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The population dynamics of the aphids, macrosiphum avenae, metopolophium dirhodum, and rhopalosiphum… Woodgate, Rossalynn C. 1977

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THE POPULATION DYNAMICS OF THE APHIDS, UACR0S1PHUM AI/EWAE, MET0?0L0?HIUM VIRHOVUM, AND RHOVMOSIVHUU PAV1 ON OATS, AI/EWA SATIl/A CV FRASER ROSSALYNN C. WOODGATE (nee NORMAN) B.S.A., UNIVERSITY OF BRITISH COLUMBIA, 1967. A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE THE FACULTY OF GRADUATE STUDIES (Department of P l a n t Science) We accept t h i s t h e s i s as conforming t o the r e q u i r e d standard THE UNIVERSITY OF BRITISH COLUMBIA By xn 1977 - feossaiyiiri C.< Woodgate, 4977. In p r e s e n t i n g t h i s t h e s i s in p a r t i a l f u l f i l m e n t o f the r e q u i r e m e n t s f o r an advanced d e g r e e at the U n i v e r s i t y o f B r i t i s h C o l u m b i a , I a g r e e t h a t t h e L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s t u d y . I f u r t h e r a g r e e t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by the Head o f my Depar tment o r by h i s r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be a l l o w e d w i t h o u t my w r i t ten pe rm i ss i on . Depa rtmen t The U n i v e r s i t y o f B r i t i s h C o l u m b i a 2075 Wesbrook Place Vancouver, Canada V6T 1W5 ABSTRACT F r a s e r oats were used to study p o p u l a t i o n s of c e r e a l aphids i n 1972,1973 and 1974. The most commonly found s p e c i e s were Macrosiphum avenae (F.) (the E n g l i s h g r a i n a p h i d ) , Metopolophium dirhodum (Wlk.) (Rose-grain aphid) and Rhopalosiphum p a d i (L.) (Oat b i r d - c h e r r y a p h i d ) . Aphid d e n s i t y was h i g h e s t i n 1972, lower i n 1973 and lowest i n 1974. G e n e r a l l y M. avenae had the h i g h e s t p o p u l a t i o n w i t h M. dirhodum s l i g h t l y lower. Rhopalosiphum p a d i was found i n f r e q u e n t l y w i t h low numbers. Fec u n d i t y t r i a l s , used to examine whether the d i f f e r e n c e between numbers of s p e c i e s was because of a d i f f e r e n c e i n f e c u n d i t y , d i d not s u b s t a n t i a t e f i e l d r e s u l t s but i n s t e a d i n d i c a t e d t h a t R.padi should have produced the h i g h e s t p o p u l a t i o n . D i f f e r e n c e s were not found i n the p o p u l a t i o n dynamics when the oats were p l a n t e d i n s o l i d b l o c k s i n s t e a d of rows. A one month delay i n the p l a n t i n g date d i d not change the t o t a l aphid p o p u l a t i o n but d i d r e s u l t i r a higher p r o p o r t i o n of M. avenae than found i n any other p l o t . C o c c i n e l l i d numbers i n two out of the three years were co n s i d e r e d n e g l i g i b l e . In 1972 r a i n was thought to d e s t r o y many c o c c i n e l l i d b e f o r e they reached l a r g e enough numbers to g r e a t l y a f f e c t the aphid p o p u l a t i o n . No d i r e c t samples were taken of hymenopterous p a r a s i t e s but any found were c o l l e c t e d and i d e n t i f i e d . Water experiments were conducted to study the e f f e c t of water on the f e c u n d i t y of R. p a d i . I t . was concluded - t h a t R. padi d e f i n i t e l y p r e f e r r e d wet c o n d i t i o n s . Temperature and r a i n f a l l r eadings were used to show the e f f e c t of weather on aphid numbers. Weather was shown to be a major r e g u l a t o r y f a c t o r i n the p o p u l a t i o n dynamics of c e r e a l aphids. C o n s i d e r a t i o n s were made f o r aphid m o r t a l i t y caused by sparrows. i v . TABLE OF CONTENTS Page TITLE PAGE i ABSTRACT i i TABLE OF CONTENTS i v LIST OF TABLES i x LIST OF FIGURES x i i ACKNOWLEDGEMENT xv INTRODUCTION 1 1. Economic Damage of Oat Aphids . . . . 1 2. Time of P l a n t i n g 4 3. P o p u l a t i o n Dynamics 5 4. In s e c t P l a n t R e l a t i o n s h i p s 8 5. Temperature R e l a t i o n s h i p s 10 6. Predator and p a r a s i t e R e l a t i o n s h i p s . . 11 7. Aim of T h i s Study 13 SECTION I - POPULATION DYNAMICS - 1972 . . . 14 1. I n t r o d u c t i o n 14 2. M a t e r i a l s and Methods 14 3. R e s u l t s and D i s c u s s i o n . . . . . . . 16 4. Conclusions, 26 SECTION I I - FECUNDITY ' . 2 9 1. I n t r o d u c t i o n 29 2. M a t e r i a l s and Methods 29 3. R e s u l t s and D i s c u s s i o n 31 A. Treatment E f f e c t s W i t h i n Species . . 31 i ) M. avenae . 31 a) M. avenae (a-1) 31 b) M. avenae (a-2) 34 c) M. avenae (a-3) 35 d) Comparison between M. avenae Treatments- • 36 i i ) ,M. dirhodum 37 a) M. dirhodum (d-1) . . . . . 37 b) M. dirhodum (d-2) 37 c) M. dirhodum (d-3) d) M. dirhodum with R. pa d i (pd-1(d)) e) Comparison between M. dirhodum Treatments i i i ) R. pa d i  a) R. padi (p-1) b) R. padi w i t h M. dirhodum (pd-l(p)) c) Comparison between R. pa d i treatments . . . . B. Treatment E f f e c t s Between Species C. C o n c l u s i o n s SECTION I I I - POPULATION DYNAMICS 1973 . 1. I n t r o d u c t i o n 2. M a t e r i a l s and Methods A. P l o t D e s c r i p t i o n s i ) P l o t A i i ) P l o t B i i i ) P l o t C B. Aphid Sampling i ) P l o t A i i ) P l o t B i i i ) P l o t C C. P l a n t Sample i ) S i z e i i ) M oisture Content i i i ) Sparrow Damage . D. N a t u r a l Enemies i ) C o c c i n e l l i d a e i i ) P a r a s i t e s 3. R e s u l t s and D i s c u s s i o n . . . . . . A. P l o t A i ) Aphids . . . . i i ) Species D i f f e r e n c e i i i ) P l a n t E f f e c t s iv) Sparrow Damage v) N a t u r a l Enemies v i . Page a) C o c c i n e l l i d a e 65 b) P a r a s i t e s 65 B. P l o t B 65 i ) Aphids 65 i i ) Species D i f f e r e n c e 66 i i i ) P l a n t E f f e c t s 66 iv) Sparrow Damage 67 v) N a t u r a l Enemies . 69 a) C o c c i n e l l i d a e 6 9 b) P a r a s i t e s 69 C. P l o t C 69 i ) Aphids 69 i i ) S p ecies D i f f e r e n c e 70 i i i ) P l a n t E f f e c t s 72 iv) Sparrow Damage 73 v) N a t u r a l Enemies 74 a) C o c c i n e l l i d a e 74 b) P a r a s i t e s 74 D. P l o t Comparisons (Conclusions) 75 SECTION IV - THE EFFECT OF PLANT AGE AND WATER STRESS ON RHOPALOSIPHUM PADI (L.) . . . . 79 1. I n t r o d u c t i o n 7 9 2. Experiment #1 ( C o n t r o l l e d C o n d i t i o n s - caged) 81 A. M a t e r i a l s and Methods 81 B. R e s u l t s and D i s c u s s i o n 83 C. C o n c l u s i o n s 86 3. Experiment #2 ( C o n t r o l l e d C o n d i t i o n s - not caged) . . . 87 A. M a t e r i a l s and Methods 8 7 B. R e s u l t s and D i s c u s s i o n . . . . . . . 88 C. C o n c l u s i o n s 90 v i i . Page 4. Experiment #3 ( U n c o n t r o l l e d Conditions) . . 90 A. M a t e r i a l s and Methods . . . . . . . 90 B. R e s u l t s and D i s c u s s i o n 91 C. Co n c l u s i o n s 93 SECTION V - POPULATION DYNAMICS 1974 95 1. I n t r o d u c t i o n 95 2. M a t e r i a l s and Methods 96 A. P l o t A 96 i ) D e s c r i p t i o n and Aphid Sampling . . . 96 i i ) Sparrow Cages 9 6 B. P l o t B 97 i ) D e s c r i p t i o n and Aphid Sampling . . . 97 C. P l a n t Sampling 97 D. S o i l Samples 97 E. P a r a s i t e s 98 3. R e s u l t s and D i s c u s s i o n 98 A. P l o t A 98 i ) Aphid Samples 98 i i ) Species D i f f e r e n c e 101 i i i ) Sparrow Cages 104 iv) P l a n t E f f e c t s 106 v) S o i l Sample R e s u l t s 107 v i ) P a r a s i t e s 108 B. P l o t B 108 i ) Aphid Samples 108 i i ) Species D i f f e r e n c e 110 i i i ) P l a n t E f f e c t s 112 iv ) S o i l Sample R e s u l t s 113 v) P a r a s i t e s 113 4. Co n c l u s i o n s 114 SUMMARY 115 BIBLIOGRAPHY i x . LIST OF TABLES PAGE 1. The number of oat t i l l e r s sampled f o r aphids on each sample date o f 1972 2. The i d e n t i f y i n g code of treatments used t o study the f e c u n d i t y of the th r e e aphid s p e c i e s 30 3. P r e - r e p r o d u c t i v e , r e p r o d u c t i v e , and p o s t -r e p r o d u c t i v e p e r i o d s f o r v a r i o u s treatments of t h r e e s p e c i e s of oat aphid 35 4. The i n t r i n s i c r a t e of i n c r e a s e i n ?/? day, the g e n e r a t i o n time i n days, the d o u b l i n g time i n days and the net r e p r o d u c t i v e r a t e of e i g h t treatments imposed upon three s p e c i e s of oat aphids 37 5. The i n t r i n s i c r a t e of i n c r e a s e i n ?/? day, the net r e p r o d u c t i v e r a t e , the g e n e r a t i o n time i n days, and the d o u b l i n g time i n days of three s p e c i e s of oat aphids, M. avenae, M. dirhodum and R. p a d i . 46 6. The mean number of nymphs produced per female f o r each treatment 47 7. The percentage of oat t i l l e r s (Avena s a t i v a cv F r a s e r ) i n f e s t e d w i t h aphids i n 1973 59 8. Oat husks wi t h seeds, as norma], and oat husks without seeds due t o sparrow damage i n 1973... 63 9. The percentage of the aphid sample l o c a t e d i n the head of the oat t i l l e r s , Avena , s a t i v a cv F r a s e r , s e l e c t e d randomly from t h r e e p l o t s of oats i n 1973 64 10. The number of a p h i d s / t i l l e r sampled and the number of a p h i d s / t i l l e r estimated i f sparrows had not been p r e s e n t i n p l o t A i n 1973 64 X. PAGE The number of a p h i d s / t i l l e r sampled and the number of a p h i d s / t i l l e r estimated i f sparrows had not been p r e s e n t i n p l o t B i n 1973. 68 The number of a p h i d s / t i l l e r sampled and the number of a p h i d s / t i l l e r estimated i f sparrows had not been p r e s e n t i n p l o t C i n 1973 73 The nine treatments a p p l i e d t o Rhopalosiphum  p a d i and o a t s , Avena s a t i v a cv F r a s e r , i n c o n t r o l l e d c o n d i t i o n s 82 An a n a l y s i s of v a r i a n c e of the mean number of nymphs produced by each of nine treatments d e s c r i b e d i n Table 13 84 The mean and standard e r r o r of the mean of p o p u l a t i o n s of R. p a d i r a i s e d on th r e e s i z e s of p l a n t s which had been t r e a t e d w i t h t h r e e d i f f e r e n t amounts of water The mean number w i t h standard e r r o r of R. pa d i on p l a n t s which had been t r e a t e d w i t h t h r e e d i f f e r e n t amounts of water An a n a l y s i s o f v a r i a n c e of the mean number of nymphs produced by R. pa d i d u r i n g t h r e e water treatments under c o n t r o l l e d c onditions...89 The mean and standard e r r o r of the mean of p o p u l a t i o n s of R. p a d i e s t a b l i s h e d on p l a n t s which had been t r e a t e d w i t h three d i f f e r e n t amounts of water An a n a l y s i s of v a r i a n c e of the mean number of nymphs produced by R. p a d i d u r i n g three water treatments under v a r i a b l e c o n d i t i o n s x i . PAGE 20. Duncan's New Multiple Range Test showing s i g n i f i c a n t water treatments under variable conditions.. 93 21. The percentage of oat t i l l e r s , Avena  sativa cv Fraser infested with aphids i n p l o t A i n 1974... 100 22. The percentage of oat t i l l e r s , Avena sativa cv Fraser infested with aphids i n p l o t B i n 1974. 109 x i i . LIST OF FIGURES PAGE 1. T o t a l c e r e a l a p h i d s / t i l l e r found i n an oat p l o t , (Avena. s a t i v a cv Fra s e r ) i n 1972, and the average t i l l e r h e i g h t of t i l l e r s s e l e c t e d from the p l o t 17 2. The number of a d u l t s / t i l l e r of th r e e s p e c i e s of aphids; Macrosiphum avenae, Metopo1ophiurn  dirhodum and Rhopalosiphum p a d i ; on a p l o t of o a t s , (Avena s a t i v a cv Fra s e r ) d u r i n g 1972 19 3. The number of c o c c i n e l l i d a d u l t s and l a r v a e found per h e c t a r e i n a p l o t o f o a t s , (Avena s a t i v a cv F r a s e r ) i n 1972 20 4. The p r e c i p i t a t i o n a t U.B.C. weather s t a t i o n f o r May, June, J u l y and August i n 1972 22 5. The t o t a l number of aphids and the number of c o c c i n e l l i d l a r v a e and a d u l t s per cm of l e a f found i n a p l o t of o a t s , (Avena s a t i v a cv F r a s e r i n 1972 23 6. T o t a l a p h i d s / t i l l e r / i n d i v i d u a l sample of an oat p l o t , (Avena s a t i v a cv F r a s e r ) sampled i n 1972. 24 7. Average t i l l e r h e i g h t / i n d i v i d u a l sample i n 197 2 of an oat p l o t , (Avena., s a t i v a cv Fr a s e r ) sampled i n 1972 25 8. D a i l y a g e - s p e c i f i c s u r v i v a l f o r Macrosiphum  avenae (The E n g l i s h G r a i n Aphid) 32 9. D a i l y a g e - s p e c i f i c f e c u n d i t y f o r Macrosiphum  avenae, (The E n g l i s h G r a i n Aphid) 33 10. D a i l y a g e - s p e c i f i c f e c u n d i t y f o r Metopolophium dirhodum (The Rose-Grain Aphid) 38 x i i i . PAGE 11. D a i l y a g e - s p e c i f i c s u r v i v a l f o r Metopolophium dirhodum, (The Rose-Grain Aphid) 39 12. D a i l y a g e - s p e c i f i c f e c u n d i t y f o r Rhopalosiphum p a d i , (The Oat B i r d - C h e r r y Aphid) 43 13. D a i l y a g e - s p e c i f i c s u r v i v a l f o r Rhopa1osiphum p a d i , (The Oat B i r d - C h e r r y Aphid) 44 14. T o t a l c e r e a l a p h i d s / t i l l e r found on 3 p l o t s of o a t s , (Avena . s a t i v a cv F r a s e r ) i n 1973... 55 15. The number of a d u l t s / t i l l e r of th r e e s p e c i e s of c e r e a l aphids; Macrosiphum avenae, Metopolophium dirhodum and Rhopalosiphum p a d i found d u r i n g 1973 i n p l o t A, p l a n t e d w i t h o a t s , (Avena s a t i v a cv F r a s e r ) 57 16. The average t i l l e r h e i g h t of t i l l e r s s e l e c t e d from 3 p l o t s o f o a t s , (Avena s a t i v a cv Fra s e r ) i n 1973 61 17. The number of a d u l t s / t i l l e r s of three s p e c i e s of c e r e a l aphids; Macrosiphum avenae, Metopolophium dirhodum and Rhopalosiphum p a d i found d u r i n g 197 3 i n p l o t B, p l a n t e d w i t h o a t s , (Avena • s a t i v a cv F r a s e r ) 62 18. The number of a d u l t s / t i l l e r o f th r e e s p e c i e s o f c e r e a l aphids; Macrosiphum avenae, Metopolophium dirhodum and Rhopalos iphum p a d i found during 197 3 i n p l o t C, p l a n t e d with o a t s , (Avena s a t i v a cv F r a s e r ) 71 19. T o t a l c e r e a l a p h i d s / t i l l e r found on two p l o t s of o a t s , (Avena s a t i v a cv F r a s e r ) i n 1974 99 20. The p o p u l a t i o n / t i l l e r of three s p e c i e s of oat aphids; Macrosiphum avenae, Metopolophium  dirhodum and Rhopalosiphium p a d i found d u r i n g 1974 i n p l o t A, p l a n t e d w i t h o a t s , (Avena s a t i v a cv Fra s e r ) 102 The p o p u l a t i o n / t i l l e r of three species of oat aphids; Macrosiphum avenae, Metopolophium dirhodum and Rhopalosiphum  padi found during 1974 i n plot B, planted with oats, (Avena, ; sativa cv Fraser) XV. ACKNOWLEDGEMENT I would l i k e to express my sincere thanks to Dr. W.G. Wellington and Dr. Bryan D. Frazer, who supervised the study, provided guidance, and the f a c i l i t i e s needed. For the advice and.helpful c r i t i c i s m of Dr. V.C. Runeckles and Dr. Judith H. Myers I am indeed g r a t e f u l . I would l i k e also to thank Mr. D. Pearce for his help i n the f i e l d , and to the many people at Canada Department of Agriculture, Vancouver, who gave t h e i r assistance. The study was financed by Canada Department of Agriculture (Grants to Dr. W.G. Wellington). Thanks go also to Dr. M. Mackauer for ide n t i f y i n g the parasites and hyperparasites. 1. INTRODUCTION Regulated p o p u l a t i o n s never i n c r e a s e i n d e f i n i t e l y and r a r e l y reach, e x t i n c t i o n . F a c t o r s c o n t r i b u t i n g to p o p u l a t i o n r e g u l a t i o n f a l l i n t o two c a t e g o r i e s - i n t r i n s i c and e x t r i n s i c . The former are c h a r a c t e r i s t i c s o f the organisms such as t h e i r b ehavior and p h y s i o l o g y . E x t r i n s i c o r environmental f a c t o r s are such t h i n g s as weather, food, and pre d a t o r s and p a r a s i t e s . The importance of i n t r i n s i c and e x t r i n s i c f a c t o r s i n p o p u l a t i o n r e g u l a t i o n has long been a p o i n t of c o n t e n t i o n among p o p u l a t i o n e c o l o g i s t s (general r e f e r e n c e - Krebs (1972)). My study i n v e s t i g a t e s the f a c t o r s a s s o c i a t e d w i t h the r e g u l a t i o n of p o p u l a t i o n numbers of the three dominant s p e c i e s of aphids found worldwide on oat s , Avena 1 s a t i v a ; Macrosiphum avenae (F.) (Homoptera, Aphididae) (The E n g l i s h G r a i n A p h i d ) , Metopolophium dirhodum (Walker) (Rose-grain aphid) and Rhopalosiphum p a d i (L.) (Oat b i r d - c h e r r y aphid) (Forbes and F r a z e r (1973), Forbes e t a l . (1973), Lambers (1939), Lambers (1947), Palmer (1952) and Richards (1960). 1. Economic Damage of Oat Aphids Although records of economic damage by aphids on oats and e x t e n s i v e c o n t r o l programs do e x i s t , Forbes (1962) found t h a t these aphids d i d not reduce g r a i n y i e l d s of oats 2. and o n l y s l i g h t l y reduced straw y i e l d s i n B r i t i s h Columbia. He thought t h a t the economic i n j u r y l e v e l f o r the aphids would have to be h i g h e r than the 47 aphids per t i l l e r observed i n h i s study. Kolbe (1970) i n West Germany however, r e p o r t e d t h a t an average i n c r e a s e i n y i e l d of 15% was o b t a i n e d by u s i n g chemicals to c o n t r o l the aphids on o a t s , v a r . Flamingskrone, when d e n s i t y on the u n t r e a t e d p l o t s was 46 aphids per stem. He thought t h a t the e r a d i c a t i o n of aphids was e s s e n t i a l i f there was s u s p i c i o n t h a t the c e r e a l crops were i n f e c t e d by v i r u s d i s e a s e s . S t e r n (1967) and S t e r n and Bowen (1967) s t u d i e d the economic treatment l e v e l of aphids on b a r l e y i n C a l i f o r n i a , and concluded t h a t the value of the e x t r a y i e l d r e s u l t i n g from chemical c o n t r o l exceeded the c o s t of treatment. Wood (1965) i n Oklahoma found t h a t even heavy i n f e s t a t i o n s of M. avenae d i d not r e s u l t i n any l o s s of y i e l d as the aphids were u s u a l l y c o n t r o l l e d by p r e d a t o r s and p a r a s i t e s . Z e c l a n t (1969) i n France found t h a t M. avenae reduced y i e l d s by 600 Kg per ha, but he a l s o thought chemical c o n t r o l unnecessary. He a t t r i b u t e d the r e d u c t i o n i n y i e l d to a decrease i n the number of g r a i n s r a t h e r than to weight l o s s of g r a i n s . As there was no r e d u c t i o n i n the weight l o s s or i n the baking q u a l i t y i t was not economically necessary to use chemical c o n t r o l . T h i s c o n c l u s i o n i s c o n t r a r y to Wratten (1975), who s a i d t h a t aphids do not reduce e i t h e r s p i k e l e t or 3. g r a i n number, as these are normally determined by p r e - a n t h e s i s f a c t o r s . He found t h a t f i e l d caged p o p u l a t i o n s of M. avenae and M. dirhodum reduced g r a i n weight of wheat by 14% and 7%, r e s p e c t i v e l y , and t h a t both reduced g r a i n p r o t e i n by 1%. He thought the l o s s of p r o t e i n was probably due to n i t r o g e n d r a i n f o r which the p l a n t c o u l d not compensate. As l i t t l e as a 1% r e d u c t i o n i n g r a i n p r o t e i n can a f f e c t the s u i t a b i l i t y o f wheat v a r i e t i e s f o r breadmaking. In F i n l a n d , RautapaS (196 8) found M. avenae reduced y i e l d s and changed brewing q u a l i t y of b a r l e y , and t h a t R. p a d i and M. avenae reduced the e x t r a c t content of malt but had no s i g n i f i c a n t e f f e c t on o t h e r brewing q u a l i t i e s (Rautapaa, 1972). These aphids a l s o reduced the y i e l d of b a r l e y through a r e d u c t i o n i n the weight of the g r a i n . In 1966, Rautapaa used outdoor cages around s p r i n g wheat and showed t h a t heads s e v e r e l y damaged by aphids, y i e l d e d an average of 0.129 g/head compared w i t h 0.999gfor undamaged heads. Although s t a r c h q u a l i t y was not a f f e c t e d , there was an adverse a f f e c t on g l u t e n q u a l i t y and on g e r m i n a t i o n . Aphids i n f e s t i n g the head a t f l o w e r i n g caused g r e a t e r l o s s e s i n y i e l d than those i n f e s t i n g the p l a n t s o n l y d u r i n g the p e r i o d of r i p e n e s s . In 1974, Kolbe and Linke found t h a t 20-30 aphids per head of wheat can cause l o s s e s up to 10% compared to up to 30% f o r 150 aphids per head. Apablaza and Robinson (1967) found i n l a b o r a t o r y 4. t e s t s t h a t aphids s e v e r e l y i n j u r e d or k i l l e d s e e d l i n g s of b a r l e y , wheat and oats, causing r e d u c t i o n s i n the k e r n e l weight of g r a i n even when p l a c e d on p l a n t s i n advanced stages of growth. George (19 24) concluded t h a t aphids have l i t t l e e f f e c t on y i e l d u n t i l the g r a i n begins to f i l l , when they can reduce y i e l d . He determined t h a t sampling w i t h s u c t i o n t r a p s w i l l g i v e an adequate warning a t the begin n i n g of a crop i n f e s t a t i o n , thus making repeated v i s i t s and sampling unnecessary. G r i g o r o v (196 7) s t u d i e d the e f f e c t of chemical c o n t r o l on M. avenae. Because of hig h m o r t a l i t y to pre d a t o r s and p a r a s i t e s a f t e r the use of an organo-phosphate insecticide, M. avenae i n c r e a s e d i n d e n s i t y t o high d e n s i t i e s a f t e r an o r i g i n a l 95% k i l l . Hamilton and R i e c k h e f e r (1969) found i n l a b o r a t o r y t e s t s with malathionand p a r a t h i o n t h a t predators were l e s s v u l n e r a b l e to the i n s e c t i c i d e s than the aphids, so they thought t h a t an i n t e g r a t e d program of c o n t r o l o f c e r e a l aphids u s i n g these m a t e r i a l s had promise. (Forbes (196 2) had s t a t e d t h a t non-s e l e c t i v e m a t e r i a l s such as malathion and p a r a t h i o n should not be used, as they are t o x i c to pre d a t o r s and p a r a s i t e s , but t h a t i f chemical c o n t r o l became necessary systemics should be used). 2. Time of P l a n t i n g The time o f p l a n t i n g has been thought by some to a f f e c t the e x t e n t o f g r a i n aphid i n f e s t a t i o n . Green (1966) 5. found t h a t b a r l e y sown i n the e a r l y s p r i n g i n Oregon developed s m a l l e r p o p u l a t i o n s of M. avenae, but l a r g e r p o p u l a t i o n s of M. dirhodum, than t h a t sown l a t e . Those two s p e c i e s as w e l l as R. p a d i became e s t a b l i s h e d on p l a n t s a t a l a t e r growth stage i n early-sown p l o t s than i n those sown l a t e r . M. avenae peaked i n numbers when the b a r l e y p l a n t s headed out but disappeared two weeks l a t e r i n J u l y . Green thought t h i s disappearance was due to p l a n t m a t u r i t y accompanied by h i g h temperature and low r e l a t i v e humidity. In h i s study, M. dirhodum a l s o d e c l i n e d s h a r p l y i n the l a t e r p a r t of J u l y , but M. dirhodum was not found u n t i l l a t e r i n the s p r i n g and only reached p o p u l a t i o n d e n s i t i e s as h i g h as M. avenae on the e a r l y - s e e d e d f i e l d . Both s p e c i e s p e r s i s t e d l a t e r on p l a n t s i n f i e l d s p l a n t e d e a r l y or i n midseason than on those p l a n t e d l a t e r . R. p a d i entered f i e l d s l a t e r than the other s p e c i e s . Green thought R. p a d i might have p r e f e r r e d p l a n t s i n which the lower leaves had begun to senesce and l o s e t h e i r c h l o r o p h y l l . R. padi showed more e r r a t i c p o p u l a t i o n trends g e n e r a l l y * p e a k i n g i n June when c o n d i t i o n s were c o o l e r than i n J u l y . T h i s s p e c i e s was most numerous on young p l a n t s i n l a t e - s e e d e d f i e l d s . 3• P o p u l a t i o n Dynamics Jones (197 2) used emergence cages to t r a p l a t e aphids i n B r i t a i n . The cages were l a r g e enough to accommodate r i p e n i n g oat and wheat p l a n t s and were surmounted by a removable 6. g l a s s t r a p c o n t a i n i n g a cone to prevent the aphids from f l y i n g back to the p l a n t s . Macrosiphum avenae, S. f r a g a r i a e , M. dirhodum and R. p a d i o c c u r r e d i n d i f f e r e n t p r o p o r t i o n s every year, but the timing of peak emergence was c o r r e l a t e d w i t h weather c o n d i t i o n s . G e n e r a l l y , seasons with more sunshine and warmer weather had e a r l i e r development of peak numbers. She noted t h a t p r e d a t o r s , p a r a s i t e s and the i n d i r e c t a c t i o n of hyper-p a r a s i t e s have an important i n f l u e n c e on y e a r l y a p h i d p o p u l a t i o n f l u c t u a t i o n s . Dean (1973) examined the i n i t i a l c o l o n i z a t i o n of c e r e a l s by oat aphids and t h e i r movement w i t h i n the crop. He used s u c t i o n and water traps to assess a l a t a e numbers. Rhopalosiphum  pa d i invaded the f i e l d f i r s t and remained the most common s p e c i e s u n t i l e a r l y June, when a l l s p e c i e s became more abundant. Metopolophium dirhodum was the f i r s t to become s c a r c e i n l a t e J u l y , while R. p a d i and M. avenae stayed u n t i l mid-August. A f t e r mid-June there was no apparent r e l a t i o n s h i p between maximum aphid d e n s i t i e s on the c e r e a l s and the number of a l a t a e caught i n the s u c t i o n t r a p . Movement of M. avenae w i t h i n the crop was s t u d i e d by p l a c i n g h e a v i l y i n f e s t e d potted p l a n t s w i t h i n the f i e l d . The aphids moved i n every d i r e c t i o n and d i d not seem to be i n f l u e n c e d by the d i r e c t i o n of the wind, although (Dean, 1973) when d i s t r i b u t i o n i n r e l a t i o n t o s h e l t e r was s t u d i e d , aphid movement seemed c l o s e l y r e l a t e d to wind d i r e c t i o n . The a d u l t s mainly invaded areas 7. shaded by hedges and woodland. Dean (1973) a l s o s t u d i e d the d i s t r i b u t i o n of aphids on t i l l e r s and concluded t h a t , u n t i l mid-June, when heading b e g i n s , the i n s e c t s were a l l on the l e a v e s . T h e r e a f t e r , 76% of M. avenae were on the heads with a few on the f l a g l e a f and two lower l e a v e s . E i g h t y - f i v e p e r c e n t of M. dirhodum were on the lower leaves w i t h a few on the f l a g l e a f and head, while R. padi and M. f e s t u c a e were o n l y found on the lower l e a v e s . Aphids seen moving up and down the stem were not probing or f e e d i n g . These r e s u l t s confirmed h i s e a r l i e r c o n t e n t i o n s t h a t when stu d y i n g c e r e a l aphids, the e n t i r e p l a n t must be c o l l e c t e d i n o r d e r to o b t a i n an unbiased estimate because the d i f f e r e n t aphid s p e c i e s l i v e on d i f f e r e n t p a r t s of the p l a n t (Dean and L u u r i n g , 1970). Dean (1974) found R. p a d i abundant i n the a i r but not on the crop, and suggested t h a t t h i s d i s c r e p a n c y arose because i n B r i t a i n the s p e c i e s i s mainly a grass aphid. Jones (1972) a l s o suggested t h a t the h i g h e r number of t h a t s p e c i e s i n the f i e l d s she s t u d i e d i n 19 71 might be due to the presence of a l a r g e r number of grasses between the wheat p l a n t s t h a t year. Dean f u r t h e r suggested t h a t changes i n c e r e a l v a r i e t i e s , farm p r a c t i c e or c l i m a t e might be b e n e f i c i a l to R. p a d i . Sparrow (1974) s t u d i e d spring-sown c e r e a l s i n non-s h e l t e r e d s i t e s . He, l i k e Dean, found t h a t p o p u l a t i o n s d i d not peak u n t i l the crops had headed out i n J u l y or August, two 8. to three weeks bef o r e r i p e n i n g . He suggested t h a t above average temperatures would be r e q u i r e d i n May and June f o r the aphids to have i n c r e a s e d s u f f i c i e n t l y by the end of June to s i g n i f i c a n t l y a f f e c t y i e l d s . Malyk and Robinson (1971) s t u d i e d the f a t e s of i n d i -v i d u a l f i e l d c o l o n i e s of c e r e a l aphids. Although l a r g e numbers of c o l o n i e s were found p r i o r to p l a n t heading, many were des t r o y e d by wind, r a i n and p r e d a t o r s . Reproduction r a p i d l y i n c r e a s e d the t o t a l aphid p o p u l a t i o n , but the p r o p o r t i o n of i n f e s t e d p l a n t s decreased. Young p l a n t s c o u l d be k i l l e d by aphid i n f e s t a t i o n s , but i f the p l a n t s werenearly mature when i n f e s t e d , the aphid population, d e c l i n e d as the p l a n t s r i p e n e d . 4. I n s e c t P l a n t R e l a t i o n s h i p s Many f a c t o r s , i n c l u d i n g the s p e c i e s , v a r i e t y and age of the p l a n t , the temperature, and p r e d a t o r s and p a r a s i t e s , which may a f f e c t aphid numbers have been s t u d i e d s e p a r a t e l y by many people. Eastop (19 73) d e s c r i b e d i n s e c t / p l a n t r e l a t i o n s h i p s and i n c l u d e d p l a n t p r e f e r e n c e s of aphids. Ten percent of the aphids l i s t e d a l t e r n a t e s e a s o n a l l y between two groups of p l a n t s which are o f t e n b o t a n i c a l l y d i s t i n c t . The host range sometimes v a r i e s with temperature; e.g., a t medium temperatures aphids are more s p e c i f i c than a t h i g h or low temperatures. Hosts of c e r e a l aphids i n Great B r i t a i n are l i s t e d by 9. George (1974). The primary host o f M. dirhodum i s Rosa spp. on which i t overwinters as the egg stage. Rhopalos iphum p a d i a l s o overwinters i n the egg stage on i t s primary host, Prunus padus, and i n warm areas i t overwinters v i v i p a r o u s l y on grasses and P. padus. Macrosiphum avenae has as i t s primary h o s t v a r i o u s gramineae on which i t overwinters v i v i p a r o u s l y as w e l l as i n the egg stage. The apterous stage o f R. p a d i was found to p r e f e r oats to young b i r d c h e r r y (P. padus) and a c t u a l l y s u r v i v e d b e s t on o a t s , while the gynopae p r e f e r r e d b i r d c h e r r y and o n l y s u c c e s s f u l l y produced o f f s p r i n g on the primary host, P. padus (Dixon, 1971). Dean (1973) compared the bionomics o f the three c e r e a l aphids on b a r l e y , o a t s , and wheat. Metopolophium  dirhodum and M. avenae were most abundant on b a r l e y , l e s s on oats and l e a s t on wheat. Rhopalosiphum p a d i i n c r e a s e d most r a p i d l y on b a r l e y and l e a s t on o a t s . Metopolophium  dirhodum and M. avenae had s h o r t e r g e n e r a t i o n times and h i g h e r s u r v i v a l r a t e s on b a r l e y w h i l e R. pa d i d i d b e s t on wheat. When gi v e n a f r e e c h o i c e i n l a r g e outdoor cages R. p a d i were found on b a r l e y . When M. avenae were r e a r e d (Apablaza and Robinson (1967)) on wheat, b a r l e y , and oats i n the l a b o r a t o r y , the b a r l e y and wheat p l a n t s d i e d b e f o r e they produced seed. C o n c u r r e n t l y M. avenae showed no p r e f e r e n c e of b a r l e y over wheat or wheat over o a t s . P l a n t age was observed by Abernathy and Thurston (1969) as an i n t e g r a l p a r t of i n s e c t / p l a n t r e l a t i o n s h i p . 10. In a study of the r e s i s t a n c e of tobacco to Myzus p e r s i c a e , the green peach aphid, a r e s i s t a n t exudate N i c o t i a n a was o n l y p r e s e n t i n o l d e r p l a n t s . T h i s demonstrates the importance of always u s i n g p l a n t s of the same age to study a p h i d / p l a n t r e l a t i o n s h i p s , because the c o n s t i t u e n t s of the p l a n t are c o n t i n u a l l y changing as the p l a n t matures. I f animals are s i m u l t a n e o u s l y s u b j e c t e d to p l a n t s of v a r y i n g ages i n one experiment, b i o l o g i c a l d i f f e r e n c e s c o u l d appear t h a t were u n r e l a t e d to the o r i g i n a l i n t e r e s t of the t e s t . 5. Temperature R e l a t i o n s h i p s Dean (1974) s t u d i e d the e f f e c t of temperature on c e r e a l aphids. He found development to^be f a s t e s t a t 25°C f o r R. p a d i , 20°C f o r M. dirhodum and 22.5°C f o r M. avenae, although the g r e a t e s t number of nymphs were born a t 2 0 ° C Above 15°C, R. p a d i ' s developmental r a t e a c c e l e r a t e d f a s t e r than the o t h e r s . The maximum temperature l i m i t , a t which a l l nymphs d i e d b e f o r e completing t h e i r development, was 2 7.5°C f o r M. dirhodum and 30°C f o r R. p a d i and M. avenae. In g e n e r a l , the p r e - r e p r o d u c t i v e a d u l t stage was l o n g e s t i n M. avenae and s h o r t e s t f o r R. p a d i . Dean (1974) r e a r e d c e r e a l aphids on w i n t e r wheat s h e l t e r e d from wind, r a i n and snow. M. avenae s u r v i v e d b e s t , M. dirhodum l e s s w e l l and R. p a d i l e a s t . When exposed, none s u r v i v e d . He found the l o n g e v i t y of M. avenae v a r i e d 11. i n v e r s e l y w i t h temperature, and f e c u n d i t y v a r i e d d i r e c t l y , w i t h temperature. In A l b e r t a , Harper (1968) found R. p a d i a c t i v e l y i n f e s t i n g b a r l e y g r a i n s t o r e d outdoors on p o l y e t h e l e n e sheets a t -17°C. They d i e d from unknown causes i n l a t e March. 6. Predator and P a r a s i t e R e l a t i o n s h i p s The study of n a t u r a l enemies i s g a i n i n g r e c o g n i t i o n as a t o o l i n understanding p o p u l a t i o n dynamics. Dean and W i l d i n g (1971) found the pathogenic f u n g i , Entomophthora  a p h i d i s , E. planchoniana and E. t h a x t e r i a n a i n f e c t i n g M. dirhodum and M. avenae i n B r i t a i n . High humidity and r a i n f a l l were thought to weaken M. dirhodum making i t more v u l n e r a b l e to a t t a c k . During t h a t season fungus i n f e c t i o n proved to be more important than p r e d a t o r s or p a r a s i t e s i n r e d u c i n g aphid numbers. Although the a t t a c k was too l a t e to prevent i n f e c t i o n o f the p l a n t s by v i r u s , i t d i d prevent aphid numbers from i n c r e a s i n g s u f f i c i e n t l y to reduce the g r a i n y i e l d . F u r t h e r work (1973) showed t h a t three s p e c i e s of Entomopthora k i l l e d 53% of the M. dirhodum, 60% of the M. f e s t u c a e and 30% of the M. avenae on wheat. These f u n g i i n f e c t e d twice as many a d u l t a l a t a e as o l d nymphs and apterous a d u l t s , and seemed to i n c r e a s e a f t e r heavy r a i n s . Three imported p a r a s i t e s of the s p o t t e d a l f a l f a aphid, T h e r i o a p h i s maculata (Buckton) obtained from Europe and the 12. Middle E a s t , (Van,, den Bosch e t a l . , 1959) were s u c c e s s f u l l y mass produced and r e l e a s e d i n many areas throughout C a l i f o r n i a . D e s p i t e interfe'Eence ' from i n s e c t i c i d e s , c o c c i n e l l i d s and aphid d i s e a s e s , the p a r a s i t e s e s t a b l i s h e d themselves and are h e l p i n g to c o n t r o l t h i s aphid. A d e t a i l e d account of the impact of n a t u r a l enemies has been gi v e n by Hagen and van den Bosch (196 8). They d i s c u s s the use of pathogens, p a r a s i t e s , and p r e d a t o r s as agents of n a t u r a l c o n t r o l s and c i t e many examples from the l i t e r a t u r e . They s t r e s s the importance of c o l l e c t i n g data to f o l l o w up i n s e c t r e l e a s e s and conclude t h a t of the mass of l i t e r a t u r e o n l y a h a l f a dozen papers had s i g n i f i c a n t i n f o r m a t i o n . Methods of r e a r i n g n a t u r a l enemies of aphids f o r s t u d y i n g t h e i r p o t e n t i a l impact are d i s c u s s e d by F r a z e r e t a l . (1974). Campbell and Mackauer (1973) found t h a t Aphidius  e r v i e r v i (Haliday) and A. s m i t h i (Sharma and Subba Rao) two i n t r o d u c e d p a r a s i t e s of the pea aphid, A. c y r t h o s i p h o n pisum were a f f e c t e d by c l i m a t e . A. s m i t h i i s dominant i n hot and dry c o n d i t i o n s , whereas A. e r v i i s dominant i n wet and m i l d c o n d i t i o n s . Dean (1974) s t u d i e d the e f f e c t s of p a r a s i t e s and p r e d a t o r s on M. dirhodum and M. avenae. C o c c i n e l l i d s and s y r p h i d s were the most numerous p r e d a t o r s . P a r a s i t e s g e n e r a l l y gave b e t t e r c o n t r o l than p r e d a t o r s . P r e d a t o r s , p a r t i c u l a r l y s y r p h i d s , become more important i n J u l y . 13. Small populations of the black bean aphid, Aphis  fabae (Scop.), are at greater r i s k from predators than large ones, (Way and Banks (1968)). The smaller populations of aphids remain crowded on a few colonized shoots which allows a few natural enemies to destroy them before many alatae are produced. Larger aphid populations disperse more widely on the plant. 7. Aim of t h i s Study The most commonly found species of aphids on oats i n B r i t i s h Columbia are: M. avenae (F.), M. dirhodum (Walker) and R. padi (L.) (Forbes (1962)). Seasonal differences i n populations have been noted, and factors such as weather, vegetation, predators, parasites and disease are known to a f f e c t the aphid numbers, but not necessarily to control them. These seasonal differences i n f i e l d aphid populations and some of the factors known to a f f e c t them were examined. Some hypotheses a r i s i n g from the f i e l d studies were tested by laboratory experiments. SECTION I POPULATION DYNAMICS - 1972 1. I n t r o d u c t i o n A p l o t of o a t s , Avena s a t i v a cv F r a s e r , p l a n t e d on May 12, 1972 on the campus of the U n i v e r s i t y of B r i t i s h Columbia, was used to study the p o p u l a t i o n dynamics of c e r e a l aphids. The U n i v e r s i t y i s l o c a t e d i n southwestern B r i t i s h Columbia a t 49°11* l a t i t u d e and 123°10' l o n g i t u d e . F r a s e r o a t s , developed f o r t h i s area was chosen because of i t s a b i l i t y to r e s i s t l o d g i n g . The p l o t c o n s i s t e d of 34 rows, one metre a p a r t and 26.5 metres l o n g . The oats were sampled f o r aphids a t i r r e g u l a r i n t e r v a l s from June 10 u n t i l August 23. The i n t e r v a l depended on the weather, but u s u a l l y was approximately one week. Sampling c o u l d not be done d u r i n g or immediately a f t e r a heavy r a i n , as the r a i n washed the aphids o f f the p l a n t s and they r e q u i r e d one to two days to r e - e s t a b l i s h themselves. 2. M a t e r i a l s and Methods Sampling c o n s i s t e d of choosing t h r e e , 7.5 cm long sampling s i t e s per row by u s i n g a t a b l e of random numbers. Samples from three adjacent rows were combined to make one sample u n i t of nine 7.5 cm s i t e s . The outer two rows on each s i d e of the p l o t and the 2 m a t the end of each row were 15. excluded from sampling to reduce border i n f l u e n c e s . Those found to be i n f e s t e d were cu t a t s o i l l e v e l , p l a c e d i n t o cardboard c a r t o n s i n i t a l l y , and i n t o p l a s t i c bags l a t e r i n the season when the p l a n t s were l a r g e r . In the l a b o r a t o r y , a s m a l l p a i n t b r u s h (#000 sable) was used to t r a n s f e r the aphids from the t i l l e r s to a v i a l o f 80% ethanol/5% l a c t i c a c i d s o l u t i o n f o r p r e s e r v a t i o n . Each week the h e i g h t of a randomly chosen t i l l e r from each sampling s i t e was measured and the number of leaves counted. L a t e r , the area, l e n g t h of l e a v e s , and h e i g h t o f 20 t i l l e r s were measured and the r e l a t i o n s h i p s between these v a r i a b l e s was used to estimate the l e a f area i n a l l o t h e r samples. The 20 t i l l e r s were c a r e f u l l y s e l e c t e d to r e p r e s e n t a l l s i z e s of t i l l e r s p r e s e n t i n the f i e l d a t sampling time. Fewer t i l l e r s were examined when the numbers of aphids i n c r e a s e d (Table 1). Sampling continued u n t i l August 23 when the p o p u l a t i o n of the aphids was d e c r e a s i n g s h a r p l y . The aphids i n each sample were separated i n t o nymphs and a d u l t s under a microscope. The aphids were f u r t h e r d i v i d e d i n t o the three most commonly found c e r e a l aphid s p e c i e s : Macrosiphum. (sitobium) avenae ( F . ) , Metopolophium  dirhodum (Walker), and Rhopalosiphum p a d i ( L . ) . Counts were made of aphidophagous c o c c i n e l l i d a e , ( l a d y b i r d b e e t l e s ) by r e c o r d i n g the t o t a l number of l a r v a e and a d u l t s observed w h i l e walking up and down the rows a t a standard r a t e . 16. Table 1. Number of oat t i l l e r sampled f o r aphids on each sample date i n 1972 Date No. of T i l l e r s June 15 3222 June 16 957 June 21 796 June 29 907 J u l y 4 1088 J u l y 7 1069 J u l y 14 1019 J u l y 18 210 J u l y 25 210 Aug. 1 60 Aug. 6 60 Aug. 14 30 Aug. 18 30 Aug. 23 20 3. R e s u l t s and D i s c u s s i o n Aphid numbers i n c r e a s e d throughout the season from 0.07 a p h i d s / t i l l e r on June 16 to a maximum of 8 8 . 8 / t i l l e r on August 14, a f t e r which time they began to d e c l i n e ( F i g . 1). There was a l s o a s l i g h t decrease i n numbers between June 29 and J u l y 4. T h i s decrease may have r e s u l t e d from the abrupt change i n weather a t the time. June had been o v e r c a s t almost every day wit h f r e q u e n t p r e c i p i t a t i o n u n t i l June 30 when the sky c l e a r e d and remained c l e a r u n t i l J u l y 6. The average temperature of 71°F + 2.83 o 18. was 9.5 F higher during these s i x c l e a r days than the 61.5°F + 1.59 reached the s i x previous days. When the population s i z e was expressed as aphids/cm of t i l l e r h eight, a s i m i l a r p a t t e r n was followed w i t h .004 aphids/cm on June 16 to 1.22/cm on August 14. The r e l a t i v e abundance of the three species remained constant throughout the season ( F i g . 2). Macrosiphum avenae accounted f o r 63.5% of the ad u l t s found, ranging from .005 per t i l l e r on June 16 to 3.67 per t i l l e r on August 14. Metopolophium dirhodum accounted f o r 33.5% of the a d u l t s per sample, ranging from . 0 0 3 / t i l l e r on June 16 to 1 . 6 3 / t i l l e r on August 14. Although R. padi a r r i v e d e a r l i e s t i n the f i e l d w i t h 0 . 0 1 / t i l l e r on June 15, i t reached a peak of only • 2 3 / t i l l e r on August 14 and accounted f o r only 2.94% of the t o t a l aphid population. (The " t o t a l " aphid population i n c l u d e s the population of a l l three s p e c i e s ) . The numbers of c o c c i n e l l i d l a r v a e observed i n the p l o t on a per hectare b a s i s ranged from 669 on J u l y 25 to 4,909 on August 1 (F i g . 3). Thereafter the numbers decreased. The d e n s i t y of adul t s was 711.5/ha on the f i r s t sampling date of June 21. The a d u l t numbers then decreased to 85.4/ha on J u l y 18, a f t e r which they rose to 45,849/ha by August 18. Only one a d d i t i o n a l F i g . 2 . The number of a d u l t s / t i l l e r of t h r e e s p e c i e s of a p h i d s ; Macros iphum avenae, Metopolophiutn dirhodum and Rhopalosiphum p a d i ; on a p l o t of o a t s , (Avena s a t I v a cv F r a s e r ) d u r i n g 1972 5.0, June J u l y August D a t e Number of C o c c i n e l l i d l a r v a e and a d u l t s per h e c t a r e o o — i — i — O o o o o o Ln O O O O 3 Co • U> •d • M o H r t 3* (II o M l 3 C o a OJ r t CO H <« 0 y—\ M l > <: o a o a n D> o H -3 CQ rt Bl M r t i— 1 H* < a. Co Cu O o. < c M r t H CO Co co to n> 3 a —^' Co 3 H < I- 1 CO VO n> ro M , o d • 3 a •3 n> M 3* m o r t B> H. ft) 4 O i l c c I- 1 O i l M l O i l c CO r t f > Co O -i-t C Co r t ft Co i * sample was taken on August 23, which i n d i c a t e d a decrease to 28,004.6 a d u l t s / h a . The decrease i n a d u l t c o c c i n e l l i d d e n s i t y from J u l y 5 to J u l y 18 may be e x p l a i n e d by the continuous r a i n f a l l from J u l y 7 to J u l y 12 w i t h p a r t i c u l a r l y heavy r a i n on J u l y 11 and 12 ( F i g . 4 ) . 2 When the number of c o c c i n e l l i d / c m o f l e a f i s 2 compared wi t h the t o t a l aphids/cm of l e a f , i t i s e v i d e n t t h a t the r a p i d i n c r e a s e i n aphid d e n s i t y from J u l y 7 to August 3 c o i n c i d e s with a decrease i n b e e t l e d e n s i t y ( F i g . 5 ) . P o s s i b l y the r a i n had a more d e t r i m e n t a l a f f e c t on the b e e t l e s than on the aphids, and the r e s u l t i n g d e c l i n e i n b e e t l e numbers then allowed the aphids to i n c r e a s e . The d e c l i n e i n l a r v a l c o c c i n e l l i d d e n s i t y c o i n c i d e d w i t h the i n c r e a s e i n the number of a d u l t s which i m p l i e s t h a t new eggs were not h a t c h i n g ( F i g . 3 ) . F i g u r e 6 shows through the d i v i s i o n o f each weekly u n i t sample i n t o i t s ten separate samples t h a t the aphid p o p u l a t i o n was never d i s t r i b u t e d evenly throughout the f i e l d . Each weekly u n i t sample i s a combined t o t a l of samples from 3 adj a c e n t rows. During most of the sampling season, the E a s t s i d e of the p l o t had more aphids. The p r e v a i l i n g w e s t e r l y wind may have encouraged a movement i n t h i s d i r e c t i o n but i t seems more l i k e l y , when compared wi t h F i g . 7, t h a t as the p l a n t s were t a l l e r , l u s h e r , and h e a l t h i e r i n t h i s area, they were able to 22. Fig.4. The p r e c i p i t a t i o n at U.B. C. weather station for May, June, July and August i n 1972 2.401 2. 201 2.00 1.80 1.60 /—\ CO cu o 1.40 C •rl v ' o 1.20 •rl 4-1 cd •H 1.00 p. •H O cu u 80 60 40 20 10 20 May 10 20 June D ate AA_ 10 20 July 10 20 August 23. F i g . 5 . The t o t a l number of a p h i d s and t h e number of c o c c i n e l l i d l a r v a e and a d u l t s per cm^ of l e a f found i n a p l o t of o a t s , (Avena-- s a t i v a cv F r a s e r i n 1972 June J u l y August D a t e F i g . 6 . T o t a l a p h i d s / t i l l e r / i n d i v i d u a l s a m p l e o f p l o t , ( A v e n a s a t i v a c v F r a s e r ) s a m p l e d i n 1 9 7 2 a n o a t lDi-O.i 3 iv 0") 1 - 0 1 i m p l e n o . D a t e 1 2 3 4 5 6 7 8 9 1 0 J u n e 16 / / / 7 / / / / / / • / / / / / / y / / s • s s / s / y s s / / 8 9 1 J u n e 2 1 1 2 3 4 5 6 7 8 9 1 0 J u n e 29 24.b F i g . 6 . T o t a l a p h i d s / t i l l e r / i n d i v i d u a l sample of an oat p l o t , (Avena > s a t i v a cv F r a s e r ) sampled i n 1972 10.0 •3. i-o J3 < ,01 -/ / / / • / / / / / / / / / / / / / / / / 1 / / / / / / / / / / ' / / / / / / / / / / / / / / / / / ' / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / <> / 7 / / / / / / / / / / / / / / / / - r / / / / / / / / / / / / / / / / / / / / / kl Sample no.1 2 3 4 5 6 7 8 D a t e J u l y 4 -/ / / / / i> / / ; / / / <• / / / / / / / / / / / / / y / 9 10 1 2 3 4 5 6 7 J u l y 7 8 9 10 1 2 3 4 5 6 7 J u l y 14 8 9 10 24. c 24.d Fig.6. Total a p h i d s / t i l l e r / i n d i v i d u a l sample of an oat plot. (Avena , sativa cv Fraser) sampled in 1972 Xi Cu 100 10 J u cu 01 2 1.0 . 1 .01 Sample no Date / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / s / / / k i kf k i k i ki PI. ki ki ki k i k i ki k i ki ki k ki ki ki ki ki ki ki ki ki ki ki ki ki ki k i ki ki ki k i kl ki k ki ki ki ki 1 2 3 4 5 6 7 8 9 10 August 18 1 2 3 4 5 6 7 8 9 10 August 23 25 a F i g . 7 . Average t i l l e r h e i g h t / i n d i v i d u a l sample i n 1972 of an o a t p l o t , (Avena tj s a t i v a cv F r a s e r ) sampled i n 1972 80 60 40 20 Sampl D e no. a t e 1 2 3 4 5 6 7 June 16 8 9 10 1 I s B ; ; ; / / / y / / 1 V > / / / 2 3 4 5 6 7 8 9 June 21 10 1 2 3 4 5 6 7 8 9 10 1 2 June 29 3 4 5 6 7 J u l y 4 8 9 10 90-70" 50 -30 10-Sample no. 0 a t e / / / / y 1 2 3 4 5 6 7 J u l y 7 8 9 10 1 2 3 / / / / / ' y / / / / / / / / / / / / / / / / / / y / y / y / / / y y / y y y y y y y y y y s y 4 5 6 J u l y / / / /' / / / / / / / / / y y y y y y y y y y y y y y 2 / y y y y V y kl ki 14 4 5 6 August 7 8 1 y y y y y y y y y y y y y y y — y y y y y y y y y y y y y y 1-1 y y y y y y y y y y y y y y y y y y s y y / y y y y y y y y y s y y y y y s y y y y y s y y y y y y y y y y / y y s s / y y y y y y y y y y y y y y y y y ,- y •* y y y y y y y y y y y y y y y y y y y * s y y y y y y y y , y / y y y y y y y y y y y y y y y y y y y y y y y y y y y y y y y 4 5 6 7 8 9 U August 6 25.b p i g . 7 . Average t i l l e r height/individual sample in 1972 of an oat p l o t , (Avena,; sativa cv Fraser) sampled in 1972, 90 7d 5d 3d 10 Sample no. 1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 8 9 10 Date August 14 August 18 August 23 support a h i g h e r p o p u l a t i o n . Observations a t the time noted t h a t the westernmost rows matured e a r l i e r , becoming dry, yel l o w , and s e e m i n g l y . u n a t t r a c t i v e to the aphid. The r e s t o f the p l o t was s l i g h t l y lower i n topography, and the p l a n t s grew t a l l e r , remained green and l u s h longer, and probably c r e a t e d a h i g h e r humidity more s u i t a b l e f o r the aphids. The p l a n t h e i g h t s had evened out ac r o s s the p l o t by August 23. The e f f e c t of p l a n t growth on the aphid p o p u l a t i o n i s shown by comparing the t o t a l a p h i d s / t i l l e r w ith the p l a n t h e i g h t ( F i g . 1). The aphid p o p u l a t i o n i n c r e a s e d r a p i d l y u n t i l August 1, and t h e r e a f t e r a t a reduced r a t e t o reach i t s peak on August 14. On August 1 the p l a n t s were growing more s l o w l y than b e f o r e . 4. C o n c l u s i o n s Through the season p o p u l a t i o n s of the three s p e c i e s of aphids i n c r e a s e d s t e a d i l y and peaked on l y once. Rhopalosiphum p a d i was the f i r s t s p e c i e s recorded i n samples i n the s p r i n g , but remained a t low numbers and became even lower on J u l y 4 and J u l y 7. Macrosiphum  avenae was always the most abundant, with M. dirhodum onl y s l i g h t l y l e s s abundant. The r a p i d i n c r e a s e i n aphid p o p u l a t i o n d u r i n g the season may have been a i d e d by the e f f e c t s o f heavy r a i n on the b e e t l e s i n J u l y , as i t appears t h a t the b e e t l e p o p u l a t i o n t h e r e a f t e r never a t t a i n e d h i g h enough numbers to g r e a t l y a f f e c t the aphids. An i n c r e a s e i n temperature and a l a c k of p r e c i p i t a t i o n may e x p l a i n the sudden decrease i n aphid numbers between June 29 and J u l y 4. C o o l e r c o n d i t i o n s accompanied by some p r e c i p i t a t i o n which f o l l o w e d the d e c l i n e prompted a continuous i n c r e a s e i n aphid numbers. The h i g h e r aphid numbers on the e a s t e r n s i d e of the p l o t may have been due to the b e t t e r c o n d i t i o n s a f f o r d e d by the t a l l e r p l a n t s . The numbers of aphids l e v e l l e d out l a t e r , when the p l a n t s were growing more s l o w l y . N u t r i e n t s may have been l e s s a v a i l a b l e to the aphids a t t h i s time as the p l a n t s were r e a c h i n g m a t u r i t y and becoming d r i e r . T h i s continued maturation of the p l a n t s probably caused the e v e n t u a l d e c l i n e i n aphid numbers. These r e s u l t s d i f f e r from those obtained a t U.B.C. on V i c t o r y oats from 1957 to 1960 (Forbes/ • (1962)). Macrosiphum  avenae reached two d e f i n i t e peaks i n s t e a d of one. Metopolophium dirhodum had one peak o n l y but had a p p r e c i a b l y lower p o p u l a t i o n s i n two years than d i d M. avenae. Rhopalosiphum p a d i o c c u r r e d i n a p p r e c i a b l e numbers i n o n l y two out of the f o u r y e a r s . In a l l cases, the h i g h e s t d e n s i t i e s were recorded i n J u l y , one month e a r l i e r than the p r e s e n t o b s e r v a t i o n s . R e s u l t s from England showed R. p a d i to be the most 28. numerous and with two periods of peak abundance, while M. avenae reached i t s highest density only once, i n August. Again the populations of both M. avenae and M. dirhodum were declining by the end of July (Dean. (1973)). Populations of M. avenae on barley i n Oregon also declined by the end of July (Green (1966)), even though four d i f f e r e n t planting dates ranging from A p r i l 2 to May 10 were used i n t h i s study. Further work might show whether some of these differences r e s u l t from d i f f e r e n t planting' dates on d i f f e r e n t v a r i e t i e s of oats. SECTION TI FECUNDITY 1. I n t r o d u c t i o n The p o p u l a t i o n dynamics i n 1972 showed constant d i f f e r e n c e s i n f i e l d d e n s i t i e s between the three s p e c i e s of oat aphids. F a c t o r s p o s s i b l y r e s p o n s i b l e f o r the d i f f e r e n c e s which were examined a t t h a t time were p l a n t growth, c o c c i n e l l i d numbers and weather c o n d i t i o n s , but they o f f e r e d few e x p l a n a t i o n s f o r the o b s e r v a t i o n s . The f o l l o w i n g experiments on the f e c u n d i t y of the aphids t h e r e f o r e , were conducted to determine whether the r e l a t i v e abundance of the three s p e c i e s c o u l d be e x p l a i n e d by d i f f e r e n t i a l f e c u n d i t y . F e c u n d i t y t r i a l s were a l s o used to e v a l u a t e the r o l e of competition, by o b t a i n i n g i n f o r m a t i o n about the r e p r o d u c t i v e p o t e n t i a l of each s p e c i e s r e a r e d s e p a r a t e l y and i n c o n j u n c t i o n w i t h one another. 2. M a t e r i a l s and Methods A code d e v i s e d d u r i n g the experiments w i l l be used throughout the d i s c u s s i o n s to r e p r e s e n t the e i g h t treatments f o r s t u d y i n g the f e c u n d i t y of the three aphid s p e c i e s (Table 2 ) . 30. Table 2. The i d e n t i f y i n g code of treatments used to study the f e c u n d i t y o f the three aphid s p e c i e s Treatment No. Number of A d u l t A p h i d s / P l a n t Species Code 1 1 M. avenae a-1 2 2 M. avenae a-2 3 3 M. avenae a-3 4 1 M. dirhodum d-1 5 2 M. dirhodum d-2 6 3 M. dirhodum d-3 7 1 R. P a d i p-1 8 1 M. dirhodum pd-1 1 R. padi One F r a s e r oat seed was p l a n t e d i n each o f two hundred 5 cm (2") diameter p l a s t i c p o t s . The s o i l used was a c q u i r e d l o c a l l y and had n u t r i e n t s added to i t . Seeds were allowed to germinate and grow to a h e i g h t of approximately 7 cm i n a greenhouse. Twenty p l a n t s were s e l e c t e d f o r each treatment and p l a c e d together i n a p l a s t i c tray containing water where they were kept c o n s t a n t l y damp. One a d u l t of each s p e c i e s was then removed from the r e a r i n g stock and allowed to produce young. The stock c o l o n i e s had been s t a r t e d from f i e l d c o l l e c t e d aphids no more than two months e a r l i e r . A 5 cm (2") p l a s t i c tube covered with saran s c r e e n i n g (40 x 60 mesh/inch) was p l a c e d over each p l a n t to cage the aphids. When an a d u l t produced s e v e r a l nymphs i t was removed. Depending on the treatment, one, two or three numphs were l e f t on the p l a n t , and when they reached m a t u r i t y , t h e i r d a i l y f e c u n d i t y was recor d e d as f o l l o w s . Each day every p l a n t was c a r e f u l l y examined f o r f i r s t - i n s t a r nymphs which were removed and counted. T h i s procedure was repeated u n t i l the a d u l t d i e d . For treatment #8, the f e c u n d i t i e s o f the R. padi a d u l t and M. dirhodum a d u l t on the same p l a n t were recorded s e p a r a t e l y . P l a n t s were trimmed r e g u l a r l y to a h e i g h t of 20 cm. Weak p l a n t s were r e p l a c e d , but i n most cases on l y one p l a n t was needed f o r the e n t i r e l i f e of the aphid. A F o r t r a n program w r i t t e n by F r a z e r (19 7 2) was used to c a l c u l a t e Ro, the net r e p r o d u c t i o n r a t e ( i n ? / ? /generation) T, the g e n e r a t i o n time, ( i n days); r , the f i n i t e r a t e of i n c r e a s e , ( i n / ? /day); rm, the i n t r i n s i c r a t e o f i n c r e a s e , ( i n •? / -J- /day); and DT, the dou b l i n g time, ( i n d a y s ) . A l l treatments were analyzed a t the 5% l e v e l of s i g n i f i c a n c e . 3. R e s u l t s and D i s c u s s i o n A. Treatment E f f e c t s W i t h i n Species i ) M. avenae a) M. avenae (a-1) reached m a t u r i t y between 10 and 11 days from b i r t h (Table 3). The maximum mean d a i l y f e c u n d i t y of 1.6 3 ^ / /day o c c u r r e d on the 13th day wit h the f i r s t a d u l t dying one day- l a t e r ( F i g s . 8 and 9 ) . E i g h t y per cent of a l l progeny F i g . 9. D a i l y age - s p e c i f i c f e c u n d i t y f o r Macrosiphum avenae (The E n g l i s h g r a i n a p h i d ) a-1 a-2< a-3 34. were produced by the 38th day, when only 16% of the adults were s t i l l a l i v e . Reproduction ceased by the 47th day with the l a s t adult dying on the 4 8th. b) M. avenae (a-2) did not reach maturity u n t i l the 13th and 14th days, which was l a t e r than (a-1) (Table 3, F i g . 8). The lower maximum mean d a i l y fecundity of 1.34?-/? /day was not produced u n t i l the 21st day, when 37% of the adults had died (Fig. 9) . A higher mean progeny production of 2.5 %/ •? /day occurred on the 43rd, 44th and 47th days, but was not as representative a mean, as only two out of the 19 females were s t i l l a l i v e at thi s time (Fig. 9). Eighty per cent of th e i r progeny were not produced u n t i l the 45th day, when only 10% of the adults were s t i l l a l i v e . (A-2) reproduced longer than (a-1) and did not cease reproduction u n t i l the 55th day, when the l a s t adult died, (Fig. 8). Table 3. P r e - r e p r o d u c t i v e p e r i o d , r e p r o d u c t i v e p e r i o d and p o s t - r e p r o d u c t i v e p e r i o d f o r v a r i o u s treatments of the oat aphids Treatment P r e - r e p r o d u c t i v e Reproductive P o s t - r e p r o d u c t i v e code p e r i o d (days) p e r i o d (days) p e r i o d (days) a-1 10.6813.42 19 .58±8.16 .1581.335 a-2 13.21+2.79 17 .21±6.62 1.5811.94 a-3 6.4411.53 17 .515.06 2.6312.15 d-1 9.06±1.43 16 .4±6.05 2.8912.66 d-2 9.72±.95 22 .33±4.0 2.9412.08 d-3 9.0±.82 26 .1±4.98 3 .2812.36 d wi t h P 9.212.13 17 .6714.53 1.731.929 p-1 5.95±2.33 19 .015.58 3.013.09 p w i t h d 8.93±2.17 17 .7318.07 2.41.888 Mean + standard d e v i a t i o n c) M. avenae (a-3) had the s h o r t e s t pre-r e p r o d u c t i v e p e r i o d of the three M. avenae treatments. They reached m a t u r i t y on the average of 6 to 7 days from b i r t h , which was f o u r days l e s s than (a-1) and 7 days l e s s than (a-2) (Table 3). The f i r s t a d u l t d i e d on the 5th day, e a r l i e r than (a-1) and (a-2) ( F i g . 8). On the 13th day, as with (a-1), the maximum mean d a i l y progeny p r o d u c t i o n o f 1.52-?/? /day was reached when 69% of the a d u l t s were s t i l l a l i v e ( F i g s . 8 and 9) . Again a h i g h e r mean of 4 ?/ -? /day oc c u r r e d on the 43rd day, but cannot be c o n s i d e r e d r e p r e s e n t a -t i v e as o n l y one female was a l i v e a t t h i s time. E i g h t y p e r c e n t of the progeny was produced by the 40th day, when only 6% of the a d u l t s were s t i l l a l i v e . Reproduction ceased by the 45th day and the l a s t a d u l t d i e d on the 4 8th ( F i g . 8). d) Comparison between M. avenae treatments An a n a l y s i s of v a r i a n c e and a c h i square t e s t showed t h a t (a-3) had a s i g n i f i c a n t l y lower p r e - r e p r o d u c t i v e p e r i o d than (a-1) and (a-2) and a s i g n i f i c a n t l y longer p o s t - r e p r o d u c t i v e p e r i o d . I t produced the h i g h e s t i n t r i n s i c r a t e o f i n c r e a s e , (Table 4) probably because of the lower p r e - r e p r o d u c t i v e p e r i o d (Table 3). The g e n e r a t i o n time and d o u b l i n g time are a l s o lowest i n t h i s treatment. The net r e p r o d u c t i v e r a t e of 13.3 8?/ ^ g e n e r a t i o n i s lower than both (a-1) and (a-2) w i t h 25.49 and 1 7 . 0 3 ? / ^ g e n e r a t i o n r e s p e c t i v e l y . These data s t r o n g l y suggest t h a t c o m p e t i t i o n does e x i s t even a t a low d e n s i t y of 3 / t i l l e r . Table 4. The i n t r i n s i c r a t e o f i n c r e a s e i n •+/-f/day, the g e n e r a t i o n time i n days, the dou b l i n g time i n days, and the net r e p r o d u c t i v e r a t e of e i g h t treatments imposed upon 3 s p e c i e s of oat aphids Treatment I n t r i n s i c r a t e G eneration time (days) Doubling time (days) Reproductive r a t e a-1 0.22 14.43 3.09 25.49 a-2 0.18 16.16 3.95 17 .03 a-3 0.26 10.11 2.70 13.38 d-1 0.32 12.27 2.18 49.44 d-2 0.27 14.20 2.61 43.58 d-3 0.30 12.86 2.31 47.11 pd-l(d) 0.32 12.36 2.20 49.20 p-1 0.50 7.88 1.39 51.14 pd-l(p) 0.33 12.10 2.13 51.51 i i ) M. dirhodum a) M. dirhodum (d-1) reached m a t u r i t y between 9 and 10 days from b i r t h (Table 3). A l l the a d u l t s were s t i l l a l i v e when the g r e a t e s t mean progeny p r o d u c t i o n of 5.72 /day was reached on the 11th day ( F i g . 1 0 ) . The f i r s t a d u l t d i d n ' t d i e u n t i l the 13th day, and over 83% of them were s t i l l a l i v e on the 21st day, when 80% of the progeny had been produced ( F i g s . 10 and 11). They ceased r e p r o d u c t i o n on the 32nd day, but the l a s t a d u l t d i d n ' t d i e f o r . a n o t h e r s i x days. b) M. dirhodum (d-2) a l s o reached m a t u r i t y F i g . 10. D a i l y age - s p e c i f i c f e c u n d i t y f o r M e t o p o l o p h u i m dirhodum. (The Rose - g r a i n a p h i d ) 6 5 A 3 2 1 Age - s p e c i f i c s u r v i v a l (Lx) 40. between the 9th and 10th days from b i r t h (Table 3). E i g h t y - n i n e p e r c e n t of the a d u l t s were s t i l l a l i v e on the 16th day when the mean d a i l y progeny p r o d u c t i o n of 3.33 /day was reached. D-2,took lon g e r than (d-1) to a t t a i n a lower mean progeny p r o d u c t i o n . The lower f e c u n d i t y o f t h i s treatment might have been due to an a d u l t dying i n i t s second day. E i g h t y p e r c e n t o f the progeny p r o d u c t i o n o c c u r r e d w i t h i n the f i r s t 26 days when 67% of the a d u l t s were s t i l l a l i v e . A l l a d u l t s had ceased r e p r o d u c t i o n by the 40th day, and a l l had d i e d by the 43rd day ( F i g . 11). c) M. dirhodum (d-3) a l s o reached m a t u r i t y on average by the 9th day (Table 3 ) . The g r e a t e s t mean d a i l y progeny p r o d u c t i o n o f 3.54 $ / ? / d a y was a t t a i n e d on the 14th day ( F i g . 10) when o n l y two a d u l t s (10.5%) had d i e d ( F i g . 11). I t took twice as long, 53 days, compared wi t h (d-2) to produce 80% of the nymphs. E i g h t y - e i g h t p e r c e n t of the progeny had d i e d by t h i s time. The l a s t a d u l t l i v e d f o r 53 days and had stopped r e p r o d u c t i o n a day e a r l i e r ( F i g . 11) . d) M. dirhodum wi t h R. p a d i ( p d - l ( d ) ) . The f e c u n d i t y o f one M. dirhodum a d u l t when r e a r e d with one R. pa d i a d u l t was the same as the o t h e r three M. dirhodum treatments. Average m a t u r i t y was again reached between 9 and 10 days from b i r t h (Table 3 ) . The g r e a t e s t mean progeny p r o d u c t i o n o f 4. /day o c c u r r e d on the 12th day when a l l M. dirhodum a d u l t s were s t i l l a l i v e ( F i g . 11). T h i s maximum mean d a i l y progeny p r o d u c t i o n was hi g h e r than (d-2) and (d-3) but lower than (d-1). E i g h t y p e r c e n t o f the progeny were produced by the 24th day, when 87% of the a d u l t s were s t i l l a l i v e . Reproduction ceased on the 33rd day and the l a s t a d u l t d i e d three days l a t e r . e) Comparison between M. dirhodum treatments An a n a l y s i s o f v a r i a n c e and a c h i square t e s t showed o n l y one s i g n i f i c a n t d i f f e r e n c e between M. dirhodum treatments. The r e p r o d u c t i v e p e r i o d s of (d-1), w i t h 16.4 days, and (dp-l(d)) w i t h 17.67 days, were s i g n i f i c a n t l y l e s s than (d-2) and (d-3) (Table 3). D-2 had a lower i n t r i n s i c r a t e of i n c r e a s e than the o t h e r s , as w e l l as a lower net r e p r o d u c t i v e r a t e (Table 4 ) . I t took longer to produce a new g e n e r a t i o n and longer to double i t s p o p u l a t i o n , but the observed d i f f e r e n c e s were not g r e a t enough to be s t a t i s t i c a l l y s i g n i f i c a n t . T h e r e f o r e crowding M. dirhodum on a p l a n t d i d not a l t e r i t s f e c u n d i t y , but d i d i n c r e a s e the r e p r o d u c t i v e p e r i o d which i n t u r n decreases the i n t r i n s i c r a t e of i n c r e a s e . 42. i i i ) R. pa d i a) R. pa d i (p-1) reached m a t u r i t y between 5 to 6 days e a r l i e r than any o t h e r treatment (Table 3 ) . I t reached i t s maximum mean d a i l y f e c u n d i t y o f 4.76 /day on the 8th day, a l s o e a r l i e r than the ot h e r treatments, and when a l l the a d u l t s were s t i l l a l i v e ( F i g s . 12 and 13). E i g h t y p e r c e n t of the progeny were produced by the 18th day, when 90% o f the a d u l t s were s t i l l a l i v e . Reproduction ceased on the 41st day and the l a s t a d u l t d i e d one day l a t e r . b) R. pa d i w i t h M. dirhodum (pd-l(p)) reached m a t u r i t y three days l a t e r than (p-1). I t s maximum mean d a i l y f e c u n d i t y , reached three days l a t e r than (p-1), was s i m i l a r to the 4.27 /day noted e a r l i e r . A h i g h e r mean d a i l y f e c u n d i t y of 5 ? / ? /day was reached on the 37th day, but was not r e p r e s e n t a t i v e , as onl y one (10%) of the a d u l t s was a l i v e a t t h a t time. E i g h t y p e r c e n t of the progeny were not produced u n t i l the 24th day, 6 days l a t e r than (p-1), when onl y 60% of the a d u l t s were s t i l l a l i v e . Reproduction ceased by the 42nd day and the l a s t a d u l t d i e d on the 45th. c) Comparison between (p-1) and (pd-l(p)) The p r e - r e p r o d u c t i v e p e r i o d of (pd-l(p)) t e s t e d w i t h a t - t e s t a t the 5% l e v e l was s i g n i f i c a n t l y l o n g e r than (p-1) (Table 3). T h i s i n t u r n a f f e c t e d the i n t r i n s i c r a t e of i n c r e a s e , which was lower i n (pd-l(p)) than (p-1) (Table 4). The g e n e r a t i o n time and d o u b l i n g time were longer i n (pd-1(p)) but the net r e p r o d u c t i v e r a t e was the same. Thus c o m p e t i t i o n between one R. p a d i a d u l t and one M. dirhodum a d u l t was evidenced by a l e n g t h e n i n g of the g e n e r a t i o n time, though the net r e p r o d u c t i v e r a t e d i d not change. Treatment E f f e c t s Between Species A t - t e s t a t the 5% l e v e l showed some s i g n i f i c a n t d i f f e r e n c e s between s p e c i e s . The pre-r e p r o d u c t i v e p e r i o d p e r i o d f o r M. avenae was longer than t h a t f o r e i t h e r M. dirhodum or R. p a d i . Metopolophium dirhodum's p r e - r e p r o d u c t i v e p e r i o d was a l s o s i g n i f i c a n t l y longer than t h a t of R. p a d i . Macrosiphum avenae had a lower i n t r i n s i c r a t e of i n c r e a s e and a lower net r e p r o d u c t i v e r a t e . I t took longer f o r M. avenae to produce a new g e n e r a t i o n and longer to double i t s p o p u l a t i o n than the o t h e r two s p e c i e s . The p o s t - r e p r o d u c t i v e p e r i o d of M. avenae was s i g n i f i c a n t l y s h o r t e r than t h a t of e i t h e r M. dirhodum or R. p a d i . The lower p r e - r e p r o d u c t i v e r a t e o f M. dirhodum compared wi t h R. padi's r e s u l t e d i n a lower 46. i n t r i n s i c r a t e of i n c r e a s e . But the net r e p r o d u c t i v e r a t e , g e n e r a t i o n time, and d o u b l i n g time of M. dirhodum were a l l h i g h e r than R. p a d i , though lower than M. avenae. R. p a d i thus had the h i g h e s t i n t r i n s i c r a t e of i n c r e a s e and the s h o r t e s t g e n e r a t i o n and do u b l i n g times, which gave i t the h i g h e s t net r e p r o d u c t i v e r a t e of the three s p e c i e s (Table 5 ) . Table 5. The i n t r i n s i c r a t e of i n c r e a s e i n /day, the net r e p r o d u c t i v e r a t e , the g e n e r a t i o n time i n days and the d o u b l i n g time of three s p e c i e s o f oat aphids, M. avenae, M. dirhodum and R. p a d i Treatment I n t r i n s i c r a t e £/£/day Net Reproductive r a t e Generation time (days) Doubling time (days) x M. avenae 0.21 17 .22 13.42 3.27 x M. dirhodum 0.30 48.12 12.93 2.31 x R. p a d i 0.43 51.48 9.09 1.60 The mean t o t a l number of nymphs produced per female f o r each treatment i s shown i n Table 6. These means are supported by those of Dean (1973), which a l s o showed R. p a d i h i g h e s t and M. avenae the lowest. Dean r e p o r t e d 54.6 R. padi nymphs/female, 5 2.9 f o r M. dirhodum and 36.8 f o r M. avenae on 47. Blender oats i n controlled conditions. Table 6. The mean number of nymphs produced per female for each treatment Treatment Mean t o t a l young per female (a-1) 25.74 (a-2) 25.63 (a-3) 21.39 (d-1) 51.56 (d-2) 47.71 (d-3) 52.75 (pd-l(d)) 47.88 (P-D 49.12 (pd-l(p) ) 54.69 48. C. C o n c l u s i o n Competition r e s u l t s when more than one aphid i s p r e s e n t on a p l a n t i r r e s p e c t i v e of the second aphid. An i n c r e a s e i n the number of a d u l t s of M. avenae i s c o r r e l a t e d i n a decrease i n the p r e - r e p r o d u c t i v e p e r i o d and a s i g n i f i c a n t l y l onger p o s t - r e p r o d u c t i v e p e r i o d . The r e p r o d u c t i v e p e r i o d of one M. dirhodum a d u l t t r e a t e d alone or w i t h one R. p a d i a d u l t was s i g n i f i c a n t l y s h o r t e r than w i t h more than one M. dirhodum a d u l t . When the f e c u n d i t y of one R. padi a d u l t was examined with one M.dirhodum a d u l t , i t s pre-r e p r o d u c t i v e p e r i o d was longer than when i t was t e s t e d alone. The p r e - r e p r o d u c t i v e p e r i o d f o r M. avenae was longer than t h a t of M. dirhodum o r R. p a d i . Dean (1974) a l s o found t h a t the p r e - r e p r o d u c t i v e stage was s h o r t e s t i n R. p a d i and l o n g e s t i n M. avenae. The p r e - r e p r o d u c t i v e p e r i o d f o r M. dirhodum was a l s o s h o r t e r than R. p a d i ' s . These r e s u l t s do not support the f i e l d r e s u l t s from 1972, which showed l a r g e p o p u l a t i o n s of M. avenae, s l i g h t l y lower p o p u l a t i o n s of M. dirhodum, and very few R. p a d i . The l a b o r a t o r y r e s u l t s show R. p a d i with the h i g h e s t f e c u n d i t y , M. dirhodum s l i g h t l y lower, and M. avenae s t i l l lower. F u r t h e r samplings of f i e l d p o p u l a t i o n s and l a b o r a t o r y work were designed to examine e n v i r o n -mental f a c t o r s a f f e c t i n g the aphids and to determine f a c t o r s other than f e c u n d i t y t h a t might i n f l u e n c e the p o p u l a t i o n dynamics of R. p a d i , M. avenae and M. dirhodum. 50. SECTION I I I  POPULATION DYNAMICS - 197 3 1. I n t r o d u c t i o n F i e l d sampling i n 1972 r e v e a l e d l a r g e d i f f e r e n c e s i n p o p u l a t i o n numbers between the th r e e s p e c i e s of oat aphids. The f e c u n d i t y t r i a l s negated the hypothesis t h a t R. p a d i was lowest i n f i e l d abundance because of low f e c u n d i t y . The f i e l d p l o t work of 1973 was done to see what d i f f e r e n c e s c o u l d occur between years and to formulate a d d i t i o n a l hypotheses. 2 . M a t e r i a l s and Methods A. P l o t D e s c r i p t i o n s 2 Three p l o t s , each 930 m were p l a n t e d with F r a s e r o a t s . i ) P l o t A was p l a n t e d on May 2 and sprayed w i t h 2-4D e s t e r form on May 28 to combat weeds. P l a n t i n g was done i n 25 rows, each one metre apar t so as to d u p l i c a t e the p r e v i o u s season's work as c l o s e l y as p o s s i b l e . i i ) P l o t B was seeded on May 3 wit h a h e r b i c i d e a p p l i c a t i o n on May 28. The p l o t was d r i l l e d 51. i n s o l i d b l o c k s , each 2.25 m wide, w i t h one metre between them to r e p r e s e n t normal farming p r a c t i c e s . Each b l o c k c o n s i s t e d o f 13 d r i l l rows. i i i ) P l o t C was p l a n t e d i n 8 s o l i d b l o c k s on May 28, approximately one month a f t e r the other two p l o t s . The purpose of the p l o t was to determine whether the time o f p l a n t i n g would have a b e a r i n g on the r a t i o o f the three aphid s p e c i e s p r e s e n t . No h e r b i c i d e was a p p l i e d . B. Aphid Sampling i ) P l o t A - A random number t a b l e was used to choose t h r e e , 8 cm long s e c t i o n s from each row. A l l p l a n t s i n each s e c t i o n were examined f o r aphids. I f aphids were prese n t on a p l a n t , i t was cut' at s o i l l e v e l , put i n t o a cardboard c a r t o n or p l a s t i c bag and taken to the l a b o r a t o r y . The aphids were then t r a n s f e r r e d , w i t h a small # 0 00 s a b l e p a i n t b r u s h , t o v i a l s of an 80% a l c o h o l / 5 % l a c t i c a c i d s o l u t i o n f o r i d e n t i f i c a t i o n . Sample s i z e decreased as the numbers of aphids i n c r e a s e d as i n 1972. i i ) P l o t B - Sampling f o l l o w e d the same method as f o r p l o t A. E i g h t of the 12 b l o c k s were chosen a t random f o r sampling w i t h 12 sampling s i t e s i n each block making up one sample. When the aphid p o p u l a t i o n became very dense, p l a n t s were taken, e i t h e r from a d i a g o n a l s t r i p a c r o s s the whole p l o t i n c l u d i n g o n l y the 8 randomly chosen b l o c k s , or of a d i a g o n a l s t r i p a c r o s s each separate b l o c k . i i ) P l o t C - Sampling f o l l o w e d the same procedure as t h a t used i n p l o t s A and B. I r r e g u l a r p l a n t growth d i d not permit a s a t i s f a c t o r y d i a g o n a l sample a c r o s s the whole p l o t , so the b l o c k s were always sampled s e p a r a t e l y . P l a n t Samples i ) S i z e T i l l e r s from each sample were c o l l e c t e d and measured on each sample day. The l e n g t h and number of l e a v e s were recorded. i i ) ^Percentage. Moisture Re p r e s e n t a t i v e p l a n t s t h a t had been measured were weighed and oven d r i e d to c a l c u l a t e t h e i r moisture content. i i ) Sparrow Damage F i e l d o b s e r v a t i o n i n 197 2 i n d i c a t e d t h a t sparrows ate the oats as soon as the seed matured. The m a j o r i t y o f aphids were on the heads at t h i s time so the sparrows c o u l d have had a g r e a t i n f l u e n c e on aphid numbers. In 1973, as soon as sparrows became e v i d e n t , a count was made each sample day of the number o f g r a i n l e s s husks, to o b t a i n an estimate of sparrow damage. N a t u r a l Enemies i ) C o c c i n e l l i d a e One hundred randomly s e l e c t e d samples^each .3m i n l e n g t h , of p l a n t s ' were searched f o r c o c c i n e l l i d a d u l t s and l a r v a e , t h r e e times i n p l o t A and twice i n plots B and C. Sampling was then stopped when the numbers became n e g l i g i b l e . i i ) P a r a s i t e s Hymenopterous p a r a s i t e s were not d i r e c t l y sampled f o r although an estimate of the p a r a s i t e s present i n the f i e l d was o b t a i n e d . A l l stems c o l l e c t e d f o r aphid samples were checked f o r p a r a s i t i z e d aphids (mummies). Each mummy c o l l e c t e d was put s e p a r a t e l y i n t o a # 3 g e l a t i n c a p s u l e . A f t e r the p a r a s i t e had emerged, i t was mounted wi t h the mummified aphid and then i d e n t i f i e d . R e s u l t s and D i s c u s s i o n A. P l o t A i ) Aphids Aphids were f i r s t found i n p l o t A on May 22 when .06 a p h i d s / t i l l e r were recovered ( F i g . 14). The p l a n t s averaged 10.61 cm h e i g h t at the time. Numbers decreased to .04 a p h i d s / t i l l e r on June 1 and reached a peak of 21.9 a p h i d s / t i l l e r on J u l y 23. No change i n p a t t e r n was found when aphid numbers were c a l c u l a t e d as aphids/cm of t i l l e r h e i g h t . The decrease i n numbers from May 5 to June 7 may have been due p a r t l y to a low average grass minimum temperature a t t h a t time of 37.38 + 1.54 °F, compared to 45.5 + 1.35 °F averaged over the e i g h t days 55. F i g . 1 4 . T o t a l c e r e a l a p h i d s / t i l l e r found on 3 p l o t s of o a t s , (Avena'-, s a t i v a cv F r a s e r ) i n 1973 P l o t A tK P l o t B P l o t C » O i l , , , , , , , . , . , . , 1 11 21 3 10 20 30 10 20 30 9 19 29 8 May June J u l y August Septetnb D a t e p r e v i o u s t o the decrease, and an average of 43.88 + 2.35 °F over the e i g h t days f o l l o w i n g the decrease. Dean (1974) r e p o r t e d t h a t these three aphid s p e c i e s produced most nymphs at 20°C (68°F), and M. dirhodum (which was the prodominant s p e c i e s e a r l y i n the season) at 10°C (50°F). Because the aphids were on small p l a n t s , and near the ground, they would have been a f f e c t e d by the grass minimum temperature (which was as low as 32°F (0°C) on May 28). These low temperatures may have caused some aphids t o drop o f f and t h e r e f o r e not be c o l l e c t e d i n the samples and would have i n h i b i t e d p o t e n t i a l nymph p r o d u c t i o n . i i ) Species D i f f e r e n c e Metopolophium dirhodum produced the f i r s t a d u l t s and had . 0 0 7 / t i l l e r by June 14 ( F i g . 15). They continued to predominate u n t i l J u l y 23, when the peak of .73 02 a d u l t s / t i l l e r a c t u a l l y was lower than t h a t of M. avenae. Meto.po 1 ophium dirhodum disappeared by the l a s t sample date of August 27. 57. F i g . 1 5 . The number of a d u l t s / t i l l e r of t h r e e s p e c i e s of c e r e a l a p h i d s ; Macrosiphum avenae, Metopolophium dirhodum and Rhopalosiphum  p a d i found d u r i n g 1973 i n p l o t A, p l a n t e d w i t h o a t s , (Avena, s a t i v a cv F r a s e r ) May June J u l y August D a t e Macrosiphum avenae was recorded next, and on June 21 had .006 a d u l t s / t i l l e r . T h e i r numbers were lower than M. dirhodum u n t i l J u l y 2 3 when they reached t h e i r peak of .9683 a d u l t s / t i l l e r . There were s t i l l .286 a d u l t s / t i l l e r on August 27 when the l a s t sample was taken. No a d u l t s o f R. p a d i were recorded u n t i l J u l y 9, when . 0 0 2 3 / t i l l e r were found. They continued t o be presen t i n low numbers and reached two peaks, .016 a d u l t s / t i l l e r on J u l y 23 and . 0 3 6 / t i l l e r on August 13. None were recorded on the l a s t sampling date of August 27. The 1973 a d u l t aphid p o p u l a t i o n i n p l o t A was composed of 53.96% M. dirhodum, 43.17% M. avenae and 2.88% R. p a d i . i i ) P l a n t E f f e c t s Movement o f aphids throughout p l o t A was slow a t f i r s t , as shown by the percentage of sampled t i l l e r s i n f e s t e d w i t h aphids (Table 7). 59. Table 7. The percentage of oat t i l l e r s (Avena. s a t i v a cv Fr a s e r ) i n f e s t e d w i t h aphids i n 1973. Date T i l l e r s w i t h T i l l e r s without Percentage Aphids Aphids of T i l l e r s w i t h Aphids P l o t A May 22 3 May 25 14 June 2 3 June 7 17 June 14 27 June 21 60 June 2 8 234 J u l y 9 296 J u l y 16 103 J u l y 23 63 J u l y 30 34 Aug 6 63 Aug 13 84 P l o t B May 25 7 June 18 21 June 26 52 J u l y 5 169 J u l y 10 129 J u l y 17 46 J u l y 24 32 J u l y 31 48 Aug 7 32 Aug 14 9 6 P l o t C June 18 0 June 25 9 J u l y 3 54 J u l y 11 221 J u l y 18 64 J u l y 25 48 Aug 1 21 Aug. 8 32 Aug 15 96 Aug 22 32 97 3.0% 188 6.93 81 3.57 711 2.34 730 3.57 630 8.70 442 43.79 132 69.16 2 98.1 0 100 8 80.95 0 100 0 100 217 3.13% 436 4.60 312 13.94 456 27.04 72 64.18 2 95.83 0 100 0 100 0 100 0 100 305 0% 469 1.88 527 9.29 260 45.95 8 88.89 0 100 3 87.5 0 100 0 100 0 100 60. There was a r a p i d i n c r e a s e i n movement from June 21 to June 28, when the percentage of t i l l e r s i n f e s t e d i n c r e a s e d almost f i v e times. Probably the i n c r e a s e i n the number of a d u l t aphids r e c o v e r e d , p a r t i c u l a r l y M. dirhodum noted then would account f o r some of t h i s i n c r e a s e d i n f e s t a t i o n ( F i g . 15). The p l a n t s i n c r e a s e d s t e a d i l y i n h e i g h t from 10.61 cm on May 22 to almost 90 cm by J u l y 30 ( F i g . 16). They averaged f o u r l e a v e s each throughout., the season. The water content o f the p l a n t s was 84.7% water on May 22 and i t decreased to 57.7% by August 6 when they had headed out and were maturing ' ; -The p l a n t s were s t i l l growing on J u l y 23 when the h i g h e s t number of a p h i d s / t i l l e r were recorded. T h e i r moisture content had decreased by more than 20% by t h i s time and had produced seed heads. Observations had shown t h a t p l o t A s t a r t e d to head out by June 27. The degree of m a t u r i t y reached by J u l y 2 3 c o u l d i n h i b i t the aphid numbers from i n c r e a s i n g f u r t h e r . i v ) Sparrow Damage By J u l y 30, 1973, f l o c k s of sparrows were again e a t i n g the oat seeds and presumably any aphids on 61. 62 Fig.17. The number of a d u l t s / t i l l e r of three species of cereal aphids; Macros iphum avenae, Metopolophium d irhodum and Rhopalosiphum  padi found during 1973 in plot B;r planted with oats, (Avena sativa cv Fraser) u co Xi (X < 001 May June July Date 63. the seeds. Weekly counts of the number of husks that did not contain a seed showed that the percentage of husks without seeds increased from 22.65% on July 30 to 70.88% on August 13 (Table 8). When aphids on July 30 were recorded separately for the leaves and heads, 45% of the aphids were on the heads (Table 9). Table 8. Oat husks with seeds, as normal, and oat husks without seeds due to sparrow damage i n 1973. Date Husks with Husks without Percentage Seeds Seeds of Husks without Seeds Plot A July 30 August 6 August 13 816 837 428 Plot B July 31 August 7 August 14 Plot C 818 567 804 August 8 August 15 August 22 Sept 4 701 1223 601 21 239 830 1032 22.65% 49.79:. 70.68 212 255 387 20.58% 31.02 32.49 5 486 453 62 .71% 28.44 42.98 74.70 64. Table 9. The percentage of the aphid sample l o c a t e d i n the head of the oat t i l l e r s , Avena s a t i v a cv F r a s e r s e l e c t e d randomly from 3 p l o t s of oats i n 1973. Date P l o t No. Leaves Heads % i n Heads J u l y 30 A 233 188 44.66 J u l y 31 B 516 687 57.12 Aug 1 C 57 133 70 These o b s e r v a t i o n s i n d i c a t e t h a t sparrows i n P l o t A decreased the peak p o p u l a t i o n , even though they d i d not a f f e c t the time of the peak (Table 10). Table 10. The number of a p h i d s / t i l l e r sampled and the number of a p h i d s / t i l l e r estimated i f sparrows had not been pres e n t i n p l o t A i n 1973. Date A p h i d s / T i l l e r A p h i d s / T i l l e r (with Sparrows) (without Sparrows) J u l y 30 Aug 6 Aug 13 11.69 5.49 3.18 12.88 6.72 3.5 v) Natural Enemies a) Coccinellidae Numbers of C o c c i n e l l i d adults and larvae were low, never exceeding . 0 0 4 / t i l l e r a l l season. Species present were Cycloneda p o l i t a (Casey), Coccinella  t r i f a s c i a t a (Mulsant), C. c a l i f o r n i c a (Mannerheim), C. undecimpunctata (L.) and Adalia bipunctata (L.). b) Parasites Parasite species present were Aphidius obscuripes (Ashmead) and Praon americanum (Ashmead). Hyperparasites present were Charips sp., Coruna  cl a r a t a (Walker), Asaphes sp. and Asaphes vulgaris (Walker). Plot B i) Aphids Aphids f i r s t appeared i n plot B on May 25. The population was then .045 a p h i d s / t i l l e r , increasing to a peak of 25 . 0 6 / t i l l e r on July 31 (Fig. 14). Numbers decreased to 3.48 a p h i d s / t i l l e r by the l a s t sampling date of August 14. When the population i s plotted as aphids/cm of plant a similar pattern i s evident with-: .-..004" .aphids/cm;.>6h :.May .25:,: a . tpeak o f .297 a p h i d s / cm on J u l y 31 and .041 aphids/cm August 14, the l a s t sampling date. i i ) Species D i f f e r e n c e Mac rosiphum avenae was the f i r s t s p e c i e s t o be recorded, and had .009 a p h i d s / t i l l e r on June 18 ( F i g . 17). T h e i r numbers r o s e u n t i l J u l y 31 when they reached a peak of 1.8 6 a d u l t s / t i l l e r . On August 14, the l a s t sampling date .094 a d u l t s / t i l l e r were re c o r d e d . Metopolophium dirhodum a d u l t s were found f i r s t on June 18 when .011 a d u l t s / t i l l e r were r e c o v e r e d . They reached a peak of .917 a d u l t s / t i l l e r on J u l y 24, decreasing to .146 a d u l t s / t i l l e r on the l a s t sampling date of August 14. In a l l o n l y two R. pa d i a d u l t s were found i n p l o t B and were expressed as .028 a d u l t s / t i l l e r on J u l y 31 and .0104 a d u l t s / t i l l e r on August 14. Of the 1973 t o t a l a d u l t aphid p o p u l a t i o n , i n p l o t B 56.12% were M. avenae, 4 3.09% were M. dirhodum and .8 0% were R. p a d i . i i i ) P l a n t E f f e c t s Although p l o t B was p l a n t e d i n s o l i d b l o c k s , the r a t e o f movement of aphids through i t was s i m i l a r to p l o t A (Table 7), as i n d i c a t e d by the percentage of t i l l e r s i n f e s t e d w i t h aphids on each sample day. Aphids moved g r a d u a l l y throughout p l o t B, so t h a t a l l p l a n t s were i n f e s t e d by J u l y 24, one week before the aphid p o p u l a t i o n peaked. The oat t i l l e r s grew from 10.7 6 cm on May 25 to 81.89 cm by J u l y 17 when growth l e v e l l e d o f f u n t i l August 14, when they reached 85.26 cm i n h e i g h t ( F i g . 16). They averaged f o u r l e a v e s throughout the growing season. The water content of the p l a n t s was 83.93% on May 25 but had decreased to 54.97% on August 14, when they were mature. The r a t e of p l a n t growth slowed"down"two weeks before the aphids peaked. Seed heads s t a r t e d t o form by June 30, when more than 24.18% of the p l a n t moisture had been l o s t . iv) Sparrow Damage Large f l o c k s of sparrows were observed e a t i n g oat seeds i n p l o t B by J u l y 25. Succeeding samples had the number of husks w i t h and without seeds 68. recorded s e p a r a t e l y (Table 8). Far fewer seeds were eaten by sparrows i n p l o t B than i n p l o t A, probably because p l o t B was adjacent t o b u i l d i n g s and a busy p a r k i n g l o t . The percentage of husks without seeds i n c r e a s e d from 20.58% on J u l y 31 to o n l y 32. 49%on August 14. Plants from the J u l y 31 sample had the aphids on the seed head recorded s e p a r a t e l y , r e v e a l i n g t h a t 57.12% of the aphids were found on t h e heads (Table 9). Damage from sparrows changed the peak number of aphids from 25.06 a p h i d s / t i l l e r to a p o t e n t i a l of 28.00 a p h i d s / t i l l e r , but was not enough to d e l a y the peak date (Table 11). Table 11. The number of a p h i d s / t i l l e r sampled and the number of a p h i d s / t i l l e r estimated i f sparrows had not been pres e n t i n p l o t B i n 1973. Date A p h i d s / T i l l e r Estimated A p h i d s / T i l l e r (with Sparrows) (without Sparrows) J u l y 31 Aug 7 Aug 14 25.06 12.94 3.48 28.00 15.23 4.13 69. v) N a t u r a l Enemies a) C o c c i n e l l i d a e Few C o c c i n e l l i d a d u l t s or l a r v a e were found i n p l o t B i n 1973. The h i g h e s t number recorded was o n l y . 0 0 0 7 / t i l l e r . Species p r e s e n t were C o c c i n e l l a c a l i f o r n i c a (Mannerheim) , C. undecimpunctata (L) and C o c c i n e l l a t r i f a s c i a t a (Mulsant). b. P a r a s i t e s P a r a s i t e s p e c i e s p r e s e n t were Aphidius o b s c u r i p e s (Ashmead) and Praon americahum. Hy p e r p a r a s i t e p r e s e n t were Charips sp., Coruna  c l a r a t a (Walker), Asaphes sp. and Asphes v u l g a r i s (Walker). C. P l o t C i ) Aphids The f i r s t samples to c o n t a i n aphids i n p l o t C were taken on June 25, w i t h .061 a p h i d s / t i l l e r ( F i g . 14). The p o p u l a t i o n peaked with 21.06 aphids/ t i l l e r on J u l y 25 and decreased to .17 a p h i d s / t i l l e r on September 4. The aphid p o p u l a t i o n f o l l o w e d a s i m i l a r t r e n d when c a l c u l a t e d as aphids/cm o f t i l l e r h e i g h t w i t h .0025 aphids/cm on June 25, a peak o f .03 aphids/cm on July 25 and .0016 aphids/cm on September 4, the l a s t sampling date. In both cases the population decreased from July 25 to August 1 and increased again to a second smaller peak on August 15. i i ) Species Differences Macrosiphum avenae and M. dirhodum adults were found on July 3, with populations of .019 and .003 a d u l t s / t i l l e r respectively (Fig. 18). Macrosiphum  avenae maintained the highest number of adults throughout the sampling season and peaked with 1.4 0 a d u l t s / t i l l e r on July 25. The l a s t sample, on September 4 contained no adults of any species. Metopolop h i urn dirhodum peaked with .29 a d u l t s / t i l l e r on July 25, had no adults i n the next two samples, and then again increased to .063 a d u l t s / t i l l e r for two weeks. Rhopalosiphum padi adults were recovered only twice i n the sampling season. On July 11, .0002 a d u l t s / t i l l e r were counted i n the sample. The 1973 adult aphid population i n plo t C was composed of 88.62% M. avenae, 9.88% M. dirhodum and 1.5% R. padi. 71. Fig.18. The number of a d u 1 t s / t i l l e r of three species of cereal aphids; Macros iphum avenae, Metopolophium dirhodum and Rhopalos iphum  padi found during 1973 in plot C, planted with oats, (avenay; sativa cv Fraser) 2 i June July August September Date i i ) Plant E f f e c t s Plot C was d i f f i c u l t to sample because of i r r e g u l a r plant growth. The height of plants side by side d i f f e r e d as much as 150 cm. While plot C was growing, there was no r a i n and the plants became wilt e d Eventually the aphids were found only on the t a l l lush plants and then only i n the southeast corner, where the plants were healthiest because of a leaking i r r i g a t i o n pipe. A single water sprinkle was set i n the southwest corner for one hour on each of two days, August 13 and August 20. The addition of the water may have been advantageous to aphid development, as there was an increase i n the population of M. dirhodum adults on August 15 and 22. The aphids spread throughout plot C within one month. The average height of the plants increased from 24.94 cm on June 25 to 122.99 cm by August 8 (Fig. 16). Although seeded one month l a t e r , they started to head out and reached t h e i r maximum height only one week aft e r plots A and B. Macrosiphum avenae moves to the heads when they are formed, (Forbes (1962)) and as plot had heads longer than the other two plots, t h i s habit may account for the large number of M. avenae i n t h i s p l o t . 73. The moisture content o f the p l a n t s decreased from 86.91% on June 25 to 65.8% by August 22 They had l o s t o n l y 8.64% moisture from June 25 to J u l y 25 when the aphids peaked. i v ) Sparrow Damage No sparrows were i n p l o t C when the aphids peaked (Table 8 ) . They e v e n t u a l l y ate 74.9% of the seed, but as th e r e were o n l y .17 a p h i d s / t i l l e r by then t h e i r e f f e c t on the aphid p o p u l a t i o n would have been n e g l i g i b l e (Table 12). Table 12. The number of a p h i d s / t i l l e r sampled and the number of a p h i d / t i l l e r estimated i f sparrows had not been present i n p l o t C iri .1973. Estimated Date A p h i d s / T i l l e r A p h i d s / T i l l e r (with Sparrows) (without Sparrows) Aug 8 8.62 8.67 Aug 15 9.13 10.94 Aug 22 6.38 8.29 Sept 4 .17 .254 v) Natural Enemies a) Coccinellidae The numbers of Coccinellidae adults were low throughout the sampling season and .0037 adults/ t i l l e r recorded on July 12 was the highest population found. No coccinellidae larvae were found i n the plot but adult species present were Coccinella t r i f a s c i a t a (Mulsant), C. t r i f a s c i a t a and C. undecimpunctata (L). b) Parasites Parasite species present were Aphidius  obscurlpes (Ashmead) and Praon americanum (Ashmead). Hyperparasites present were Charips, sp., Coruna c l a r a t a (Walker), Asaphes sp. and Asaphes  vulgaris (Walker). D PLOT COMPARISON (CONCLUSIONS) Throughout the 1973 season, one peak was reached by the t h r e e aphid s p e c i e s i n each p l o t . P l o t B peaked l a t e s t but had the h i g h e s t t o t a l aphid p o p u l a t i o n , w h i l e p l o t s A and C were almost the same. The p o p u l a t i o n decrease i n p l o t A from May 25 t o June 7 was p o s s i b l y due to the combined e f f e c t of a drop i n maximum and low g r a s s minimum temperatures. P l o t B showed no i n c r e a s e i n aphids/cm d u r i n g t h i s time, w h i l e p l o t C had not y e t been sampled. The t o t a l a d u l t aphid p o p u l a t i o n i n the t h r e e p l o t s was comprised of 59.02% M. avenae , 39.17% M. dirhodum and 1.82% R. p a d i . The percentage of a d u l t s per p l o t was s i m i l a r f o r a l l t h r e e p l o t s , although p l o t A was h i g h e s t w i t h 7.94%, whereas p l o t B had 7.85% and p l o t Q 7.76%. The f i r s t a d u l t s found were the M dirhodum l o c a t e d i n p l o t A. Four days l a t e r , both M. avenae and M. dirhodum were found a t the same time i n p l o t s B and C. Only i n p l o t A d i d M. dirhodum have a h i g h e r p o p u l a t i o n than the two other s p e c i e s . Macrosiphum dirhodum a l s o entered the f i e l d f i r s t , so i t i s p o s s i b l e t h a t the s p e c i e s to enter the f i e l d f i r s t would have the eve n t u a l h i g h e s t p o p u l a t i o n . I t i s a l s o p o s s i b l e t h a t the p o t e n t i a l number of M. avenae was decreased as a r e s u l t of sparrows e a t i n g the seeds, as M. avenae was found on the seeds and sparrows had emptied 70.6% of the husks by August 13. In p l o t B,only 32.49% of the seed husks were damaged by sparrows a t t h i s time, and o n l y 28.44% i n p l o t C. Almost s e v e n t y - f i v e percent of the husks were without seeds by the l a s t sampling date of September 4 i n p l o t C, but t h i s l e v e l of damage would have had minimal a f f e c t on the aphid p o p u l a t i o n , as very few aphids were l e f t . The p o p u l a t i o n dynamics of M. avenae and M. dirhodum d i f f e r e d l i t t l e i n p l o t B. Although p l o t C was p l a n t e d one month a f t e r p l o t s A and B, the aphid p o p u l a t i o n i n the former reached t h e i r peak numbers at the same time as those i n the l a t t e r p l o t s . The numbers of each s p e c i e s i n r e l a t i o n t o each other was very d i f f e r e n t i n p l o t as 89% of the a d u l t s found were M. avenae,10% were M. dirhodum and 1.5% were R. p a d i , compared w i t h 43.17% and 56.12% M. avenae, 53.96% and 43.09% M. dirhodum, and 2.88% and .80% R. pa d i r e s p e c t i v e l y . Rhopalosiphum p a d i entered a l l t h r e e p l o t s l a t e i n the season and t h e i r numbers remained low throughout p l o t s A and B. In p l o t C, however, many R. padi were observed aft e r the l a s t sampling date i n the v i c i n i t y of the leaky i r r i g a t i o n pipe. Thus they appeared to prefer wetter conditions than were generally available i n 1973. The planting of p l o t C one month afte r plots A and B stimulated faster development of both the aphids and the plants. Plot C had the fastest growth rate, with the maximum height being reached only one week af t e r the other two p l o t s . The plants were t a l l e r , with more leaves. They contained more moisture, with 73.4% a f t e r 86 days of growth although they appeared very wilted throughout the growing season. After 96 days, plot A had 57.8% moisture and p l o t B had 57.2% moisture i n the plants. The f a s t growth rate of p l o t C may possibly have been a r e s u l t of f e r t i l i z e r added to the p l o t area during the previous season. The aphids spread faster throughout plot C than i n the other p l o t s , so they infested 100% of the t i l l e r s there similtaneously with the 100% l e v e l s i n the other p l o t s . No difference between plots could be attributed to the method of planting* Aphids moved throughout the p l o t faster i n p l o t C, planted i n blocks, but so was p l o t B. Plot A had more ladybird beetles than plots B and C, but the 78. numbers were n e g l i g i b l e i n a l l t h r e e p l o t s i n 1973. In summary, M. dirhodum was f i r s t t o e n t e r p l o t A i n the s p r i n g and was f o l l o w e d a f t e r one week by M. avenae. During the same week, M. dirhodum and M. avenae together entered p l o t B, e n t e r i n g p l o t C two weeks l a t e r . Sparrows lowered the p o t e n t i a l aphid d e n s i t y i n a l l three p l o t s . When M. dirhodum entered a f i e l d f i r s t , as i n p l o t A, i t had the h i g h e r p o p u l a t i o n of the t h r e e s p e c i e s . The method of planting, s o l i d b l o c k s or rows made no d i f f e r e n c e t o the p o p u l a t i o n dynamics of the three s p e c i e s . A l a t e r planting date, as i n p l o t C, d i d not change the t o t a l p o p u l a t i o n of the aphids, but changed the r a t i o of the t h r e e s p e c i e s t o one another. The h e r b i c i d e a p p l i e d t o p l o t s A and B d i d not enhance the aphid numbers as suggested by Adams and Drew (1965). They r e p o r t e d t h a t a h e r b i c i d e a p p l i c a t i o n w i l l i n c r e a s e the aphid p o p u l a t i o n by d e p r e s s i n g c o c c i n e l l i d a d u l t s and k i l l i n g the l a r v a e . SECTION IV THE EFFECT OF PLANT AGE AND WATER STRESS ON RHOPALOSIPHUM PADI (L.) 1. I n t r o d u c t i o n During two c o n s e c u t i v e summers, 1972 and 1973, th r e e s p e c i e s of aphids were found i n samples of oats at U.B.C. M. avenae and M. dirhodum were the most abundant s p e c i e s , w i t h R. p a d i appearing r a r e l y and i r r e g u l a r l y , i n the samples. In 1972, R. p a d i was the f i r s t s p e c i e s t o e n t e r the f i e l d i n the s p r i n g when the p l a n t s were on l y 2 0-25 cm h i g h . They were prese n t i n low numbers f o r th r e e weeks, and then were not observed or c o l l e c t e d u n t i l the middle of J u l y , a f t e r which time they remained i n low numbers throughout the season. Rhopalosiphum p a d i were not recorded i n 197 3 u n t i l J u l y 9, when some were p i c k e d up i n samples i n p l o t A. Samples from p l o t C contained R. p a d i by J u l y 11, and p l o t B by J u l y 31. Again they remained i n low numbers throughout the sampling season. I t was thought t h a t the two main f a c t o r s i n f l u e n c i n g the R. p a d i p o p u l a t i o n s c o u l d be weather c o n d i t i o n s and the s p e c i e s of host p l a n t . Many authors mention t h a t R. p a d i prefers other crops to o a t s . Dean (1974) found R. padi to be 80. abundant i n the a i r but not on the crop, and suggested that crop v a r i e t i e s grown i n England may not be palatable to R. padi as i t i s mainly a grass aphid. He further suggested that changes i n cereal v a r i e t i e s or climate might encourage i t to become a pest. Dixon and Glen (1971) say that the appearance of immigrants of R. padi seems to be related to the cessation of the spring growth period of b i r d cherry (Prunus padus L.), the primary host. They say R. padi w i l l stay on sucker growth throughout the summer months and leave for grasses only i n the absence of such suckers. Dean (1973) found that R. padi, under controlled conditions, produced fewernymphs on oats than on both wheat and barley. Greene (1966) observed R. padi on more plant species than M. avenae and M. dirhodum and also mentioned that R. padi prefer cool conditions and were even observed a l i v e and crawling i n snow. Jones (1972) mentioned that an increase i n R. padi in plots i n 1971 was probably due to the presence of other grasses between the wheat plants. Dean (197 3) found R. padi to have low numbers i n the f i e l d but to increase when caged as a r e s u l t of the more humid conditions. 81. I t was observed i n f i e l d experiments i n 1972 (Se c t i o n and 1973 ( S e c t i o n III) t h a t R. p a d i r e t u r n e d to the oat p l o t s i n the l a t e summer when the surrounding grasses became very dry but the oat p l a n t s were s t i l l s u c c u l e n t . I t was a l s o observed i n 1973 t h a t one wet s e c t i o n of an oat p l o t (see S e c t i o n I I I ) had an i n c r e a s i n g number of R. padi l i v i n g on i t l a t e i n t o the f a l l . Watering experiments were proposed to determine whether moisture was a l i m i t i n g f a c t o r i n the development of R. p a d i . The e f f e c t o f p l a n t age co u l d a l s o be examined by i n i t i a l l y a p p l y i n g three d i f f e r e n t water treatments to thr e e ages of p l a n t s . 2. Experiment # 1 ( C o n t r o l l e d C o n d i t i o n s - caged) A. M a t e r i a l s and Method Two F r a s e r oat seeds were seeded i n t o 9 cm diameter p l a s t i c pots f i l l e d w i t h 32 g o f mica-peat ( a commercial peat m o s s - v e r m i c u l i t e s o i l mix) and set i n a p e t r i d i s h . T h i s mix was used i n l a b o r a t o r y s t u d i e s because of i t s u n i f o r m i t y . Two seeds were sown i n each of n i n e t y p o t s . T h i r t y pots were sown on January 19, 30 on February 19 and 30 on March 12. Each pot was g i v e n 50 ml of water a t each watering and gi v e n equal amounts of water u n t i l the p l a n t s had grown to 10 cm i n h e i g h t . The f i r s t seed t o germinate was kept and the other, i f i t germinated was d i s c a r d e d . The plantswhen 10 cm 82. h i g h were segregated i n t o the f o l l o w i n g treatments: 1. Wet - Pots s a t i n water a t a l l times. 2. Damp - 50 ml of water was a p p l i e d t o the s o i l whenever a l l t r a c e of water had disappeared from the p e t r i d i s h . 3. Dry - 50 ml of water was a p p l i e d to the s o i l when the p l a n t showed s i g n s of w i l t i n g . A l l added water was recorded. The 30 p l a n t s i n each watering treatment were sub d i v i d e d i n t o groups of ten p l a n t s from each seeding date so t h a t t h e r e were th r e e p l a n t - h e i g h t treatments (Table 13). 1 Mature - The p l a n t had headed out. 2. Medium - Averaged 30 cm i n h e i g h t . 3. Short - Averaged 25 cm i n h e i g h t . Table 13. The nine treatments a p p l i e d t o Rhopalosiphum  pa d i and o a t s , Avena s a t i v a cv F r a s e r , i n c o n t r o l l e d c o n d i t i o n s . Wet Damp Dry Mature Medium Short Mat - wet Mat - damp Med - wet Med - damp Short - wet Short - damp Mat - dry Med - dry Short - d r y 83. Nine p l a n t s , one from each treatment, were p l a c e d randomly i n each of ten f l a t s . The f l a t s were moved. 1 d a i l y under a l i g h t bank (which was on f o r 16 hours each day) to ensure equal l i g h t i n g . When the p l a n t s had reached the r e q u i r e d t e s t h e i g h t , each one r e c e i v e d an R. pa d i a d u l t chosen randomly from the r e a r i n g stock, which had been maintained f o r fo u r months. Each p l a n t was then covered w i t h an 8 cm p l a s t i c tube (open a t the top because R. pa d i w i l l s t ay a t the base of the p l a n t , B e l v e t t e t a l (1965)), where the a d u l t was allowed to d e p o s i t one nymph bef o r e i t was removed. When t h i s nymph reached m a t u r i t y , i t s f e c u n d i t y was recorded f o r 16 c o n s e c u t i v e days. P r e l i m i n a r y f e c u n d i t y s t u d i e s (see S e c t i o n ;II) had shown t h a t most R. pa d i a d u l t s had produced 80% of t h e i r progeny d u r i n g the f i r s t 16 days. B. R e s u l t s and D i s c u s s i o n When analyzed a t the 5% l e v e l o f s i g n i f i c a n c e , the r e s u l t s showed t h a t s i g n i f i c a n t l y d i f f e r e n t f e c u n d i t i e s of R. pa d i were produced by g i v i n g the p l a n t s d i f f e r e n t amounts of water. S i g n i f i c a n t d i f f e r e n c e s were a l s o shown when p l a n t s o f d i f f e r e n t ages, as measured by t h e i r h e i g h t s , were used to re a r the aphids (Table 14). The mean and standard e r r o r o f the mean are l i s t e d i n Table 15. 84. Table 14. An analysis of variance of the mean number of nymphs produced by each of nine treatments described i n Table 13. Source of Vari a t i o n Sums of Squares Degrees of Freedom Mean Squares F Treatment 18220.889 8 2277.611 6 .54* Water 2974.689 2 1487 .344 4 .27* Height 12443.889 2 6221.944 17 .86* Water x height 2802. 311 4 700.578 2 .01 Block 3314.011 9 368.223 1 .06 Experimental Error 24390.889 70 348.441 Total 45925.789 81 Si g n i f i c a n t at the 5% l e v e l . 85. Table 15. The mean and standard error of the mean of populations of R. padi raised on three sizes of plants which had been treated with three d i f f e r e n t amounts of water. Treatment Standard Error of the Mean Mean Small-dry Small-damp Small-wet 5.98 4 .46 7.16 30.1 38.4 44.1 Medium-dry Medium-damp Medium-wet 5.88 2.45 5.90 47 40.1 44 .5 Large-dry Large-damp Large-wet 1.98 6.47 7.04 2.2 14.7 32.2 Combined -small Combined-medium Combined-large 3.60 2.94 3.95 37 .53 43.87 16.27 Combined-dry Combined-damp Combined-wet 4.43 3.46 4 .01 26.43 31.07 40.27 86. F u r t h e r a n a l y s i s by Duncan's New M u l t i p l e Range Tes t showed there was a s i g n i f i c a n t d i f f e r e n c e i n the number o f R. p a d i produced between a l l t h r e e watering treatments; wet > damp > dry, and between a l l t h r e e h e i g h t treatments; medium > small > l a r g e . Twenty-one percent o f the water was giv e n t o the dry treatments, which produced 27.04% of the progeny. The damp p l a n t s r e c e i v e d 34.98% of the water and produced 31.78% of the aphids, whereas 4 4.3% of the water was used by the wet p l a n t s , which produced 41.19% of the aphid young. More water d i d not r e s u l t i n a higher f e c u n d i t y when p l a n t s i z e was c o n s i d e r e d . The small p l a n t s r e c e i v e d 18.39% o f the water and produced 38.39% of the progency. Twenty-eight percent of the water was g i v e n to the medium p l a n t s to produce the most progeny (44.87%), whereas the l a r g e p l a n t s w i t h 53.81% of the water produced o n l y 16.74% of the young. C. C o n c l u s i o n s These r e s u l t s support the i n i t i a l h y p o t hesis t h a t R. pa d i p r e f e r wet c o n d i t i o n s . The l e a s t numbers of aphids was produced on the l a r g e p l a n t s , p o s s i b l y because they had headed out and had reached m a t u r i t y by the end of the experiment. I t was shown e a r l i e r , (see S e c t i o n s I and IV) 87. t h a t the water content of the p l a n t s decreased as they matured, r e g a r d l e s s of p r e c i p i t a t i o n . The p l a n t s of medium h e i g h t produced more than the s m a l l e r p l a n t s , p o s s i b l y because the n u t r i e n t content o f the small p l a n t s was not enough to support a l a r g e aphid p o p u l a t i o n . 3. Experiment # 2 ( C o n t r o l l e d C o n d i t i o n s - not caged) A. M a t e r i a l s and Methods In.the f i r s t experiment the p l a n t s were caged t o prevent the aphids from e s c a p i n g . The purpose of t h i s second experiment was to see whether s i m i l a r r e s u l t s c o u l d be obtained i f the aphids were f r e e to choose any a v a i l a b l e p l a n t . Only water r e l a t i o n s were t e s t e d on p l a n t s o f s i m i l a r s i z e . ( P l a n t s averaging 30 cm i n h e i g h t were used because they had produced a h i g h e r f e c u n d i t y i n experiment # 1 ) . Two F r a s e r oat seeds were p l a n t e d i n each of 15, 8 cm diameter p l a s t i c pots f i l l e d w i t h 47 g of mica-peat, to which 50 ml of water were added. A d d i t i o n a l water was added t o a l l pots as r e q u i r e d . The pots were l e f t to germinate under a l i g h t bank i n the greenhouse. F o r t y days l a t e r they were p l a c e d i n a random d e s i g n under the same c o n t r o -l l e d c o n d i t i o n s as those i n experiment # 1 . A p e t r i d i s h was p l a c e d under each pot to c o n t a i n the water, and the pots 88. were set about 30 cm a p a r t . The p l a n t s i n the dry treatment were gi v e n water o n l y when they showed s i g n s of w i l t i n g . The damp p l a n t s were g i v e n 50 ml whenever the p e t r i d i s h d r i e d , and the p l a n t s i n the wet treatment sat c o n s t a n t l y i n water. Water was always added from the top. Although no oat p l a n t s or v i s i b l e r e s i d u e s of oat p l a n t s had been i n t h i s room f o r over two months, R. p a d i a l a t e s began to appear on the p l a n t s w i t h i n two or t h r e e days. No aphid > was removed from the p l a n t s u n t i l a l l r e s i d e n t s were counted on the 43rd day. B. R e s u l t s and D i s c u s s i o n Records show t h a t R. p a d i a l a t e s moved i n i t i a l l y to the d r y p l a n t s from unknown sources. A f t e r 4 3 days, the wet p l a n t s averaged 214 aphids/ p l a n t , whereas the damp p l a n t s had an average of 150 aphids/ p l a n t . These c o n t r a s t e d w i t h an average of o n l y 20 a p h i d s / p l a n t on the d r y p l a n t s (Table 16). 89. Table 16. The mean number w i t h standard e r r o r of R. pa d i on p l a n t s which had been t r e a t e d w i t h t h r e e d i f f e r e n t amounts of water. Treatment Mean Standard E r r o r No. Aphids of the Mean Dry 20 4.33 Damp 150 15.37 Wet 214 19.46 An a n a l y s i s of v a r i a n c e and a Least S i g n i f i c a n t D i f f e r e n c e Test showed t h a t the wet and damp treatments supported s i g n i f i c a n t l y more aphids than the dry treatments (Table 17). Table 17. An A n a l y s i s of v a r i a n c e of the mean number of nymphs produced by R. p a d i d u r i n g three water treatments under c o n t r o l l e d c o n d i t i o n s . Source o f V a r i a t i o n Sums of Degrees o f Mean F t e s t Squares Freedom Squares Treatment 89757.51 2 44878.76 8.898* Experimental E r r o r 56112.2 11 5101.11 T o t a l 145869.71 13 * S i g n i f i c a n t at the 5% l e v e l . 90. C. C o n c l u s i o n s These r e s u l t s support those i n experiment # 1, i n which the dry p l a n t s a l s o had a s t a t i s t i c a l l y lower f e c u n d i t y . In a d d i t i o n , t h i s experiment shows t h a t these r e s u l t s w i l l occur when the aphids are a b l e t o s e l e c t the p l a n t as w e l l as when they are f o r c e d to l i v e on a p a r t i c u l a r p l a n t . The hi g h d e n s i t y of aphids on the p l a n t s i n i t i a t e d the p r o d u c t i o n of a l a t e adults,, so even though the p l a n t s were not t o u c h i n g , the aphids c o u l d move away from them. 4. Experiment # 3.(Uncontrolled C o n d i t i o n s ) A. M a t e r i a l s and Methods Experiment # 3 compared the p l a n t p r e f e r e n c e of f i e l d aphids w i t h those used i n experiments 1 and 2, which had been r e a r e d f o r s e v e r a l g e n e r a t i o n s i n the l a b o r a t o r y . Two oat seeds were p l a n t e d i n each o f f i f t e e n , 9 cm diameter pots f i l l e d w i t h mica-peat. Only the f i r s t seed t o germinate was kept. The experiment was c a r r i e d out from J u l y 12 u n t i l August 21 on the i n s i d e window s i l l o f an E a s t - f a c i n g l a b o r a t o r y . The l i g h t and heat t h e r e f o r e were p r o v i d e d n a t u r a l l y and were u n c o n t r o l l e d . Water was added as i n experiment # 2. 91. "Nursery" p l a n t s f o r r e a r i n g aphids were kept about th r e e metres from the t e s t p l a n t s and a l l i n s t a r s o f R. p a d i brought i n from the f i e l d were r a i s e d on them. The aphids had to reach the t e s t p l a n t s u n a s s i s t e d . F o r t y days a f t e r the t e s t p l a n t s had been p l a c e d on the window s i l l , the l i v e aphids remaining on them were counted. B. R e s u l t s and D i s c u s s i o n An average of 4 60.4 a p h i d s / p l a n t and 436.4 aphids/ p l a n t were r e s p e c t i v e l y , ^maintained by the wet and damp treatments. The dry p l a n t s produced an average of o n l y 59.4 a p h i d s / p l a n t (Table 18). Table 18. The mean and standard e r r o r of the mean of p o p u l a t i o n s of R. p a d i e s t a b l i s h e d on p l a n t s which had b e e n - t r e a t e d w i t h three d i f f e r e n t amounts of water. Treatment Mean Standard E r r o r (No. Aphids) of the Mean Wet Damp Dry 460.4 436.4 59.4 19.46 15.37 4 .33 The numbers of aphids recorded on the wet and damp p l a n t s were s i g n i f i c a n t l y h i g h e r than on the dry ones ( A n a l y s i s o f V a r i a n c e and Duncan's New M u l t i p l e Range T e s t (Tables 19 and 2 0 ) ) . Table 19. An a n a l y s i s o f v a r i a n c e o f the mean number of nymphs produced by R. p a d i d u r i n g three water treatments under v a r i a b l e c o n d i t i o n s Source of V a r i a t i o n Sums of Degrees o f Mean F Test Squares Freedom Squares Treatment 505843.333 2 252921.7 19.32* Experimental E r r o r 157093.6 12 13091.133 T o t a l 662936.933 14 S i g n i f i c a n t a t the 5% l e v e l . Table 20. Duncan's New M u l t i p l e Range Te s t showing s i g n i f i c a n t water treatments under v a r i a b l e c o n d i t i o n s Treatment Wet Damp Dry Wet Damp ns Dry * S i g n i f i c a n t at the 5% l e v e l . These r e s u l t s w i t h f i e l d - c o l l e c t e d aphids support those obtained w i t h l a b o r a t o r y r e a r e d aphids i n experiments # 1 and # 2, where R. p a d i p o p u l a t i o n s on wet and damp p l a n t s were a l s o s i g n i f i c a n t l y h i g h e r than those found on the dry p l a n t s . C. C o n c l u s i o n s These three experiments a l l support the hypothesis t h a t R. p a d i p r e f e r wet c o n d i t i o n s . T h e i r p r e f e r e n c e f o r wet c o n d i t i o n s would be a p a r t i a l e x p l a n a t i o n to t h e i r presence i n oat p l o t s mainly i n the e a r l y s p r i n g and l a t e ns summer when c o o l e r n i g h t temperatures produce heavy dews and more humid c o n d i t i o n s . These r e s u l t s are c o n t r a r y to those of Adams and Drew (1969) i n which, when water was a p p l i e d to p l a n t s , R. p a d i decreased r e p r o d u c t i o n and i n c r e a s e d l o n g e v i t y , but they are supported by Dean's (1973) r e s u l t s which showed t h a t the f e c u n d i t y of R. p a d i i n c r e a s e d when i t i s caged, a c o n d i t i o n t h a t would i n c r e a s e the humidity. P l a n t s of medium h e i g h t (averaging 3 0 cm) supported the l a r g e s t R. p a d i p o p u l a t i o n s . T a l l e r ones were probably too mature and dry, even when s i t t i n g i n water, and the s m a l l e r ones were too small to support l a r g e aphid p o p u l a t i o n s . Greene (1966) r e p o r t e d t h a t R. p a d i appeared i n f i e l d s i n Oregon l a t e r than M. avenae and M. dirhodum. He says t h i s d i f f e r e n c e may r e s u l t from the p l a n t s becoming more f a v o r a b l e when the lower l e a v e s begin to senesce and l o s e t h e i r c h l o r o p h y l l . SECTION V POPULATION DYNAMICS - 1974 1 • I n t r o d u c t i o n Two p l o t s o f oats were p l a n t e d i n 1974. P l o t A was p l a n t e d i n rows i n the same l o c a t i o n as i n 1973. Plans were made to i r r i g a t e s e c t i o n s o f p l o t A t o f i e l d t e s t the assumptions t h a t R. p a d i p r e f e r r e d wet p l a n t c o n d i t i o n s , but the p l a n t s never became dry enough i n 1974. Small cages, 30 cm x 3 0 cm i n diameter and 90 cm hig h , were put i n the f i e l d t o attempt f e c u n d i t y t r i a l s , but were d i s c a r d e d when the r a i n kept washing the aphids o f f the p l a n t s , w h i l e the p l a n t s became i n f e c t e d w i t h mildew making them im p o s s i b l e to use. In 197 2 and 197 3 l a r g e numbers o f sparrows were e a t i n g the oats,so to t e s t t h e i r e f f e e t * two cages 2.4 m .:• x 3 m i n diameter and 1.8 m hi g h , designed to keep sparrows o f f the p l a n t s , were p l a c e d over two s e c t i o n s of three rows. P l o t B was l o c a t e d i n the same area as 1972 to see whether the l a r g e aphid p o p u l a t i o n of 88.8 a p h i d s / t i l l e r r e c o r d e d i n 1972 would r e c u r . (The p o p u l a t i o n d e n s i t y i n 1972 was almost four times t h a t of 1973, but i t was not known whether the higher p o p u l a t i o n stemmed from a d i f f e r e n c e i n l o c a t i o n or an annual d i f f e r e n c e i n numbers). In 1974, more time was a v a i l a b l e f o r i d e n t i f y i n g s p e c i e s a t a l l developmental stages. 2. M a t e r i a l s and Methods A. P l o t A i ) D e s c r i p t i o n and Aphid Sampling,, On May 13, 1974, P l o t A was seeded i n 47 rows, one meter a p a r t and 3 2.2 m l o n g . The outer two rows on each s i d e and one metre a t the end of each row were excluded from the sampling. Aphid p o p u l a t i o n s were sampled i n the same manner as i n p r e v i o u s y e a r s . i i ) Sparrow Cages P l a n t s w i t h i n the two sparrow cages were i n c l u d e d i n the p l o t samples u n t i l sparrows were seen i n the p l o t s and then they were sampled s e p a r a t e l y , each time by t a k i n g two stems per row or s i x per cage. B. P l o t B i ) D e s c r i p t i o n and Aphid Sampling On May 27, p l o t B was p l a n t e d i n 13 rows, one metre apa r t and 7.5m l o n g . I t was l o c a t e d about one m i l e south of p l o t A i n the same area as the 197 2 p l o t . Again one metre around the p l o t was l e f t as a guard area and not sampled. Aphid p o p u l a t i o n s again were sampled i n the same manner as i n the p r e v i o u s two seasons. A l l i n s t a r s were immediately separated i n t o s p e c i e s i n s t e a d of being s t o r e d i n v i a l s . C. P l a n t Sampling A sample of s i x to ten stems was c o l l e c t e d each week f o r r e c o r d i n g the h e i g h t i n cm and the number of l e a v e s . These t i l l e r s were then weighed, oven d r i e d and weighed again to o b t a i n t h e i r moisture content. D. S o i l Samples F i v e samples of s o i l were c o l l e c t e d a t a depth of 10-15 cm i n both p l o t s a t v a r i o u s times, u s u a l l y , on aphid sampling days. The s o i l was put i n 98. a i r t i g h t aluminum c o n t a i n e r s , weighed, oven d r i e d and weighed again t o estimate the moisture content of each p l o t a t approximate r o o t depth. E. P a r a s i t e s A l l sampled oat stems were checked f o r p a r a s i t i z e d aphids (mummies) and any found were put i n g e l a t i n c a p s u l e s , a l l o w e d t o emerge, mounted and l a t e r i d e n t i f i e d . 3. R e s u l t s and D i s c u s s i o n A. P l o t A i ) Aphid Samples Aphids were f i r s t found i n p l o t A on June 5 when there were . 0 0 3 / t i l l e r . They i n c r e a s e d t o a peak of 14.29 a p h i d s / t i l l e r by J u l y 15 and had decreased again t o 2 . 1 0 / t i l l e r by August 7, the l a s t sampling date ( F i g . 19). The p o p u l a t i o n f o l l o w s s i m i l a r trends when expressed as aphids/ cm of t i l l e r h e i g h t . Movement of aphids throughout the p l o t was measured by the percentage . of aphid i n f e s t e d Fig.19. Total cereal a p h i d s / t i l l e r found on two plots o f oats, Avena. sativa cv Fraser) in 1974 100. t i l l e r s on each sampling date (Table 21). On June 5, the f i r s t sampling date, o n l y 1.74% of the t i l l e r s w e r e i n f e s t e d w i t h aphids. By J u l y 2, the l a s t date on which the number of i n f e s t e d t i l l e r s was counted, almost 50% had aphids on them. Table 21. The percentage of oat t i l l e r s (Avena. s a t i v a cv F r a s e r ) i n f e s t e d w i t h aphids i n p l o t A i n 1974. Date Percentage of T i l l e r s w i t h Aphids June 5 1.74% June 10 2.14 June 13 3.90 June 17 5.07 June 24 20.44 J u l y 2 49.14 101. i i ) Species Difference Over half (51.65%) of the aphid population was M. avenae (Fig 20). They were f i r s t recorded on June 5, with . 0 2 / t i l l e r , and had increased to a peak of 7 . 6 7 / t i l l e r on July 29. By the l a s t sampling date of August 27, only 1 . 3 8 / t i l l e r were l e f t . The M. avenae were di s t r i b u t e d uniformly through-out the plot, and did not produce many alates u n t i l August 19 when 51% of the 4th ins t a r aphids were alates. There were 1.05 4th instar a p h i d s / t i l l e r c o l l e c t e d that day. Few alate adults were recovered i n the samples; presumably they had l e f t the pl o t . Metopolophium dirhodum made up 4 6.4 9% of the t o t a l aphid population only s l i g h t l y lower than M. avenae. There were fewer M. dirhodum recovered in the f i r s t sample, (.01/tiller) but they increased more rapidly to reach a higher peak of 9.14 / t i l l e r on July 15, two weeks e a r l i e r than M. avenae peaked. The M. dirhodum decreased more rapidl y than M. avenae, only . 1 6 7 / t i l l e r by August 27, the l a s t sampling date. There was a s l i g h t increase i n population between August 9 and 19 which might have been due to 102. F i g . 2 0 . T h e p o p u l a t i o n / t i l l e r o f t h r e e s p e c i e s o f o a t a p h i d s ; M a c r o s ip'hu'm a v e n a e , Me t o p o l o p h i u m d i r h o d u m a n d R h o p a l o s I p h u m p a d i f o u n d d u r i n g 1 9 7 4 i n p l o t A , p l a n t e d w i t h o a t s , ( A v e n a e s a t i v ; c v F r a s e r ) May J u n e J u l y A u g u s t D a t e 103. .38 cm o f r a i n on August 11 and 12 a f t e r a lengthy d r y p e r i o d . Out of a p o p u l a t i o n of 2.24 4th i n s t a r / t i l l e r , over 56% were a l a t e s on J u l y 15, when M. dirhodum peaked. The t o t a l aphid p o p u l a t i o n peaked at t h a t time with 14.30 aphids/ t i l l e r , which was probably h i g h enough to overcrowd the M. dirhodum, i n d u c i n g them to produce a l a t e s . S i x t y - f o u r percent of the t o t a l aphid p o p u l a t i o n then was M. dirhodum. As w i t h M. avenae, on l y a few a l a t e a d u l t s were rec o v e r e d . The p o p u l a t i o n of R. p a d i was low throughout the sampling season, a c c o u n t i n g f o r o n l y 1.86% of the-t o t a l aphid p o p u l a t i o n . On June 5, there were . 0 0 3 / t i l l e r , but none were recovered i n the next two samples. The f l u c t u a t i n g p o p u l a t i o n i n c r e a s e d to . 9 0 5 / t i l l e r by August 19 and then decreased to .548 a p h i d s / t i l l e r by August 27, the l a s t sampling date. The f l u c t u a t i n g behavior of the R. p a d i p o p u l a t i o n was due to t h e i r low numbers. P r i o r t o J u l y 15, the h i g h e s t number of c o l l e c t e d stems i n f e s t e d with R. p a d i was o n l y 5 out of 1065 stems, too low to p r o v i d e a r e p r e s e n t a t i v e sample. There were .032 R. p a d i / t i l l e r on J u l y 15 and 32.35% of 104. these were 4th i n s t a r a l a t e s . No a d u l t s were. recovered i n the next sample. Presumably they had l e f t the p l o t . Rhopalcsiphum p a d i i n i t i a l l y do seem t o p r e f e r other g r a s s e s t o o a t s , Dean (1974), Greene (1966), Jones (1972), but many of the nearby u n c u l t i v a t e d g r a s s e s such as Orchard grass ( D a c t y l i s glomerata ( L ) ) , had r i p e n e d by August, causing the R. pa d i t o move onto the o a t s , which were greener and more s u c c u l e n t . The R. pa d i p o p u l a t i o n continued to i n c r e a s e , produced very few a l a t e s (43% of 4th i n s t a r a t the most) and by August 27 made up 26.14% of the t o t a l aphid p o p u l a t i o n . i i i ) Sparrow Cages The data from both cages are combined t o g i v e a l a r g e r sample. The cages were l a r g e and covered o n l y i n 3/4" ch i c k e n wire, so except f o r e x c l u d i n g sprarrows, the c o n d i t i o n s i n s i d e the cage should have been the same as those o u t s i d e . The caged oats supported a l a r g e r p o p u l a t i o n than d i d the open p l o t ( F i g . 21). On August 13, when the t o t a l aphid p o p u l a t i o n had decreased t o 5.38 a p h i d s / t i l l e r , the p o p u l a t i o n i n s i d e the cages 105. had r i s e n to 21.92 a p h i d s / t i l l e r , which was 1.53 times higher than the peak p o p u l a t i o n of the whole p l o t . On the l a s t sampling date o f August 27, the caged p o p u l a t i o n was s t i l l 12.5 a p h i d s / t i l l e r , whereas o u t s i d e the cage i t had decreased t o 2.10/ t i l l e r . Most of the. caged aphid p o p u l a t i o n was M. avenae, as i n the whole p l o t , but the p o p u l a t i o n of M. avenae was l a r g e r i n s i d e the cages. T h i s d i f f e r e n c e was g r e a t e s t on August 19, when 70.90% of the t o t a l aphid p o p u l a t i o n i n p l o t A was M. avenae, whereas i n s i d e the cages, M. avenae made up 90.67% of the aphid p o p u l a t i o n . F i f t y p ercent o f the caged 4th i n s t a r were a l a t e s by August 27 when the oats were very r i p e and dry. On t h a t date o n l y 4 6.67% o f the aphids i n s i d e the cages were M. avenae, whereas i n the open p l o t 65.91% were. Presumably, the M. avenae p o p u l a t i o n had i n c r e a s e d too much to be supported on the dry heads, so the a l a t e s t h a t were produced had l e f t . There were fewer M. dirhodum i n s i d e the cages than o u t s i d e . By August 27, o n l y 1.33% of the 106. aphids i n s i d e the cage were M. dirhodum, whereas there were 7.95% o u t s i d e . There were a l s o fewer R. p a d i i n s i d e the cages than o u t s i d e , w i t h the e x c e p t i o n of the l a s t sample when R. pa d i had higher numbers i n s i d e the cages. On August 19, while the R. p a d i p o p u l a t i o n o u t s i d e the cages had i n c r e a s e d to 16.63% o f the aphid p o p u l a t i o n , there were onl y 4.67% i n s i d e . By August 27, the l a s t sampling date, the uncaged R. p a d i p o p u l a t i o n had i n c r e a s e d t o 2 6.14% while i n s i d e the cages i t was 52%. As R. p a d i are u s u a l l y found a t the base of the p l a n t i t seems u n l i k e l y t h a t the sparrows c o u l d have a f f e c t e d the p o p u l a t i o n t h a t much, but the presence of cages might have changed some unknown environmental c o n d i t i o n . i v ) P l a n t E f f e c t s The f i r s t p l a n t measurements were taken on June 5. T i l l e r s then averaged 13.53 cm hi g h , w i t h 2.86 l e a v e s . They grew u n t i l J u l y 22, r e a c h i n g a h e i g h t o f 102.34 cm with 4.86 l e a v e s . The p l a n t s , a l r e a d y headed out, continued to grow a l i t t l e more, r e a c h i n g 107.06 cm on August 13. 107. The plants were s t i l l growing when M. dirhodum peaked, but had l e v e l l e d o f f by the time M. avenae and R. padi peaked. This l e v e l l i n g did not appear to i n h i b i t the growth of the M. avenae or R. padi populations. The plants had an average moisture content of 85.07% on June 5, r i s i n g to 91.18% by June 10. Rain amounting to 3.7 cm from June 5 to June 10, would have caused t h i s increase. Rain up to 2.05 cm a day prevented the moisture content of the plants from decreasing as i t had i n 197 3 so that i t was s t i l l 80.46% on July 15, when M. dirhodum peaked. As M. dirhodum prefer the succulent oat leaves to the d r i e r head t h i s higher moisture content may have allowed them to increase more rapidly than M. avenae, which appears to be less influenced by the moisture content of the plant. The higher nutrient content of the head att r a c t s the M. avenae more than do the succulent leaves. v) S o i l Sample Results The highest s o i l moisture content (22.91%) was measured on July 18 a f t e r ten days of nearly continuous r a i n amounting to 6.4 cm. 108. The f i r s t sample, taken on June 26, contained 6.96% s o i l moisture and the l a s t , on August 27, had the lowest (5.52%). None were low enough to induce w i l t , which would have allowed the necessary i r r i g a t i o n to f i e l d t e s t the water r e l a t i o n s h y p o thesis from S e c t i o n V. v i ) P a r a s i t e s Of the 119 mummies ( p a r a s i t i z e d aphids) c o l l e c t e d i n p l o t A i n 1974, 111 or 93.28% of them emerged. The p a r a s i t e s p e c i e s present c o n s i s t e d of one Praon americanum (Ashmead), one Ephedrus c a l i f o r n i c u s (Baker), 8 Aphidius avenaphis ( F i t c h ) and 68 Aphidius o b s c u r i p e s (Ashur). The h y p e r p a r a s i t e s present c o n s i s t e d of one Coruna c l a r a t a (Walker), three Dendrocernus n i g e r (Howard) and 35 Asaphes sp. B. P l o t B i ) Aphid Samples The t o t a l aphid p o p u l a t i o n was lower i n p l o t B than p l o t A ( F i g . 19). P l o t B had .073 a p h i d s / t i l l e r 109. when f i r s t sampled on June 14. I t peaked on J u l y 25, wi t h 6.78 a p h i d s / t i l l e r , and had decreased t o 1.94/ t i l l e r by August 22, the l a s t sampling date. When expressed as aphids/cm of t i l l e r h e i g h t the same trend s appeared with .006 aphids/cm on June 14, i n c r e a s i n g to .08/cm on J u l y 25 and d e c r e a s i n g t o .019 aphids/cm by August 22. Aphid movement throughout the p l o t was slow, w i t h 2.44% of the stems i n f e s t e d on June 14 and o n l y 24.65% i n f e s t e d by J u l y 5 (Table 22). In p l o t A, almost 50% of the t i l l e r s were i n f e s t e d w i t h aphids by J u l y 2. The slower movement i n p l o t B was probably because of the lower p l a n t d e n s i t y (6.83 t i l l e r s / 7 cm compared to 9.68 t i l l e r s / 7 cm i n p l o t A). Table 22. The percentage of oat t i l l e r s , Avena s a t i v a cv F r a s e r , i n f e s t e d w i t h aphids i n p l o t B i n 1974. Date Percentage of T i l l e r s i n f e s t e d with Aphids June 14 2.44 June 18 11.65 June 21 14.23 June 25 22.60 June 28 22.63 J u l y 5 24.65 110. i i ) Species D i f f e r e n c e Macrosiphum avenae c o n t r i b u t e d 59.23% to the t o t a l aphid p o p u l a t i o n . I t was the f i r s t s p e c i e s to e n t e r the f i e l d and had .07 3 i n d i v i d u a l / t i l l e r r ecorded on June 14 ( F i g . 21). The p o p u l a t i o n i n c r e a s e d to 3 . 5 7 / t i l l e r by J u l y 25 and had decreased to .444 M a v e n a e / t i l l e r on August 22, the l a s t sampling date. The i n c r e a s e i n numbers of M. avenae from August 2 to August 9 cannot be e x p l a i n e d by any recorded data. Macrosiphum avenae was recorded u n i f o r m l y throughout the p l o t and produced a l a r g e p r o p o r t i o n of a l a t e a d u l t s throughout the sampling season. On the f i r s t and l a s t sampling dates the one M. avenae a d u l t recorded was an a l a t e , and on J u l y 12, 85% of the a d u l t s recorded were a l a t e s . Although winged M. avenae were a l s o produced i n p l o t B, they d i d not appear to l e a v e the p l o t . Me to polophiurn dirhodum entered the p l o t June 18, when . 0 2 4 / t i l l e r were recorded. The p o p u l a t i o n i n c r e a s e d to a peak of 1 . 9 7 / t i l l e r on J u l y 25 and then decreased to , 5 5 6 / t i l l e r by August 22. Metopolophium dirhodum c o n t r i b u t e d . 26.36% to the t o t a l aphid p o p u l a t i o n . 11X t F i g . 2 1 . T h e p o p u l a t i o n / t i l l e r o f t h r e e s p e c i e s o f o a t a p h i d s ; M a c r o s i p h u m a v e n a e , M e t o p o l o p h i u m d i r h o d u m a n d R h o p a l o s i p h u m  P a d l f o u n d d u r i n g 1 9 7 4 i n p l o t B , . p l a n t e d w i t h o a t s , ( A v e n a ^ s a t i v a c v F r a s e r ) 4.0 M a y J u n e J u l y A u g u s t D a t e 112. They were a l s o d i s t r i b u t e d u n i f o r m l y throughout the crop and 40% of t h e i r 4th i n s t a r s were a l a t e s . Only 24% of the a d u l t s recovered were a l a t e so a p p a r e n t l y some were l e a v i n g the p l o t . In p l o t B, R. p a d i c o n t r i b u t e d 14.41% to the t o t a l aphid p o p u l a t i o n , compared wi t h o n l y 1.86% i n p l o t A. I t had entered the p l o t by June 18, when .048 i n d i v i d u a l s / t i l l e r were recorded. Rhopa10siphum p a d i a l s o peaked on J u l y 25 with 1 . 2 4 / t i l l e r , decreased to , 3 3 3 / t i l l e r by August 16 and then i n c r e a s e d again to . 8 8 9 / t i l l e r by August 22. Rhopalosiphum p a d i a l s o was u n i f o r m l y d i s t r i b u t e d throughout the p l o t , but produced very few a l a t e s . Fourteen percent of a l l 4th i n s t a r aphids and 15% o f a d u l t s were a l a t e s , i n d i c a t i n g t h a t a higher p r o p o r t i o n of R. p a d i were s t a y i n g i n the p l o t than e i t h e r of M. avenae or M. dirhodum. i i i ) P l a n t E f f e c t s Although p l o t B was p l a n t e d two weeks a f t e r p l o t A the p l a n t s grew at s i m i l a r r a t e s to s i m i l a r h e i g h t s . They averaged 12 cm i n h e i g h t with 3 113. l e a v e s when measured f i r s t on June 14. They reached t h e i r maximum h e i g h t of 108.2 cm wit h 4.17 leav e s by August 2 and so were s t i l l growing when the aphid p o p u l a t i o n peaked on J u l y 25. The p l a n t s i n p l o t B had a s l i g h t l y higher moisture content than i n p l o t A, which may have been s u f f i c i e n t t o encourage the g r e a t e r development o f R. pa d i i n p l o t B. i v ) S o i l Sample R e s u l t s The s o i l i n p l o t B had a higher moisture content throughout the e n t i r e growing season than d i d p l o t A. The average s o i l moisture o f 15.64% i n p l o t B compared t o 11.68% i n p l o t A may have produced an environment around the base of the p l a n t s humid enough to promote development of R. p a d i . v) P a r a s i t e s F i f t y of 51 mummified p a r a s i t i z e d aphids (98.04%) emerged. The p a r a s i t e s p e c i e s c o l l e c t e d c o n s i s t e d of two Praon americanum (Ashmead), f o u r A p h i d i u s avenaphis ( F i t c h ) and 19'Aphidius' o b s c u r i p e s (Ashmead). The h y p e r p a r a s i t e s p e c i e s present were two Ch a r i p s sp. and 24 Asaphes sp. 114. 4. Conclus i o n s P l o t A i n 1974 produced more aphids than p l o t B so the hig h aphid p o p u l a t i o n o f p l o t B i n 1972 was not a r e s u l t o f p l o t l o c a t i o n alone. Movement of aphids throughout both p l o t s was f a s t e r i n p l o t A, probably because of i t s denser p l a n t growth. The higher aphid p o p u l a t i o n i n p l o t A r e s u l t e d i n the p r o d u c t i o n of a l a t e s through crowding. P l o t B with fewer aphids, had very few a l a t e s . A higher aphid p o p u l a t i o n was recorded i n s i d e than o u t s i d e the p r o t e c t e d cages, i n d i c a t i n g t h a t sparrows d i d decrease the aphid p o p u l a t i o n as much as 61%. The higher R. p a d i p o p u l a t i o n and l a c k of R. p a d i a l a t e s i n p l o t B probably r e s u l t e d from the higher s o i l moisture and p l a n t moisture content on t h a t p l o t . Many p a r a s i t i z e d R. p a d i never emerged. Some c o l o n i e s s e t up i n c o n t r o l l e d c o n d i t i o n s were completely p a r a s i t i z e d . 115. SUMMARY The population dynamics of three aphid species studied on Fraser oats i n 1972, 1973 and 1974 varied considerably over the three sampling seasons. The t o t a l aphid population peaked once during each season with the peak densities ranging from 88.8 a p h i d s / t i l l e r i n 1972 to 6.76 a p h i d s / t i l l e r i n the same area i n 1974. The r e l a t i v e abundance of the three aphid species remained constant throughout each season and between seasons. Macrosiphum avenae was the most abundant species with M. dirhodum only s l i g h t l y less so. Rhopalosiphum padi was found i n every p l o t each season, although i t s numbers were very small except i n 1974, when i t contributed 26.14% to the t o t a l aphid population of plot B. With the exception of 1972, when the peak numbers for a l l species occurred on August 14, the populations of each species usually peaked i n late July or early August. In 1974, R. padi did not reach peak numbers u n t i l August 19 in p l o t A. In a l l cases the longer a species took to a t t a i n i t s peak, the higher that peak was, so the l a t e r peak i n 1972 undoubtedly contributed to the higher aphid density recorded that year. 116. Every season the p o p u l a t i o n s * peak o c c u r r e d at about the same time as the p l a n t s reached t h e i r f u l l h e i g h t and were maturing. Forbes (1962) had a l s o found M. avenae t o be the most abundant s p e c i e s a t U.B.C, wit h M. dirhodum next most abundant and R. p a d i very s c a r c e . He recorded M. avenae wi t h two peaks, t h r e e out of the f o u r years sampled, one i n June and one i n J u l y , M. dirhodum wi t h one peak each year, once i n June and three times i n J u l y , and R. p a d i w i t h one peak one year and two peaks the other year i t was recorded, a l l i n J u l y . S u b s t a n t i a l l y h i g h e r aphid d e n s i t i e s were recorded one year than the othe r t h r e e . Jones (1972) does not a r r i v e a t the same r e l a t i v e abundance of the s p e c i e s over a f i v e - y e a r r e c o r d o f a l a t e s , but i n s t e a d f i n d s M. dirhodum t o be most abundant, wi t h M. avenae next and R. p a d i s t i l l the lowest. She a l s o records, d e n s i t i e s of one year t o be much higher than the o t h e r s . Dean (1973) found R. p a d i t o be the most numerous s p e c i e s i n the a i r but the l e a s t numerous on the c e r e a l crops. He suggested t h a t c e r e a l v a r i t i e s grown i n B r i t a i n were u n p a l a t a b l e t o R. p a d i and thus not a t t r a c t i v e to 117. migrants. In 1970, he found R. padi a r r i v i n g f i r s t i n the f i e l d , but l a t e r becoming scarce. The only relationship found between t i l l e r height and aphid density i s that the plots with the shortest t i l l e r s appear to have the highest aphid numbers per t i l l e r and per cm of t i l l e r height. The l a t e r planting date of p l o t C i n 1973 produced a higher proportion of M. avenae and an increase i n development of both aphids and plants, but did not subs t a n t i a l l y change any other aspect of the population dynamics. Greene (1966) suggested that the time of planting influences aphid populations and found populations of M. dirhodum to be much higher i n early seeded f i e l d s . Sparrow (1974) also suggested that s i g n i f i c a n t populations would have to occur before the end of June for cereal aphids to be a problem and that late seeding might prevent t h i s . D i f f e rent planting methods; i . e . s o l i d blocks vs rows, did not a f f e c t the population dynamics. Fecundity t r a i l s which produced large numbers of R. padi and small numbers of M. avenae did not support observations from f i e l d sampling which showed the opposite r e s u l t s . 118. R e s u l t s s t r o n g l y i n d i c a t e t h a t weather p l a y s a s i g n i f i c a n t r o l e i n r e g u l a t i n g the aphid numbers. In 1972, heavy r a i n s on the b e e t l e s i n J u l y prevented t h e i r p o p u l a t i o n s from r e a c h i n g h i g h enough numbers to g r e a t l y a f f e c t the aphids. In the same year, an i n c r e a s e i n temperature and l a c k of p r e c i p i t a t i o n may have caused a sudden decrease i n aphid numbers from June 29 to J u l y 4, whereas c o o l e r c o n d i t i o n s accompanied by some p r e c i p i t a t i o n f o l l o w i n g the d e c l i n e prompted the aphid numbers t o i n c r e a s e . In 1973 a p o p u l a t i o n decrease i n p l o t A from May 25 t o June 7 was thought t o be due t o a 6°F drop i n maximum temperature on May 25, accompanied by low g r a s s minimum temperatures of between 35°F and 32°F from May 25 to May 28. Rhopalosiphum p a d i was used to study one aspect of weather (water) on the aphid p o p u l a t i o n s . Watering experiments supported the hypothesis t h a t R. p a d i do p r e f e r wetter c o n d i t i o n s than the other s p e c i e s . Dean (1973, 1974) and Jones (1972) and many ot h e r s suggest t h a t grasses are the p r e f e r r e d hosts of R. p a d i . 119. The combination of water p r e f e r e n c e and non-p r e f e r e n c e to oats would e x p l a i n the behavior of R. p a d i i n the f i e l d . .Imigrants would e n t e r the f i e l d i n the s p r i n g , f i n d the oats u n p a l a t a b l e , and continue moving to surrounding p r e f e r r e d g r a s s e s . As these grasses r i p e n e d and d r i e d , the R. p a d i would r e t u r n to the l e s s r i p e and more s u c c u l e n t oat crop. Although R. p a d i do not p r e f e r o a t s , f e c u n d i t y t r i a l s have shown t h a t they c e r t a i n l y can l i v e on them. A v a i l a b l e data do not e x p l a i n the success of R. p a d i when caged (see a l s o Dean (1973)), other than the conclusive e f f e c t of the i n c r e a s e d humidity w i t h i n the cages. The y e a r l y v a r i a t i o n s cannot be e x p l a i n e d s o l e y by d i f f e r e n c e s i n weather p a t t e r n s and c o c c i n e l l i d numbers d i d not seem to have a c o n t r o l l i n g e f f e c t . The e f f e c t of other n a t u r a l enemies c o u l d not be determined, as no d i r e c t samples of them were taken. The f e c u n d i t y t r i a l s d i d not e x p l a i n the r e l a t i v e abundance of the t h r e e s p e c i e s , but the experiments on water r e l a t i o n s supported the i r r e g u l a r t i m i n g of R. p a d i . I t would be i n t e r e s t i n g t o e x p l o r e f u r t h e r the r o l e of the p l a n t as a c o n t r o l l i n g agent f o r c e r e a l aphid 120. populations.through i t s moisture content, s i z e , vigour, n u t r i t i o n a l and v a r i e t a l make-up. Further studies should consider the mortality i n f l i c t e d by sparrows. 121. BIBLIOGRAPHY 1. Abernathy, C.O. and Thurston, R. 1969. P l a n t age i n r e l a t i o n to the r e s i s t a n c e of N i c o t i a n a to the green peach aphid. J . Econ. Ent. 6 2:1356-1359. 2. Adams, J.B. and Drew, M.E. 1965. G r a i n aphids i n New Brunswick. I l l Aphid p o p u l a t i o n i n h e r b i c i d e -t r e a t e d oat f i e l d s . Can. J . Z o o l . 43 (5) : 789-794. 3. Adams, J.B. and Drew, M.E. 1969. G r a i n Aphids i n New Brunswick. IV E f f e c t s of malathion and 2,4-D amine on aphid p o p u l a t i o n s and on y i e l d s of oats and b a r l e y . Can. J . Z o o l . 47(3): 423-426. 4. 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J . , Hagen, K.S., and Holloway, J.K. 1959. The c o l o n i z a t i o n and e s t a b l i s h m e n t of imported p a r a s i t e s of the s p o t t e d a l f a l f a aphid i n C a l i f o r n i a . J . Econ. Ent . 52(1):136-141. 55. Way, M.J., and Banks. 1968. P o p u l a t i o n s t u d i e s on the a c t i v e stages of the b l a c k bean aphid, Aphis fabae Scop, on i t s w i n t e r host Euonymus europeaus L. Ann. A p p l . B i o l . 62:177-197. 56. Wood j r . E.A.. ^ 1965. E f f e c t s of head and f o l i a g e i n f e s t a t i o n of the E n g l i s h g r a i n aphid on y i e l d of Triumph wheat. Processed. Ser. Okla. A g r i c . Exp. Stn. no. 522. From Rev. A p p l . Ent. 1968. 57. Wratten, S.D. 1975. The nature of the e f f e c t s of the aphids S i t o b i o n avenae and Metopolophium dirhodum on the growth of wheat. Ann. A p p l . B i o l . 79:27-34. 58. Z e c l a n t , F. 1969. Attempts to estimate the damage caused on wheat by the presence of numerous c o l o n i e s of M. avenae. T.c.no. 1-3. pp. 17-25. From Rev. A p p l . Ent. 1970. 

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