CO C D ro CD -n'CD 7^ r n ° ZD m co _ C D ZD ^ 73 m CO . . C O Q cn CD CD m >0 ZD rn rn 73 ZZL ZD GO GO rn rn :z: ~n rn 73 ZD m co o co CO S 6 T 196 g e n e t i c a l l y or p h y s i o l o g i c a l l y . Since some f i t n e s s measure can u s u a l l y be found to f i t j u s t about any hypothesis or o b s e r v a t i o n , the above hypothesis i s not t o t a l l y s u b s t a n t i a t e d . Lodgepole may, i n a c t u a l f a c t , not be maximizing any of the p r e d i c t e d f i t n e s s measures. However, the evidence suggests t h a t i t i s worth c o n s i d e r i n g the p o s t u l a t e d concept as a p o s s i b l e e x p l a n a t i o n of the observed behavior of the D. pjanderosae-lodgepole pine system, a t e s t f o r the hypothesis might be to look at lodgepole pine i n the northern part of B r i t i s h Columbia, where mountain pine b e e t l e i s not and never has been present, and see whether r e p r o d u c t i v e c h a r a c t e r i s t i c s are d i f f e r e n t or whether another n a t u r a l h a r v e s t i n g agent i s present. The concept that p l a n t s p e c i e s may be using t h e i r i n s e c t predators to t h e i r own advantage i s a u s e f u l one to pursue. In the D. ponderosag-lodgepole pine system, the bark b e e t l e s are probably a t t a c k i n g t r e e s as soon as good a t t a c k success i s i n s u r e d , while the t r e e s may have adjusted t h i s age so t h a t t h e i r own r e p r o d u c t i v e outputs are maximized. 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