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Water mite parasitism of water boatmen (Hemiptera:Corixidae) Smith, Bruce Paul 1977

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WATER MITE PARASITISM OF WATER BOATMEN (HEMIPTERA:CORIXIDAE) by BRUCE PAUL SMITH B . S c , U n i v e r s i t y of Toronto, 1975 THESIS SUBMITTED IN PARTIAL FULFILLMENT THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n THE FACULTY OF GRADUATE STUDIES DEPARTMENT OF ZOOLOGY We accept t h i s t h e s i s as conforming to the r e q u i r e d standard THE UNIVERSITY OF BRITISH COLUMBIA September, 1977 © Bruce Paul Smith, 19 7 7 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 deg ree 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 ha t the 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 t e n p e r m i s s i o n . Depar tment o f Zoology  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 In t h i s study the consequences of water mite parasitism on water boatmen were investigated, concentrating on two host species of the genus Cenocorixa. It was established that mite parasitism severely r e s t r i c t e d egg production i n Cenocorixa b i f i d a Hung.: whether th i s should be attributed to a n u t r i t i o n a l drain or through hormonal i n t e r -vention was considered. The p o s s i b i l i t y of mite interference i n f l i g h t a b i l i t y and post-imaginal f l i g h t muscle development was also investigated. It was found that mite parasitism of C. b i f i d a i n the f i e l d varied considerably between habitats, s a l i n i t y of the lake water influencing both the mite species involved and the prevalence of mite parasitism. When tested both i n the f i e l d and laboratory, there was no apparent difference i n parasitism rates based on the sex, morph or teneral development of the host. It was concluded that individuals of a species were equally susceptible to attack. There was, however, a very d e f i n i t e difference i n s u s c e p t i b i l i t y between host species based on equivalent exposure under laboratory conditions. When C. b i f i d a and Cenocorixa expleta Uhler i n pa r t i c u l a r were compared, C. expleta was s i g n i f i c a n t l y preferred by the four main mite species i n f e c t i n g C. b i f i d a . This was sub-stantiated i n f i e l d data. Considering the prevalence of mites on C. bif I d a , and the s u s c e p t i b i l i t y of C. expleta to parasitism, the pr o b a b i l i t y of the l a t t e r being p a r a s i t i z e d approaches 100% i n lakes within the s a l i n i t y tolerance range of mites. When parasitism of these two host species was further investigated, i t became apparent that C. expleta cannot sustain mite parasitism and i n most cases, died. Past workers have noted the limited coexistence of C. expleta and C. b i f i d a . Despite both species being p h y s i o l o g i c a l -l y fresh water insects, they only cohabit lakes i n the upper s a l i n i t y range of C. b i f I d a . When the r e l a t i v e abundance of these two species was compared over the s a l i n i t y range i n which they coexist, C. expleta was rare u n t i l the upper s a l i n i t y l i m i t of mites was reached. There was a defined change i n th e i r r e l a -t i v e abundance at t h i s point, C. expleta being i n the majority when s a l i n i t y was "above this l i m i t . I t i s evident that water mites severely reduce the reproduc-t i v e success of C. expleta i n low s a l i n i t i e s . They are therefore instrumental i n influencing the outcome of any b i o l o g i c a l i n t e r -actions between C. expleta and C. b i f i d a i n these lakes. i v TABLE OF CONTENTS PAGE ABSTRACT i i TABLE OF CONTENTS i v LIST OF TABLES v i i LIST OF FIGURES v i i i ACKNOWLEDGEMENTS x i I. INTRODUCTION 1 A. The Aim 1 B. The Study Area 2 C. The Corixidae 9 D. The Mites 10 II . PHYSIOLOGY OF WATER MITE PARASITISM 14 A. Introduction 14 B. Materials and Methods 18 1. F l i g h t Experiment 18 2. F l i g h t Muscle Development Experiment 2 2 3. Egg Production 2 3 C. Results 24 1. F l i g h t Experiment 24 2. F l i g h t Muscle Development Experiment 24 3. Egg Production 24 D. Discussion 29 I I I . ECOLOGY OF WATER MITE PARASITISM 35 A. Introduction 35 V TABLE OF CONTENTS (CONTINUED) PAGE B. Materials and Methods.. 36 1. F i e l d Sampling 36 2. Host Preference Experiments 38 i . Differences i n mite preference for morph or sex groups 39 i i . Mite avoidance of par a s i t i z e d hosts... 39 i i i . Differences i n mite s u s c e p t i b i l i t y for four c o r i x i d species 39 i v . Comparison of s u s c e p t i b i l i t y to parasitism between C. b i f i d a and C. expleta 40 3. Comparison of E. infundibu1ifera #1 Growth on C. b i f i d a and C. expleta 40 C. Results 40 1. MiteoDistribution'on\C. •b i f i d a . . ". .'. . 40 i . F i e l d data 40 i i . Differences i n mite preference for morph or sex groups 69 i i i . Mite avoidance of par a s i t i z e d hosts... 70 2. Variation i n Parasitism Between Corixid Species 73 i . General 7 3 i i . Comparison of mite parasitism between C. b i f i d a and C. expleta 77 v i TABLE OF CONTENTS (CONTINUED) PAGE D. Discussion:: 89 LITERATURE CITED 100 APPENDIX I. A Key to the Water Mite Larvae Found on C o r i x i d a e i n the F r a s e r P l a t e a u Region of B. C 107 APPENDIX I I . D i s t r i b u t i o n o f Mites and Water Boatmen Encountered 116 APPENDIX I I I . H o s t - P a r a s i t e A s s o c i a t i o n Records 117 v i i LIST OF TABLES TABLE ^ PAGE I. S a l i n i t y d a t a . . . . . 6 I I . F l i g h t muscle development and p a r a s i t i s m 25 I I I . P a r a s i t i s m r a t e s of C. b i f i d a by non - r e s i d e n t mite s p e c i e s (pooled) 66 IV. Mean numbers of mites per host w i t h r e s p e c t to f l i g h t muscle development and sex 71 V. Mean numbers of mites per host with r e s p e c t t o p r e v i o u s p a r a s i t i s m 71 VI. Comparison between host s p e c i e s of p a r a s i t i s m parameters 74 v i i i LIST OF FIGURES FIGURE PAGE 1. Research area 3 2. Temperature data 7 3. EyTais-Hy drachma l i f e history 11 4. Summary of l i p i d transport and physiology 16 5. Apparatus for f l i g h t experiment 20 6. Results for f l i g h t experiment 25 7. Egg production by C. b i f i d a i n Greer Lake 27 8. Average egg production by C. b i f i d a i n Long Lake (Chilcotin) with respect to Eyl a i s spp. parasitism and f l i g h t morph 30 9. Parasitism parameters for E. i n f u n d i b u l i f e r a  #1 on C. b i f i d a i n LE 3 for the 1976/77 season 41 10. Parasitism parameters for E. i n f u n d i b u l i f e r a  #1 on C. b i f i d a i n Round-up L. for the 1976/77 season 43 11. Parasitism parameters for E. i n f u n d i b u l i f e r a  #1 on C. bifIda i n Lake Lye for the 1976/77 season 45 12. Parasitism parameters for E. i n f u n d i b u l i f e r a  #1 on C. b i f i d a i n Long Lake C h i l c o t i n for the 1976/77 season 47 13. Parasitism parameters for E. i n f u n d i b u l i f e r a  #1 on C. b i f i d a i n LE 4 for the 1976/77 season 49 i x LIST OF FIGURES (CONTINUED) FIGURE PAGE 14. Parasitism parameters for E. i n f u n d i b u l i f e r a #1 on C_. b i f i d a i n LE 5 for the 1976/77 season ^1 15. Parasitism parameters for E. i n f u n d i b u l i f e r a  #1 on C. b i f i d a i n Boitano Lake for the 1976/77 season 5 3 16. Parasitism parameters for mites on C. b i f i d a i n Near Pothole L. for the 1976/77 season 5 5 17. Parasitism parameters for mites on C_. b i f i d a i n Greer Lake for the 1976/77 season 18. Parasitism parameters for mites on C. b i f i d a i n Westwick Lake for the 1976/77 season 5 ^ 19. Parasitism parameters for Hydrachna  conjecta on C. b i f i d a i n Barkley Lake for the 1976/77 season 6 1 20. Parasitism parameters for mites on C. b i f i d a i n SP 8 for the 1976/77 season. 6 3 21. D i s t r i b u t i o n of mites p a r a s i t i c on C_. b i f i d a with respect to s a l i n i t y 67 22. Comparative s u s c e p t i b i l i t y of four host species to parasitism by E. infund1bu1ifera i l 7 5 23. A comparison between parasitism parameters for C. b i f i d a and C_. expleta i n TO Round-up Lake X LIST OF FIGURES (CONTINUED) FIGURE PAGE 24. A comparison between parasitism parameters for C. b i f i d a and C. expleta i n LE 3 80 25. A comparison of s u s c e p t i b i l i t y to parasitism between C. b i f i d a and C. expleta 82 26. The r a t i o of C. expleta to C. b i f i d a versus s a l i n i t y 85 27. A comparison of mite growth between the two Cenocorixa species 87 28. EyTal s species p a r a s i t i c on Corixidae 108 29. Hydrachna species p a r a s i t i c on Corixidae 110 x i ACKNOWLEDGEMENTS I would l i k e to express my sincere gratitude to my supervis-or, Dr. G. G. E. Scudder, for a l l his assistance with t h i s pro-j e c t . His e d i t o r i a l s k i l l s were es p e c i a l l y appreciated during the writing of thi s thesis. I also would l i k e to thank Drs. J. Adams and J. P h i l l i p s for serving on my research committee, and for t h e i r h e l p f u l advice. I am grate f u l to Drs. J. Myers and J. N. M. Smith for stimu-l a t i n g discussions and encouragement. As well, an extra thanks to Dr. Myers for reading the manuscript and o f f e r i n g suggestions. I would also l i k e to thank the various people I shared the laboratory with, for t h e i r help and f r u i t f u l discussions. In pa r t i c u l a r , John Spence and Sydney Cannings gave a great deal of assistance both i n the laboratory and the f i e l d , for which I am very g r a t e f u l . This research was supported by the National Research Council of Canada, through both an operating grant to Dr. G. G. E. Scudder, and a postgraduate scholarship to me. 1 I. INTRODUCTION. A. The Aim. At present very l i t t l e i s known about mite-arthropod associ-ations. Host-parasite records are about the only data available. There i s some information on the effects of mites on economically important arthropods, such as scutacarid mite parasitism which causes the Is l e of Wight disease i n honey bees (Hirst, 1921), but the impact of mites on insects has largely been ignored. Further, the i n t e r r e l a t i o n s h i p s of mite populations and th e i r host popula-tions are generally unexplored. For the most part information on the e f f e c t of water mites on the i r hosts i s largely speculative. There are very few accurate observations on the effects of water mite parasitism. For ex-ample, Abdel-Malek (1948) and Miyazaki (1936) showed that fecund-i t y was reduced i n para s i t i z e d mosquitoes, however Mullen (1974) states that the eff e c t s are not clear-cut. Davies (1959) con-cluded that mites may at times control Simulium decorum. Soar and Williamson (1925) recorded large numbers of dead par a s i t i z e d Corethra sp. f l o a t i n g i n lakes, and presumed the parasite load to be the cause of death. In recent times, many workers have concentrated upon the Corixidae for studying the mite-insect in t e r a c t i o n , with a r t i c l e s by Davids (197 3), Harris and Harrison (1974), Davids and Schoots (1975), and Martin (1975) . The corixid-water mite r e l a t i o n i s id e a l for study: the mites p a r a s i t i c on corixids undergo the 2 greatest size increase on the host, and the f i n a l volume r a t i o of parasite to host i s the largest recorded (Davids, 1973). It i s therefore a l o g i c a l association to study for an investigation of physiological damage to the host. The aim of thi s study was to investigate the impact of mite parasitism on water boatmen, s p e c i f i c a l l y the genus Cenocorixa. F i r s t physiologic damage to the in d i v i d u a l was assessed i n C. b i f i d a (Hung.), followed by an investigation of the ecological aspects of the rel a t i o n s h i p . In the ecology section the v a r i a -t i o n i n parasitism between environments and between host species was studied. F i n a l l y an evaluation was made of the e f f e c t of parasitism on host populations and the influence of d i f f e r e n t i a l parasite pressure between host species. B. The Study Area. This research was conducted i n the Fraser Plateau region of central B r i t i s h ' Columbia, with study s i t e s i n the v i c i n i t y of Williams Lake, Clinton, and Kamloops (Fig. 1). Most of the lakes selected for t h i s research have previously been studied by Scudder (1969a, 1969b, 1975), Topping (1969), Cannings (1973), Reynolds (19 74), and Topping and Scudder (19 77). Consequently, a fund of information on physical, chemical, and b i o t i c charac-t e r i s t i c s of these lakes was avai l a b l e . In the Williams Lake region, three study lakes (Boitano Lake, Westwick Lake, and SP 8) were located near Springhouse, while seven of the lakes (Round-up Lake,, (Phalerope Lake) , Barnes Lake (Box 4) , Lake Lye (Box 20/21) , Near Pothole Lake, Barkley Lake (Opposite Box 4), Greer Lake 3 F i g u r e 1. Research area. A. Riske Creek Region B. Springhouse Region C. C l i n t o n Region D. Kamloops Region 4 5 (Box 89), and Long Lake (Chilcotin)) were located on Becher's P r a i r i e near the town of Riske Creek. In the Clinton region, l o c a l i t i e s consisted of Long Lake (Clinton), Le 3, Le 4, and Le 5. As noted by Topping (1969), Le 1 and Le 2 have joined Long Lake (Clinton) as a r e s u l t of a r i s e i n the water table. A single lake, LB 2, was located i n Kamloops on Bachelor's Range. The lakes with Cenocorixa were selected so as to cover a broad range of s a l i n i t y . Several extra lakes were studied i n the s a l i n i t y range where C. b i f i d a and C. expleta (Uhler) coexist. Surface conductivity readings were taken i n each lake during the study period and are l i s t e d i n Table I. Conductivities are ad-justed to 25°C for standardization, and conductivities quoted i n the rest of t h i s study w i l l be maximum annual readings. These were comparable to records from previous studies. The i o n i c composition varies from lake to lake, fresher lakes being predom-inantly magnesium carbonate while i n the higher s a l i n i t y lakes sodium bicarbonate prevails (see Topping and Scudder, 1977). The lakes also varied i n f l o r a l and faunal communities, which gener-a l l y are inversely proportional i n complexity to the io n i c con-tent of the water (Hammer et a_l., 19 75). As noted by Reynolds and Reynolds (1976), the submergent vegetation i s v i r t u a l l y non-existent i n higher s a l i n i t i e s (5,900 umhos. and up i n this study), while the bottoms of most fresher lakes are densely covered. Temperature records were taken i n several lakes using monthly chart recorders (Ryan model D-30, chart 380-4) and a i r tempera-ture was recorded i n the Williams Lake v i c i n i t y (Fig. 2). From past studies (Cannings, 19 73; Jansson and Scudder, 1974; Scudder, 6 Table T. S a l i n i t y data 1976/77. APRIL MAY JULY SEPT. MAY LAKE 17-19 4-6 25-30 15-20 15-20 (18-25+) 197.6 19.7 6 19 76 1976 19 77 LB 2 — 14130+ 20470 20547 — LONG LAKE (CLINTON) - - - 17058 -BARNES LAKE 1321 10551 - 15197 10017 LE 3 - - - 13026 4361 ROUND-UP LAKE 1867 7762 10446 10700 7357 LAKE LYE 859 7159 9704 8529 6574 LONG LAKE (CHILCOTIN) - 6884 + - 8064 6887 LE 4 - - - 7986 5026 LE 5 - - - 7133 4878 BOITANO LAKE 558 5125 6730 5877 5098 NEAR POTHOLE LAKE - 3841 + - 4947 3687 GREER LAKE 2164 2743 3226 3288 2059 WESTWICK LAKE 1338 1733 2129 1923 1598 BARKLEY LAKE 784 942 883 892 845 SP 8 343 503 581 504 578 Micromhos per centimeter surface conductivity, adjusted to 25 C. 7 Figure 2. Temperature data. Presented are d a i l y tempera-ture ranges for two of the lakes monitored, and for a i r temperature at Springhouse. Temperature recorders for the lakes were placed at a depth of 30 cm. Records are for A p r i l 1976 to June 1977, with winter represented by a break i n the horizontal axis. Note that temperatures below 0°C are below the range of the temperature recorders. A. Lake Lye water temperature., B. Westwick Lake water, temperature. C. Springhouse (by Westwick Lake) a i r temperature. 8 1975), the water temperature i s quite consistent between lakes and study s i t e s , although Kamloops i s s l i g h t l y warmer and Clinton s l i g h t l y cooler than the Williams Lake region (Jansson and Scudder, 19 74). Compared to other years, the spring and summer of 19 76 were below average i n temperature, while the f a l l , winter, and following spring were above average. C. The Corixidae. Seventeen species of water boatmen can be found i n the re-search lakes, however only a f r a c t i o n of these are regularly encountered (see Appendix I I ) . This study concentrates on Cenocorixa b i f i d a and C. expleta, which were e s p e c i a l l y common in the region, and have been subjects for considerable previous work (Jansson and Scudder, 19 74; Reynolds, 1974; Scudder, 1969b, 1971, 1975; Scudder, J a r i a l and Choy, 1972; Scudder and Meredith, 1972). The Cenocorixa species are b i v o l t i v e i n high s a l i n i t y lakes (a p a r t i a l t h i r d for C. expleta i n LB 2), and monovoltive i n low s a l i n i t y habitats (Jansson and Scudder, 1974; Scudder, 19 75). They reproduce u n t i l the end of July or early August, then enter an ovarian diapause, presumably for n u t r i t i o n a l reas-ons (Jansson and Scudder, 1974; Scudder, 1975). These two species exhibit a f l i g h t muscle polymorphism, consisting of a blockage i n post-imaginal f l i g h t muscle development i n the individuals forming the f l i g h t l e s s morph (Scudder, 1971, 1975). A very high frequency of the non-flying morph i s c h a r a c t e r i s t i c of C. expleta, while C. b i f i d a exhibits this polymorphism to a lesser degree and only i n moderately saline waters,(Scudder, 1975), 10 The f l i g h t l e s s morph develops i n the f a l l , apparently only when temperatures remain below 15°C (Scudder and Meredith, 1972; Scudder, 1975) . Other common c o r i x i d species of these lakes are H e s p e r o c o r i xa laevigata (Unlet)y Cymatia americana Hussey, Sigara  b l color1p enn1s (Walley), Sigara decorate11a (Hung.), and C a l l i c o r -ixa audeni Hung. D. The Mites. Hydrachna and E y l a i s are the only two genera known to contain species p a r a s i t i z i n g water boatmen. The mites are p a r a s i t i c only as larvae, the nymph and adult being free l i v i n g predators (see Fig . 3). There are quiescent l i f e history stages, calyptostases, separating the egg, larva, nymph, and adult stages. Peculiar to mites of Hydrachnidae and Eylaidae, the nymphochrysalis (between the l a r v a l and nymphal stages), i s also associated with the host (Mitchell, 1957; Davids, 1973; Harris and Harrison, 1974). Hydrachna d i f f e r s considerably from E y l a i s despite the con-vergent l i f e history strategy. Hydrachna as a nymph and adult i s predatory on c o r i x i d eggs, and possesses an aquatic larva that a c t i v e l y hunts i t s host. E y l a i s , i n contrast, i s a predator of microcrustaceans, and i t s larvae walk upon the water's surface tension u n t i l a water boatman accidentally contacts the mite when replenishing i t s a i r supply. Hydrachna also d i f f e r s i n ov i p o s i t -ing within a i r chambers i n the stems of aquatic plants, H. skorikowi P i e r s i g being the only reported exception. Before beginning t h i s study i t was necessary to survey the mite species occurring on corixids i n t h i s region, as a l l p r e v i a 11 Figure 3. EyTals-Hydrachna l i f e history. Calyptostases are quiescent developmental stages analogous to pupation i n insects. 12 PFPion O N H O S T — P E R I O D O F F H O S T A C A L Y P T O S T A S E S ous studies had been conducted i n Europe or upper New York State It appears that the past t r a d i t i o n of applying the names or European species to North American counterparts i s often not j u s t i f i e d when l a r v a l characters are referred to. Several new species were encountered, and a detailed r e v i s i o n i s i n progress A key to the larvae of mites encountered has been included (Appendix I ) , and i t must be stressed that while I have used the European names i n several instances, these are not necessarily the same species. Nine species of mites were found on c o r i x i d hosts i n thi s region. 14 II . PHYSIOLOGY OF WATER MITE PARASITISM. A. Introduction. The e f f e c t of parasitism by water mites appears best recorded in the Corixidae. Leston (1955) suggested they could be a d i r e c t cause of mortality, while Crisp (1959) recorded interference with ovarian development. Harris (19 70) suggested mites may i n t e r f e r e mechanically and p h y s i o l o g i c a l l y with c o r i x i d f l i g h t , and propos-ed that r e s p i r a t i o n and copulation may be mechanically hindered. Davids (19 73) studied the population dynamics and ecology of the mite Hydrachna conjecta Koenike and noted reduction i n parasitism during winter on smaller hosts, and argued that damage to the host was a r e s u l t of n u t r i t i o n a l drain. More recently Davids and Schoots (1975) reported a d i r e c t c o r r e l a t i o n between host species, parasite species, and number of eggs i n the host, and presumably the n u t r i t i o n a l drain. Martin (1975) also studied mite induced host castration and suggested a hormonal rather than n u t r i t i o n a l basis. In Cenocorixa b i f i d a mite parasitism seems to prevent egg maturation i n the f l y i n g form, but non-flying females can produce at least some mature eggs when par a s i t i z e d (Jansson, 1971; Jansson and Scudder, 1974; Simpson, 1968). Young (1965a, b) presumed that the f l i g h t l e s s morph had more room i n the thorax for f a t body, used l i t t l e reserves for f l i g h t muscle development and maintenance, and had an increased metabolic e f f i c i e n c y . There i s thus a good deal of circumstantial evidence for a n u t r i -t i o n a l drain by water mite parasites which has a detrimental e f f e c t on the host. 15 During t h e i r p a r a s i t i c l a r v a l stage water mites feed on the host's haemolymph: i n insects the haemolymph serves to transfer the n u t r i t i o n a l reserves from the f a t body to the host's organs (see F i g . 4). It has been shown with other haemolymph parasites of insects that there i s a competition between the host's tissues and the parasite for these nutrients (Vinson, 19 75). In Hemiptera the n u t r i t i o n a l reserve i s stored as t r i g l y c e r i d e (Martin, 1969; Thomas, 1974). It i s relayed to the organs v i a the haemolymph as diglyceride conjugated with haemolymph proteins (Chino and G i l -bert, 1965; G i l b e r t , 1967; G i l b e r t and Chino, 1974'; Thomas, 1974). It i s reported that the long chain f a t t y acids i n the t r i - and diglycerides are necessary for ovarian yolk deposition and for both r e s p i r a t i o n and development of f l i g h t muscles (Chino and Gi l b e r t , 1965; Thomas and G i l b e r t , 1967). There appears to be evidence for shared use.of t h i s l i p i d reserve by f l i g h t muscle and ovaries, as f l i g h t muscle develop-ment i s sometimes by-passed during ovarian development (for example i n Sigara nigrolineata, Young, 1965b): i t i s well known in insects that dispersal and reproduction are temporally separ-ated because of n u t r i t i o n a l competition problems (Dingle, 1965; Johnson, 1953, 1969). If water mites exert a s u f f i c i e n t drain on the resources to compete with ovarian development i n Corixidae, i t i s reasonable to expect competition with f l i g h t muscle devel-opment and f l i g h t , as these are the two other major energy expenditures for the adult. In t h i s section the r e l a t i o n s h i p between mite parasitism and the three predicted areas of nutrient competition i s investigated. 16 F i g u r e 4. Summary of l i p i d t r a n s p o r t and p h y s i o l o g y . A. Midgut: Passes l i p i d s i n t o haemolymph as D i g l y c e r i d e s (1). Can change c h a i n l e n g t h o f f a t t y a c i d c o n s t i t u e n t s . B. Haemolymph: Tr a n s p o r t system f o r n u t r i e n t s . Brings D i g l y c e r i d e s and P h o s p h o l i p i d s to p o i n t s of development and r e s p i r a t i o n (5,6,7,8) i n c o n j u g a t i o n w i t h haemolymph p r o t e i n s . C. F a t Body: Locus of storage and conver-s i o n . Takes up s u r p l u s D i g l y c e r i d e s from the haemolymph (2) t h a t o r i g i n a t e a t the midgut. Stores l i p i d s as T r i g l y c e r i d e s , c o n v e r t i n g them to P h o s p h o l i p i d s and D i g l y c e r i d e s f o r r e l e a s e i n t o the haemo-lymph (3,4) as they are needed. D. F l i g h t Muscles: A major d r a i n of Phospho-l i p i d s d u r i n g p o s t - i m a g i n a l development (5), and of D i g l y c e r i d e s d u r i n g f l i g h t (6). E. M i t e : Feeds on haemolymph and i t s n u t r i -ents i n t r a n s p o r t (7.) . F. O v a r i e s : Demands l a r g e q u a n t i t i e s of D i -g l y c e r i d e s and P h o s p h o l i p i d s d u r i n g v i t e l l o g e n e s i s (8) . D i g l y c e r i d e s : 1,2,4,6,7,8. P h o s p h o l i p i d s : 3,5,7,8. 17 S U M M A R Y O F L I P I D T R A N S P O R T A N D P H Y S I O L O G Y 18 The r e p o r t e d f l i g h t l e s s morph advantage i s looked a t c l o s e l y , and Mart i n ' s (1975) hypothesis t h a t mites c a s t r a t e t h e i r hosts hor-monally i n order to keep them i n a d i s p e r s a l s t a t e i s c o n s i d e r e d . Evidence of a major impact of water mite p a r a s i t i s m on the ph y s i o l o g y o f C. b i f i d a i s sought. Three separate experiments were c a r r i e d out. In the f l i g h t experiment the aim was to determine i f the f l i g h t a b i l i t y of a c o r i x i d i s m o d i f i e d by mite p a r a s i t i s m . By the n u t r i t i o n a l com-p e t i t i o n h y p o t h e s i s , we would expect a r e d u c t i o n i n f l i g h t a b i l i t y f o r p a r a s i t i z e d h o s t s . I f Mar t i n ' s (1975) hypothesis was c o r r e c t , the r e v e r s e would be t r u e . The f l i g h t muscle develop-ment experiment was i n s p i r e d by the presence i n f i e l d samples of t e n e r a l c o r i x i d s b e a r i n g mature nymphochrysalids. Using the n u t r i t i o n a l d r a i n h y p othesis as a r a t i o n a l e , t h i s t e s t was de-signed to see i f mite p a r a s i t i s m was a s s o c i a t e d w i t h r e t a r d e d f l i g h t muscle development. The t h i r d experiment was undertaken to examine the e f f e c t s o f p a r a s i t i s m on egg p r o d u c t i o n . Egg pro-d u c t i o n i n hosts p a r a s i t i z e d by the three major mite s p e c i e s was compared t o t h a t of u n p a r a s i t i z e d bugs. In a d d i t i o n , a f u r t h e r i n v e s t i g a t i o n was made i n t o the r e p o r t e d d i f f e r e n c e i n egg produc-t i o n between the two f l i g h t morphs when p a r a s i t i z e d . B. M a t e r i a l s and Methods. 1. F l i g h t Experiment. F l y i n g form C. b i f i d a were c o l l e c t e d from Long Lake ( C h i l c o -t i n ) on May 5, 1977 and t r a n s p o r t e d to the l a b o r a t o r y i n i n s u l a t -ed f l a s k s . The bugs were then kept i n d e c h l o r i n a t e d water 19 without food and i n darkness at 5°C„until use. Insects were used during the f i r s t three days following t h e i r c o l l e c t i o n i n the f i e l d . This was the standard procedure used for handling corixids for experimentation. Female corixids were undergoing ovarian development at t h i s time. Individual male and female corixids were introduced into the f l i g h t chamber for a one-half-hour period during which t h e i r time spent i n f l i g h t was recorded. The f l i g h t chamber (Fig. 5) was constructed from a p l a s t i c bag 45.7 x 81.3 cm. held expanded by a wire r i n g . The bag was suspended by i t s closed end and the chamber's f l o o r was formed by attaching a 45.7 cm. square of polyethylene sheet to the bag's open end. One side of the square was not attached for a 5 cm. distance to allow access to the chamber. This assembly was sus-pended so that the f l o o r rested on the surface of a water bath held at 30°C, and a 40 watt incandescent l i g h t was placed several inches from the chamber's upper end. The chamber's design was based on the observations that a sudden change i n l i g h t i n t e n s i t y and temperature are s t i m u l i for f l i g h t i n corixids (Fernando, 1959; Popham, 1952; Scudder, 1969). A f l i g h t m i l l cannot be used with most aquatic insects as suspending them i n a i r induces a swimming behaviour rather than f l i g h t . Water boatmen that f a i l e d to f l y were examined and i f t h e i r wings were phy s i c a l l y damaged the t r a i l was discarded. Forty C. b i f i d a were tested; 10 pa r a s i t i z e d and 10 unparasit-ized of each sex. Parasitized bugs used for the test c a r r i e d one Ey l a i s spp. larva of an intermediate stage of engorgement attach-ed to the second, t h i r d or fourth abdominal tergum. The results 20 g u r e 5. A p p a r a t u s f o r f l i g h t e x p e r i m e n t . 21 22 for the two sexes were tested separately for significance with the Mann-Whitney U t e s t . 2. F l i g h t Muscle Development Experiment For t h i s experiment, teneral sexually immature C. b i f i d a were collected from Long Lake (Chilcotin) on Sept. 10, 1976. They were transported to and maintained i n the laboratory i n the same manner as for the f l i g h t experiment (page 18). i n addition, several reeds densely covered with egg masses of E y l a i s infundi-b u l i f era #1 (undescribed species) were co l l e c t e d from the same lake, and transported to the laboratory i n an insulated flask f i l l e d with lake water. In the laboratory, the mite eggs were placed at room temperature i n an open tray with enough lake water to keep the eggs submerged. To induce hatching, a 40 watt lamp was placed within 8 inches of the water's surface. When s u f f i c i e n t numbers of mites had hatched, several f l a t , rectangular tissue culture bottles (8.5 cm. x 13 cm. x 13 cm.) were p a r t i a l l y f i l l e d with lake water and innoculated with a number of mite larvae. Teneral C. b i f i d a (Stage I by Scudder, 1971) were introduced i n numbers to the tissue culture bottles, to which a square of window screening had been inserted to pro-vide a substrate for the insects to c l i n g to. The corixids were sampled at in t e r v a l s u n t i l a large proportion were parasitized by mites. One hundred water boatmen were then selected, 50 unpara-s i t i z e d and 50 p a r a s i t i z e d with one mite each, and placed i n a round rearing dish 10 cm. high by 24 cm. i n diameter. These corixids were maintained at 20-24°C i n dechlorinated water provid-ed with constant aeration. They were cleaned and fed frozen 23 brine shrimp d a i l y . After 18 days the surviving corixids were preserved and sorted with respect to f l i g h t muscle development. Scudder's (1971) stages of teneral development were used as the c r i t e r i o n , Stages I-III representing the undeveloped c l a s s . The results were tested with a Fisher Exact Probab i l i t y test for s t a t i s t i c a l s i g n i f i c a n c e . 3. Egg Production. The relationship between parasitism and egg production was investigated i n two lakes, using spring 1977 c o l l e c t i o n s of C. b i f i d a made for the ecological study (page 36). Samples were taken on A p r i l 22, May 4, May 20, and June 2. Greer Lake was one lake chosen as the three main parasites of C. b i f i d a bred i n this lake and heavily p a r a s i t i z e d the c o r i x i d species. Long Lake (Chilcotin) was also chosen, as t h i s was one of the only lakes possessing non-flying C. b i f i d a . The c orixids were dissected and the number of mature chorion-ated eggs per bug was recorded. , The water boatmen from Greer Lake were grouped as to whether they were unparasitized, p a r a s i t -ized by one of each of the three mite species (Eylais i n f u n d i b u l i -fera #1, E y l a i s d i s c r e t a Koenike, Hydrachna conjecta) or bearing an empty nymphochrysalid membrane from one of the mite species. Long Lake dissections were grouped with regard to f l i g h t morph, then further s p l i t into unparasitized, p a r a s i t i z e d with one E. i n f u n d i b u l i f e r a #1, or one E. i n f u n d i b u l i f e r a #1 membrane. In addition, the length of the mite was measured for Long Lake samples using a calibrated eyepiece g r a t i c u l e . 24 C. Results. 1. F l i g h t Experiment. F l i g h t duration varied considerably, ranging from almost two minutes to no attempt. The mean f l i g h t duration (Fig. 6) was s i g n i f i c a n t l y less for male par a s i t i z e d bugs (P<.05) but not s i g n i f i c a n t for females. The re s u l t s for the males and females were not combined as a s i g n i f i c a n t sexual difference was present. 2. F l i g h t Muscle Development Experiment. In this experiment the p a r a s i t i z e d group had 1fewer indiv i d u -als with f l i g h t muscle development past stage I I I , but a s i g n i f i -cance l e v e l of only P=.07 (see Table I I ) . The r e l a t i v e l y high mortality i n the experiment reduced the power of the test, only twenty bugs surviving. At termination, several mites had recently completed t h e i r host associated development, and were free swimming nymphs. 3. Egg Production. A reduced mean egg number per bug was observed i n a l l para-s i t i z e d groups. In Greer Lake, a l l species of mite found on C. b i f i d a were associated with a s i g n i f i c a n t reduction (Fig. 7). However, E. discreta d i f f e r e d from E. i n f u n d i b u l i f e r a #1 and H. conjecta i n that the E. discre t a p a r a s i t i z e d corixids produced some eggs i n early May. It was also noticed that E. d i s c r e t a was consistently smaller than E. i n fund ibu1i fera #1 when both were present as engorging larvae. In June there was a p a r t i a l recovery of ovarian development i n water boatmen bearing mite , nymphochrysalid membranes, although mean egg number was s t i l l l e s s . Corixids bearing E. i n f u n d i b u l i f e r a #1 membranes appeared 25 F i g u r e 6. Re s u l t s f o r f l i g h t experiment. The d i f f e r e n c e between mean f l i g h t time of p a r a s i t i z e d and u n p a r a s i t i z e d male C. b i f i d a i s s i g n i f i c a n t a t P<.05 f o r a Mann-Whitney U Tes t . The d i f f e r e n c e between the two female groups i s not s i g n i f i c a n t . P a r a s i t i z e d hosts c a r r i e d one E. i n f u n d i b u l i f e r a #1 mite each. Table I I . F l i g h t muscle development and p a r a s i t i s m . The number of C. b i f i d a t h a t had completed f l i g h t muscle development i s compared to the number t h a t had not. F i s h e r Exact P r o b a b i l i t y = .07 f o r the n u l l h ypothesis of no d i f f e r e n c e w i t h r e s p e c t to p a r a s i t i s m . P a r a s i t i z e d hosts c a r r i e d one E. i n f u n d i b u l i f e r a #1 mite each. 2 6 < Q U J • 30J ^ = .20. Z -10-z -< Female U N PARAS mZEDjPARAS ITI ZED Male Parasitized Unparasitized FLIGHT TENERAL FISHER EXACT PROBABILITY =0.0 7 27 Figure 7. Egg production by C. b i f i d a i n Greer L. with respect to parasitism. Mean egg numbers are graphed, with the sample sizes and 5% confid-ence l i m i t s above each group. The 'm' sub-s c r i p t indicates hosts bearing an empty nymphochrysalid membrane. Each par a s i t i z e d host carried one mite. 10 - I -U = U N P A R A S I T I Z E D I = I N F U N D I B U L I F E R A to oo M A Y 4 M A Y 2 0 J U N E 3 29 to be fastest i n recovery, followed by E. discr e t a and H. cohjec-ta, but the only s t a t i s t i c a l l y s i g n i f i c a n t difference was between H. conjecta and the two E y l a i s spp. In Long Lake (Chilcotin) there was a non-flying morph advant-age while the mites were on the host (see F i g . 8). S i g n i f i c a n t l y more eggs were produced by p a r a s i t i z e d non-flying bugs i n com-parison to the f l y i n g form, but there was no difference i n un-parasitized c o r i x i d s . This trend for a non-flying morph advant-age was however reversed once the mites had dropped o f f t h e i r hosts; the f l y i n g form recovered i t s ovarian development f i r s t . The average length of mites from non-flying bugs was s i g n i f i c a n t -l y less than on f l y i n g forms (Fig. 8), and t h i s suggests that the difference i n egg production was due to mites on the non-flying morph being i n an e a r l i e r stage of engorgement. D. Discussion. In general these results support the hypothesis of n u t r i t i o n -a l competition between the mite parasites and the host's organs. The f l i g h t experiment was only s i g n i f i c a n t i n the males, but this i s not too surprising. As mites compete with ovarian development, they probably obtain a large proportion of t h e i r required n u t r i -ents from those intended for egg production. In essence, the mite development replaces egg development, and i n a female host the mite only competes against f l i g h t metabolism for the further energy i t needs. As male insects do not normally have energy stores comparable to those needed for egg production (Fast, 1964; Gi l b e r t , 1967), the f l i g h t metabolism and other systems are under 30 Figure 8. Average egg production by C. b i f i d a . i n Long L. (Chilcotin) with respect to EyTais spp. parasitism and f l i g h t morph. Included above each bar i s the 5% confidence i n t e r v a l , and below each bar i s the sample s i z e . The difference between morphs of unparasitized hosts on May 20 i s s i g n i f i c a n t , as well as for the par a s i t i z e d hosts on A p r i l 22, May 20, and June 2. Mean mite length i s s i g n i f i c a n t l y d i f f e r e n t i n both cases. Each par a s i t i z e d host carried one mite. 31 APRIL22 MAY 4 o z o o 20-§ 10-cc ui 3 ILL 10 4 I I FLIGHT .NON-FLIGHT MAY 20 JUNE 2 T 10 7 10 10 T = O O SAMPLE SIZE UNPARASITIZED 10 8 O Z o o o < 10. u 5 A T 4 3 L 1 III I HHBII———»J—————L • ENGORGING MITE AMITEMEMBRANE PARASITIZED 9 5 13 7 1 9 3 2 2 Z o z 3-| 2. 1 0 L L 9 5 13 7 MITE SIZE ON PARASITIZED HOST 32 a heavier competition. In any event, we cannot accept the hy-pothesis by Martin (1975) that the mite suppresses ovarian de-velopment i n order to increase the l i k e l i h o o d of migration. By his theory the mites should be associated with an increased f l i g h t a c t i v i t y , which by thi s experiment i s c l e a r l y not the case. While the results of the f l i g h t muscle development experiment were not s i g n i f i c a n t at P=.05, I f e e l i t should not be disregard-ed, due to the small sample size and a pr o b a b i l i t y of P=.07. Also, water boatmen of Stage II or III development can be found i n July carrying mature mite nymphochrysalids. F l i g h t muscle development takes an estimated 4 to 15 days, depending on temper-ature (Scudder, 1971), while at 20-24°C mites were associated with t h e i r hosts at least 17 days, longer at lower temperatures. The presence of these Stage III corixids bearing mite nymphochrys-a l i d s would be impossible i f f l i g h t musculature was not retarded. The egg development study demonstrated that a l l three major parasites of C. b i f i d a influence egg development. Total egg production was greatly reduced, but the bugs were not permanently s t e r i l i z e d : with a l l species examined there i s a recovery of egg production after mite drop o f f . This was also recorded by Martin (1975) working with Sigara f a l l e n i . However, when any of these three mite species were i n the f i n a l stages of association with the host, there were no eggs produced. I t was noticed that the l i f e cycle of E. discreta i s s l i g h t l y staggered i n comparison with E. i n f u n d i b u l i f e r a #1, being smaller i n early spring, and becoming a nymphochrysalid and dropping o f f the host s l i g h t l y l a t e r (see also page 65). This was r e f l e c t e d i n egg production, as some water botamen bearing E. discr e t a contained mature eggs on May 4, unlike E. i n f u n d i b u l i f e r a #1 p a r a s i t i z e d bugs, but were behind E. i n f u n d i b u l i f e r a #1 i n ovarian recovery, although t h i s d i f f e r -ence was not s t a t i s t i c a l l y s i g n i f i c a n t i n t h i s study. Davids and Schoots (19 75) encountered a comparable s i t u a t i o n with H. conjecta and H. cruenta on Sigara s t r i a t a . Differences i n r e l a t i v e mite development appear also to be a factor i n comparing p a r a s i t i z e d non-flying and f l y i n g hosts. When a mite i s delayed i n development, the host can produce a few eggs in early spring, but i s l a t e r i n regaining ovarian development. It i s not surprising that the mites on the non-flying morph are behind those on the f l y i n g morph hosts; the former host develops only i n late f a l l and the mites attaching to them have very l i t t l e time for engorgement before winter. I t has been established that mites do not engorge s i g n i f i c a n t l y over the winter (Davids, 1973; Davids and Schoots, 1975), so that mites of non-flying morph hosts are behind i n development r e l a t i v e to those of f l y i n g morph hosts. I t therefore appears the non-flying morph does not gain any sele c t i v e advantage under parasitism i n t h i s case . The n u t r i t i o n a l hypothesis for host detriment appears to be adequate for explaining the parasite induced e f f e c t s on c o r i x i d hosts. While t h i s does not discount an interference i n the host's hormonal balance by the mite, i t i s unnecessary for our present understanding of t h i s r e l a t i o n s h i p . There i s no evidence for a l t e r a t i o n of secondary sexual characters as i s so often associated with hormonal castration (Askew, 1971). When we con-sider that i n c e r t a i n mite/corixid combinations eggs can s t i l l be produced i n reduced numbers (Davis and Schoots, 1975), endocrine castration i s u n l i k e l y . There appear to be c o n f l i c t i n g views i n general as to whether a parasite castration i n insects i s due to p a r a s i t i c intervention i n the endocrine system (Rockstein, 1973), or n u t r i t i o n a l drain (Engelmann, 1970). I t i s l i k e l y that both occur, but to ascribe a l l cases to one or the other i s an overly s i m p l i s t i c view. In summary, the e f f e c t of mite parasitism on corixids appears to be severe, i n t e r f e r i n g with di s p e r s a l , possibly retarding f l i g h t muscle development, and reducing fecundity of a p a r a s i t i z -ed female to a small f r a c t i o n of i t s normal production. It can be predicted that i f the incidence of parasitism i s high, mites would have a considerable impact upon the f i t n e s s of the host population. 35 II I . ECOLOGY OF WATER MITE PARASITISM. A. Introduction. Several authors have i n the past recorded population dynamics of water mite parasites and t h e i r hosts. For example, McCrae (1976) and Mullen (1974) surveyed mosquito parasitism, Davies (1959) worked with b l a c k f l i e s , and Marples (1962) studied Hydrach-na sp. on the notonectid Anisops a s s i m i l i s . The only such works on mites p a r a s i t i z i n g Corixidae were done by Davids (1973), and Harris and Harrison (1974). There has not been any study to date, however, that consider-ed the v a r i a t i o n i n parasitism between d i f f e r e n t habitats. In thi s study area, the lakes form natural gradients i n s a l i n i t y , and thus i s an obvious and convenient parameter to study. The only author to have considered v a r i a t i o n of parasitism for a given c o r i x i d species was Davids (1973), who tested for differences between the two sexes of host. Variation i n int e n s i t y of parasitism between d i f f e r e n t host species has often been recorded (Davids, 1973; Davies, 1959; Harris and Harrison, 1974; Martin, 1975; Mullen, 1974). While such possible reasons as differences i n host microhabitat, behav-iour or siz e , as well as differences i n mite preference have been suggested, there have not been any tests of these hypotheses to date. Martin (1975) suggested d i f f e r e n t levels of parasitism may i n turn a f f e c t competitive advantage between d i f f e r e n t host species, but t h i s has not been investigated further. In t h i s section I s h a l l examine v a r i a t i o n i n mite parasitism of C. b i f i d a within populations, between habitats, and compare the parasitism of C. b i f i d a with other available hosts. While host behaviour and microhabitat are undoubtedly at least i n part responsible for differences between host species, I s h a l l only be concerned with testing differences i n mite s u s c e p t i b i l i t y under equivalent exposure. In addition, the possible results of differences i n parasite pressure between the two Cenocorixa spp. w i l l be examined. As C. b i f i d a and C. expleta have a broad over-lap i n food, microhabitat and general ecology (Scudder, 1969b; Reynolds, 19 74), I f e e l a comparison of the r e l a t i v e impact of parasitism on the i r success i s meaningful. B. Materials and Methods. 1. F i e l d Sampling. Faunal samples were taken from the end of July 1976, shortly a f t e r mites had begun attaching to th e i r hosts, to early June 1977, when over 80% of the mites had dropped o f f t h e i r hosts. Co l l e c t i n g was terminated i n mid-October 1976 at freeze up, and resumed i n late A p r i l 1977 within days of the ice break-up. Samples were taken at a constant depth and location within the lake, but no attempt was made to estimate density. A "D" frame aquatic net (Wards S c i e n t i f i c , Monterey, Cal i f o r n i a ) was used throughout the study, and a white photographic developing tray f a c i l i t a t e d sorting water boatmen from the net sweepings. Sample size was kept as large as possible (up to a l i m i t of approximately 750 bugs per sample). Corixids were preserved i n 95% ethyl alcohol and i d e n t i f i e d under a dissecting microscope to species and sex, and the number and species of associated mites and mite nymphochrysalid membranes was recorded for each host. When the species i d e n t i f i c a t i o n of a mite or nymphochrysalid membrane was i n doubt, i t was mounted on a s l i d e for examination by interference contrast microscopy. In addition, mites were regularly examined under the interference contrast microscope to check the accuracy and consistency of dissecting microscope i d e n t i f i c a t i o n s . For C. b i f i d a , the prevalence (percentage parasitism), mean number of mites per host, variance of mites per host, and mean number of mites per pa r a s i t i z e d host were calculated for each mite species i n each sample. These parameters were graphed for C. b i f i d a for each lake i n which mites bred. In only four cases 1 were the sample sizes for C. b i f i d a below 50, the average sample consisting of approximately 250 of th i s species. Over 42,000 C. b i f i d a were sorted. These parameters were also calculated for other c o r i x i d species when sample sizes were s u f f i c i e n t l y large. Whenever possible, differences between the sexes and between teneral and f u l l y s c l e r o t i z e d bugs i n percent parasitism and mites per pa r a s i t i z e d host were tested for s i g n i f i c a n t differences. As well, the observed number of i n t e r s p e c i f i c multiple parasitisms (two or more species of mites on one host) were compared to the 1The following sample sizes were below 50: SP 8, July 20, 1976, N=46; SP 8, October 10, 1976, N=42; Boitano Lake, July 30, 1976, N=36; LE 4, July 26, 1976, N=48. 38 expected value when the expected frequency of parasitism was suf-f i c i e n t l y large to be tested s t a t i s t i c a l l y . When p l o t t i n g mite d i s t r i b u t i o n a l data, the presence of teneral or non-flying morph corixids with a given mite species was the only proof accepted for that mite successfully breeding in the lake. Attention was also paid to the frequency of para-sitism by mites not breeding i n the lake, which proved to be a very useful indicator of immigration. 2. Host Preference Experiments. Host preference experiments were used to try and determine: 1) i f a subset (sex or teneral stage) of the host population was susceptible to parasitism, 2) i f mites avoided a previously para-s i t i z e d host, 3) i f the variance i n parasitism between several host species was due to differences i n s u s c e p t i b i l i t y or just an a r t i f a c t of habitat differences within a lake, 4) i f differences in s u s c e p t i b i l i t y are r e a l , how do C. bifIda and C. expleta compare when exposed to the common C. b i f i d a parasites? The procedures used i n these experiments were s i m i l a r . Corix-ids for experimentation were transported and maintained u n t i l use in the standard method described e a r l i e r (page 18), and l a r v a l E. i n f u n d i b u l i f e r a #1, when used, were obtained from f i e l d c o l l e c t e d eggs as mentioned previously (page 22). Unless stated otherwise, the procedure outlined i n the f l i g h t muscle develop-ment experiment (page 22) was used for subjecting water boatmen to mite larvae and determining exposure time. If one tissue culture bottle was too small for the experiment, a round p l a s t i c rearing dish (10 cm. high x 24 cm. i n diameter) was substituted. 39 Water boatmen used i n an experiment were introduced to the tes t container simultaneously, and the water was changed several times at the end of the experiment to exclude any unattached mites. Exposure time was thus kept consistent for bugs of a given experiment. Percent parasitism and mites per host were calculated and compared i n each experiment, using contingency tables and chi-square tests respectively to esta b l i s h s t a t i s t i c a l s i g n i f i c a n c e . S p e c i f i c d e t a i l s and any deviations from the standard method are described below. i . Differences i n mite preference for morph or sex groups. For t h i s experiment f i f t e e n teneral females, sixteen teneral males, eight f l i g h t form females, and f i f t e e n f l i g h t form males of C. b i f i d a were exposed to E. i n f u n d i b u l i f e r a #1. - Both the water boatmen and mite eggs were c o l l e c t e d from Long Lake (C h i l -cotin) . The res u l t s were tested with a non-parametric ANOVA te s t . i i . Mite avoidance of par a s i t i z e d hosts. C. b i f i d a and E. inf u n d i b u l i f e r a #1 were also used i n thi s experiment, c o l l e c t e d from Long Lake ( C h i l c o t i n ) . Twenty-seven unparasitized and t h i r t y - f i v e c o r i x i d s p a r a s i t i z e d with one E. i n f u n d i b u l i f e r a #1 each were used i n thi s comparison. Results were again tested for s i g n i f i c a n t differences with a non-parametric ANOVA. i i i . Differences i n mite s u s c e p t i b i l i t y for four c o r i x i d species. This test was to estab l i s h i f differences i n in t e n s i t y of parasitism between species i s i n fac t due to differences i n attachment rates. Cymatia airier icana, H. laevigata (both c o l l e c t -ed from Greer Lake), C. expleta (from Barnes Lake), and C. b i f i d a (from Long Lake, Chilcotin) were simultaneously exposed to E. 40 i n f u n d i b u l i f e r a #1 for a 24-hour period. i v . Comparison of s u s c e p t i b i l i t y to parasitism between C. b i f i d a and C. expleta. This experiment was i n fact four separate tests, using a d i f f e r e n t mite species i n each case. C. b i f i d a for the tests were co l l e c t e d from Long Lake (Chilcotin) and C. expleta from Barnes Lake. Eggs of E. 1nfundibulifera #1 were obtained as mentioned previously, while eggs for H. skorikowi and H. conjecta were obtained from adult mites col l e c t e d i n the f i e l d and kept i n the laboratory at 20°C without food. E. d i s -creta larvae for the test were co l l e c t e d i n the f i e l d by skim-ming the surface water of SP 8. 3. Comparison of E. i n f u n d i b u l i f e r a #1 Growth on C. b i f i d a and C. expleta. Round-up Lake was chosen for t h i s study as i t possessed the largest population of C. expleta co-occurring with an endemic E. i n f u n d i b u l i f e r a #1 population. The corixids used were those c o l l e c t e d i n the f i e l d sampling (page 36). Measurements were made using a dissecting microscope equipped with a cali b r a t e d eye-piece g r a t i c u l e . C. Results. 1. Mite D i s t r i b u t i o n on C. b i f i d a . i . F i e l d data. Twelve of the f i f t e e n lakes supported breed-ing populations of mites. The analysis of mite parasitims on C. b i f i d a for these twelve lakes i s presented i n Figures 9 to 20. Five mite species were found on C. b i f i d a , however, H. cruenta #1 (undescribed species) and H. skorikowi were too few to calculate any meaningful s t a t i s t i c s . E. i n f u n d i b u l i f e r a #1, E. discre t a , 41 F i g u r e 9. P a r a s i t i s m parameters f o r E. i n f U n d i b u T i f e r a  #1 on C. b i f i d a i n LE 3 f o r the 1976/77 season. The wint e r break i s i n d i c a t e d by a do t t e d l i n e b r e a k i n g the h o r i z o n t a l a x i s . The upper curve f o r the percent p a r a s i t i s m i n l a t e May and June r e p r e s e n t s the per c e n t -age of hosts b e a r i n g empty mite nymphochrys-a l i d membranes. 43 Figure 10. Parasitism parameters for E. infundibul1fera  #1 on C. b i f i d a i n Round-up L. for the 1976/77 season. The winter break i s i n d i -cated by a dotted l i n e breaking the horizon-t a l axis. The upper curve for the percent parasitism i n late May and June represents the percentage of hosts bearing empty mite nymphochrysalid membranes. MITES PER PERCENT PARASITISM PARASITIZED HOST MITES PERHOST VARIANCE 45 Figure 11. Parasitism parameters for E. infundibul1fera  #1 on C. b i f i d a i n Lake Lye for the 1976/77 season. The winter break i s indicated by a dotted l i n e breaking the horizontal axis. The upper curve for the percent parasitism i n late May and June represents the percent-age of hosts bearing empty mite nymphochrys-a l i d membranes. P E R C E N T M I T E S P E R P A R A S I T I S M P A R A S I T I Z E D H O S T M I T E S P E R H O S T V A R I A N C E P o _ _^  to CO O Ui O In P . ? ' ' I 1 1 : 1 47 Figure 12. Parasitism parameters for E. i n f u n d i b u l i f e r a  #1 on C. b i f i d a i n Long L. C h i l c o t i n for the 1976/77 season. The winter break i s indicated by a dotted l i n e breaking the horizontal axis. The upper curve for the percent parasitism i n late May and June represents the percentage of hosts bearing empty mite nymphochrysalid membranes. 49 F i g u r e 13. P a r a s i t i s m parameters f o r E. i n f u n d i b u l i f e r a  #1 on C. b i f i d a i n LE 4 f o r the 196 7/77 season. The w i n t e r break i s i n d i c a t e d by a d o t t e d l i n e b r e a k i n g the h o r i z o n t a l a x i s . The upper curve f o r the percent p a r a s i t i s m i n l a t e May and June r e p r e s e n t s the percent-age of hosts b e a r i n g empty mite nymphochrys-a l i d membranes. P E R C E N T M I T E S P E R P A R A S I T I S M P A R A S I T I Z E D H O S T M I T E S P E R H O S T V A R I A N C E Ln O 51 Figure 14. Parasitism parameters for E. i n f u n d i b u l i f e r a #1 on C. b i f i d a i n LE 5 for the 1976/77 season. The winter break i s indicated by a dotted l i n e breaking the horizontal axis. The upper curve for the percent parasitism i n late May and June represents the percent-age of hosts bearing empty mite nympho-chrysalid membranes. 53 Figure 15. Parasitism parameters for E. i n f u n d i b u l i f e r a  #1 on C. b i f i d a i n Boitano L. for the 1976/77 season. The winter break i s indicated by a dotted l i n e breaking the horizontal axis. The upper curve for the percent parasitism i n l ate May and June represents the percentage of hosts bearing empty mite nymphochrysalid membranes. P E R C E N T M I T E S P E R 55 Figure 16. Parasitism parameters for mites on C. b i f i d a i n Near Pothole L. for the 1976/77 season. S t a t i s t i c s for the three mite species (Eylais i n f u n d i b u l i f e r a #1, E. discreta and Hydrachna conjecta) are figured. The winter break i s indicated by a dotted l i n e breaking the horizontal axis. PERCENT PARASITISM — >o to ° P ? ? MITES PER PARASITIZED HOST MITES PER HOST O o I VARIANCE O Oi 1 L_ 57 Figure 17. Parasitism parameters for mites on C. b i f i d a i n Greer Lake for the 1976/77 season. S t a t i s t i c s for the three mite species (Eylais  i n f undibulifera #1, E. discreta and Hydrachna conjecta) are figured. The winter break i s indicated by a dotted l i n e breaking the horizontal axis. P E R C E N T M I T E S P E R 00 m > Z O 59 Figure 18. Parasitism parameters for mites on C. b i f i d a i n Westwick Lake for the 1976/77 season. S t a t i s t i c s for the three mite species (Eylais i n f u n d i b u l i f e r a #1, E. discr e t a and Hydrachna  conjecta) are figured. The winter break i s indicated by a dotted l i n e breaking the horizontal axis. P E R C E N T M I T E S P E R P A R A S I T I S M P A R A S I T I Z E D H O S T M I T E S P E R H O S T V A R I A N C E W P -> o — 10 u> o u i o 1 1 1 J 1 1 1 I I 1 4 I 61 Figure 19. Parasitism parameters for Hydrachna conjecta on C. b i f i d a i n Barkley L. for the 1976/77 season. The winter break i s indicated by a dotted l i n e breaking the horizontal axis. P E R C E N T M I T E S P E R P A R A S I T I S M P A R A S I T I Z E D H O S T M I T E S P E R H O S T V A R I A N C E t o jk. o D - O O O — K J C J 0 _ K J t o O _ i o C 63 Figure 20. Parasitism parameters for mites on C. b i f i d a i n SP 8 for the 1976/77 season. S t a t i s t i c s for the three mite species (Eylais infundi-b u l i f era #1, E. discreta and Hydrachna  conjecta) are figured. The winter break i s indicated by a dotted l i n e breaking the horizontal axis. P E R C E N T M I T E S PER P A R A S I T I S M P A R A S I T I Z E D H O S T M I T E S P E R H O S T V A R I A N C E O 65 and H. c o n j e c t a were very s i m i l a r i n t h e i r p o p u l a t i o n dynamics. Attachment began i n l a t e J u l y , o f t e n with a sudden peak p e r i o d causing a very high p r e v a l e n c e , a l a r g e number of mites per host, and high v a r i a n c e . The attachment r a t e then r a p i d l y dropped and the v a r i o u s parameters l e v e l l e d o f f to an e q u i l i b r i u m . Mite attachment continued throughout August, September and i n t o Octob-er, but there was no evidence of mites a t t a c h i n g i n s p r i n g . A very s m a l l second g e n e r a t i o n was e v i d e n t as E. i n f u n d i b u l i f e r a #1 and H. c o n j e c t a were dropping o f f t h e i r hosts i n s m a l l numbers du r i n g l a t e J u l y and e a r l y August, but the v a s t m a j o r i t y of mites remained on t h e i r hosts u n t i l s p r i n g . E. 1n fund i b u l l f e r a #1 were s l i g h t l y e a r l i e r than E. d i s c r e t a and H. c o n j e c t a i n dropping o f f t h e i r h o s t s , which took p l a c e i n May and e a r l y June. C o r i x i d s b e a r i n g mites not known to breed i n a g i v e n l a k e were uncommon u n t i l a f t e r e a r l y September (Table I I I ) , which corresponds with the f a l l / s p r i n g d i s p e r s a l (Popham, 1952; Fernan-do, 1959; Scudder, 1969b). As these s t r a y mites were so frequent at t h i s time, c o n s i d e r a b l e mixing of water boatmen p o p u l a t i o n s must have o c c u r r e d and should be c o n s i d e r e d when i n t e r p r e t i n g the data. Taking the e a r l y September samples as the l a t e s t r e l i a b l e data, we can compare the d i s t r i b u t i o n of mite p a r a s i t i s m over the s a l i n i t y g r a d i e n t (see F i g . 21). E. i n f u n d i b u l i f e r a #1 was the major p a r a s i t e of C. b i f i d a i n high s a l i n i t y , t h a t i s i n lakes up to 13,000 umhos. c o n d u c t i v i t y . A d e f i n i t e peak i n p a r a s i t i s m (as high as 35% p a r a s i t i s m ) o c c u r r e d f o r t h i s mite around 7,000 to 8,000 ymhos. but i n lakes of lower s a l i n i t y t h i s mite was of 66 Table I I I . P a r a s i t i s m r a t e s of C. b i f i d a by-non - r e s i d e n t mite s p e c i e s (pooled). PERCENT PARASITISM DATE LAKE LYE ROUND-UP LONG LAKE LAKE (CLINTON) JULY 31, 1976 0.0 0.0 -AUG. 10-13, 1976 0.73 0.77 0.72 AUG. 24-26, 1976 0.58 1.30 2.01 SEPT. 7-9, 1976 1.17 1.18 4.81 SEPT. 23-24, 1976 2.66 6.92 24.77 OCT. 10, 1976 7.81 11.91 17.23 APRIL 22, 1977 6.18 8.02 -MAY 4, 1977 7.81 7.28 -MAY 20, 1977 11.48 7.79 -JUNE 3, 1977 13.92 8.19 67 Figure 21. D i s t r i b u t i o n of mites p a r a s i t i c on C. b i f i d a with respect to s a l i n i t y . The v e r t i c a l axis i s the percent parasitism for the various mites from samples taken Sept. 8, 1976. The horizontal axis represents the Sept. 15-20 surface conductivity readings for the various sample lakes. Mites not breeding i n a given lake (as defined by absence of p a r a s i t i z e d teneral hosts) have been excluded. An 'X' represents the record of a mite breeding i n the lake, but that none were found i n the Sept. 8 c o l l e c t i o n . PERCENT PARASITISM E. inf u n d i b u l i f e r a no.1 E . d i s c r e t a n O Z a c r> ^ ooo's-H! -< Z 3 o • ooo oH ooo'si-10 o ft o o o H. c o n j e c r a H . s k o r i k o w i H . c r u e n t a no . l O i 1 r K) P Ki lesser importance, and r a r e l y attained even 5% prevalence. H. skorikowi also was more successful i n high s a l i n i t y , but never reached even 1% prevalence. The upper s a l i n i t y l i m i t for t h i s species was also 13,000 ymhos. E. discreta and H. conjecta only bred i n lower s a l i n i t y lakes, up to 5,000 ymhos. E. discr e t a was found to e x i s t on teneral water boatmen i n two lakes of higher conductivity, but on only a small f r a c t i o n of one percent of the hosts. E y l a i s spp. were notably absent from Barkely Lake, which was es p e c i a l l y overgrown with submergent macrophytes. H. conjecta was very successful i n t h i s lake (over 40% of CV b i f i d a were pa r a s i t i z e d ) , and t h i s was the only lake i n which H. cruenta  #1 was found. H. cruenta #1 was, however, rarely found on C. b i f i d a , being primarily a parasite of Cy mafia' americana. In terms of t o t a l percentage of corixids bearing mites, C. b i f i d a was under a heavy parasite pressure i n the fresher waters (total parasitism up to 45%), with a second, lesser peak i n the range of 7,000-8,500 ymhos. There was a low percent parasitism of C. b i f i d a i n lakes at the t r a n s i t i o n between dominant mite species (5,000-6,000 ymhos.). Over 8,500 ymhos. the percent para-sitism was around 10-15% u n t i l the upper l i m i t for mites of 13,000 ymhos. No mites were found to breed i n Barnes Lake, Long Lake (Clinton), or LB 2 during t h i s study. i i . Differences i n mite preference for morph or sex groups. From the f i e l d data, comparisons between sexes and between tener-a l and f u l l y s c l e r o t i z e d hosts for both percent parasitism and mites per pa r a s i t i z e d host frequently showed s t a t i s t i c a l l y s i g n i f -icant differences. These differences, however, were not consis-70 tent i n d i r e c t i o n of preference even within a given lake or sample date. I t was therefore evident that testing f i e l d data would not give a trustworthy answer to thi s question. When t h i s question was tested i n the laboratory, there were differences i n mean number of mites per host between the d i f f e r -ent groups (Table IV), the rates for male f l i g h t and teneral groups being considerably d i f f e r e n t . There was,•however, l i t t l e difference for the females. When teneral development was not considered and the groups were lumped, there was no sexual d i f -ference i n mean mites per host. These re s u l t s were tested with a non-parametric ANOVA test, and the results were not s i g n i f i c a n t (P>.99). This test takes into account the numbers of mites on each i n d i v i d u a l host, and thus I att r i b u t e the va r i a t i o n i n mean mites per host to chance. i i i . Mite avoidance of pa r a s i t i z e d hosts. The expected f r e -quencies of i n t e r s p e c i f i c multiple parasitisms for the f i e l d data were r a r e l y large enough to test for significance, despite very large sample size s . There i s the requirement that an expected frequency must be f i v e or more to use a chi-square t e s t . There was rar e l y a s i g n i f i c a n t difference when these tests were possi-ble, and the d i r e c t i o n was inconsistent. When avoidance of attaching to a para s i t i z e d host was tested i n the laboratory, there was a trend towards higher attachment rates on para s i t i z e d hosts (Table V). This was, however, not. s i g n i f i c a n t (P=.20). It i s possible that some individuals are more susceptible to mite parasitism and thus are attacked at a higher rate, or that the presence of a mite increases s u s c e p t i b i l -71 Table IV. Mean numbers of mites per host with respect to f l i g h t muscle development and sex. Table V. Mean numbers of mites per host with respect to previous parasitism. 72 MALE FEMALE FLIGHT 3.7 N = 15 TENERAL 6.5 N =16 ! 4.7 N=8 I 4.9 N=15 NON—PARAMETRIC AN OVA P> .99 PARASITIZED UNPARASITIZED MALE 0.6 0.2 N=21 N = 15 1.1 0.3 FEMALE N=14 N=12 NON-PARAMETRIC ANOVA P-.20 73 i t y by weakening the host. The hypothesis for avoidance of multiple parasitism i s therefore rejected, and while a higher attachment rate to p a r a s i t i z e d hosts i s possible, I r e j e c t i t as the s i g n i f i c a n t l e v e l was too low. 2. Variation i n Parasitism Between Corixid Species. i . General. Strong differences i n i n t e n s i t y of parasitism were noticed between c o r i x i d species i n the f i e l d samples (see Table VI). In low s a l i n i t y , Cymatia americana was the major host for E. i n f u n d i b u l i f e r a #1, while C. b i f i d a was only l i g h t l y para-s i t i z e d . The' reverse was true, however, for E. d i s c r e t a . Sigara  bicoloripennis was also heavily p a r a s i t i z e d by E. discreta, but the mites showed no signs of engorgement, and resulted i n a large black spot on the host's c u t i c l e . C. b i f i d a appeared to sustain the heaviest parasitism by H. conjecta, although Q. americana, H. laevigata, and S. bicoloripennis were also strongly affected. H. conj ecta also did not appear to engorge on S_. bicoloripennis • In terms of t o t a l mite parasite pressure, C. b i f i d a appeared to be under the heaviest parasite load, followed by C. americana. H. laevigata and S. bicoloripennis were both infected by t h e i r own species of mites (E. i n f u n d i b u l i f e r a #2 on H. laevigata, E. i n f u n d i b u l i f e r a #3 and Hydrachna SP#1 on S. bicoloripennis) that were not found on any other host species. The r e s u l t s of the laboratory experiment for differences i n mite s u s c e p t i b i l i t y for four c o r i x i d species were s i g n i f i c a n t at P<.001 i n terms of both percent parasitism and mites per host (Fig. 22). Both C. expleta and Cymatia americana suffered Table VI. Comparison between host species of p a r a s i t i s m parameters. Lake SP 8, 1976 - F i e l d data Date Host E. i n f u n d i b u 1 i f e r a #1 E. d i s c r e t a H. . c o n j e c t a %P M/Par.H %P M/Par.H %P M/Par.H Aug. 25 Hespe r o c o r i x a l a e v i g a t a 6.8 1.07 7.8 1.13 4.4 1.11 Cenocorixa b i f i d a 5.9 1.00 22.6 1.52 34.3 1.51 S i g a r a b i c o l o r i p e n n i s 2.9 1.00 29.8 1.68 1.0 4.00 Sept . 8 H. l a e v i g a t a 18.3 1.05 2.5 1.33 19.2 1.48 Cymatia americana 40.9 2.07 1.5 1.00 16. 7 1.18 C. b i f i d a 2.4 1.00 22.5 1.45 30.6 1.81 S. b i c o l o r i p e n n i s 2.7 1.00 17.8 1.92 13.7 1.20 Sept . 22 H. l a e v i g a t a 3.3 1.00 10.9 1.20 9.7 3.44 C. americana 33.3 1.23 6.4 2.20 9.0 2.14 C. b i f i d a 3.3 1.00 25.2 2.00 32.5 1.98 S. b i c o l o r i p e n n i s 1.1 1.00 13.7 1.62 15. 8 1.87 %P = Percent P a r a s i t i s m ; M/Par.H = Mites Per P a r a s i t i z e d Host. Note: As i d e n t i f i c a t i o n of i n f u n d i b u l i f e r a s p e c i e s depends on a c h a r a c t e r t h a t i s o f t e n l o s t i n mounting engorged mites, I have lumped them i n t o one group. C. b i f i d a i s p a r a s i t i z e d by s p e c i e s #1 o n l y , and S. b i c o l o r i p e n n i s only by species #3. H. l a e v i g a t a , however, i s p a r a s i t i z e d by both #1 and #2. 75 Figure 2 2 . Comparative s u s c e p t i b i l i t y of four host species to parasitism by E. i n f u n d i b u l i fe ra  #1. Both results are s i g n i f i c a n t l y d i f f e r -ent at P<.001 using Chi-square tests. 76 S A M P L E S I Z E 18 19 14 77 extremely high percentages of pa r a s i t i z e d hosts, and while the mean number of mites per host were high for both species, C. expleta was much more susceptible. H. laevigata and C. b i f i d a were attacked at much lower rates, H. laevigata having a higher percent parasitism than C. b i f i d a but a lower mean number of mites per host. It i s therefore accepted that there are i n t e r -s p e c i f i c differences i n mite s u s c e p t i b i l i t y even i n equivalent exposure. i i . Comparison of mite parasitism between C. blfIda and C. expleta. In the f i e l d r e s u l t s , C. expleta i s evidently under a much higher parasite pressure from E. i n f u n d i b u l i f e r a #1 i n terms of every parasitism parameter (see Figs. 23 and 24. Note that i n four samples for LE 3 there was a sample size of less than 50 for C. expleta 2'.) . The percent parasitism i s approximately tenfold greater for C. expleta over C. b i f i d a during i n i t i a l mite attach-ment, dropping to approximately fourfold i n l a t e r samples. H. skorikowi was the only other mite species coexisting with C. expleta to any extent, but H. skorikowi was never at a high enough prevalence for meaningful s t a t i s t i c s to be calculated. When the two Cenocorixa species were exposed to four mite species i n the laboratory, C. expleta was s i g n i f i c a n t l y preferred i n a l l cases (see F i g . 25). The re s u l t s were most impressive for the two E y l a i s species, mites per host being s i g n i f i c a n t at 2The following c o l l e c t i o n s from LE 3 had sample sizes less than 50: July 26, 1976, N=19; Sept. 7, 1976, N=39; Sept. 24, 1976, N=48; Oct. 7, 1976, N=35. 78 Figure 23. A comparison between parasitism parameters for C. b i f i d a and C. expleta i n Round-up Lake . Winter i s denoted by a dotted l i n e s p l i t t i n g the horizontal axis. 80 Figure 24. A comparison between parasitism parameters for C. b i f i d a and C. expleta i n LE 3 . Winter i s denoted by a dotted l i n e s p l i t t i n g the horizontal axis. Variance of load has been omitted as i t was excessively high for C. expleta. M I T E S PER P E R C E N T P A R A S I T I S M P A R A S I T I Z E D H O S T M I T E S PER HOST o cn — ' 1 L_ / / l i I 82 Figure 25. A comparison of s u s c e p t i b i l i t y to parasitism between C. b i f i d a and C. expleta. There were s i g n i f i c a n t differences between these two species at P<.05 for each mite species tested. Chi-square tests were used. 83 100-PERCENT PARASITISM 50-E.infundibulifera E.discreta H.skorikowi H. conjecta SAMPLE A n SIZE 4 0 ° 6 9 8 1 2 0 2 0 60 76 expleta = bifida MITES PER HOST 2-1 i H E.infundibulifera E.discreta H.skorikowi H-conjecta no.l 84 P<.001, as well as for the difference i n prevalence for E. infun-d i b u l i f e r a #1. H. skorikowi and H. conjecta preferred C. expleta as well, but the difference was less dramatic (percent parasitism for both species was P<.05, mites per host was P<.001 for H. skorikowi, P<.05 for H. conjecta). An i n t e r e s t i n g relationship i s evident when the r a t i o of C. expleta to C. b i f i d a i s plotted for the lakes where they coexist ranked i n regard to increasing conductivity (Fig. 26) . A d i s t i n c t break i n the graph occurred when the mites' s a l i n i t y l i m i t s was reached, with C. expleta being much more abundant r e l a t i v e to C. b i f i d a when mites are absent. When mite prevalence i s i n the 10-15% range on C. b i f i d a , C. expleta i s moderately successful, but when around 25% on C. b i f i d a , C. expleta i s barely detectable. The r e l a t i v e abundance of C. expleta i s thus related to the success of endemic mite populations and s a l i n i t y . This graph was based on the early September samples, just p r i o r to the period of f a l l migration (see Table I I I ) . The comparison of E. i n f u n d i b u l i f e r a #1 growth on C. b i f i d a and C. expleta showed that the mites begin engorgement i n the same manner on both hosts, but most mites f a i l to grow past a certain point on C. expleta (see F i g . 27). By spring, v i r t u a l l y a l l mites on C. expleta are unengorged, while C. b i f i d a bears mite nymphochrysalids. 85 Figure 26. The r a t i o of C. expleta to C. b i f i d a versus s a l i n i t y . The r a t i o of these two species (expressed as percent C. expleta of the t o t a l Cenocorixa spp. present) was plotted against Sept. 15-20th conductivity readings for the various sample lakes. The c o r i x i d samples were made Sept. 8. 1976. Percent parasitism of C. b i f i d a by mites on th i s sample date have been included. n Percent C. expleta of Total o o I k o 1 o» o 00 p o o N r-—i m M •o T ? 5 •o o o u "o 5 * lO o LM 3 o Salinity Sal in i ty 00 Limit of M i t e s Limit of C. bif ida 87 Figure 27. A comparison of mite growth between the two Cenocorixa species. Mites are grouped i n size classes expressed as percent of t o t a l number for the p a r t i c u l a r host species. BIFIDA I EXPLETA •* *•* » X O F MITES DC UJ MITE SIZE CLASS 00 00 10 >•' 1.75 mm. +-1.25-1.75 0 .75-1 .25 T 0.25J-0.75 < 0 .25 12 32 23 63 15 72 34 36 43 51 July3! Aug.12 Aug 26 Sept.9 Sept.23 Oct.10 31 20 20 15 6 14 > s Illlllllll H i i l l Mil == 4 S A M P L E S IZE April 22 May 4 May 20 89 D. DISCUSSION When we examine the y e a r l y p o p u l a t i o n c y c l e we see t h a t w h i l e f l u c t u a t i o n s occur, there tends to be an e q u i l i b r i u m l e v e l i n each lake f o r the v a r i o u s p a r a s i t e parameters ( i . e . Boitano Lake, F i g . 15; Lake Lye, F i g . 11; LE 3, F i g . 9). The c y c l e o f t e n be-g i n s with a sudden f l u r r y of p a r a s i t i s m , r e s u l t i n g i n superpara-s i t i s m , and a high v a r i a n c e i n l o a d i n g (Long Lake, C h i l c o t i n , F i g . 12; Greer Lake, F i g . 17). However, these s u p e r p a r a s i t i z e d i n s e c t s r a p i d l y d i s a p p e a r , presumably having d i e d (Davids, 1973). Both i n t h i s study and i n t h a t of Davids (1973), dead mites were found to remain on t h e i r h o s t s , so the disappearance of super-p a r a s i t i s m cannot be a t t r i b u t e d to mites dying and dropping o f f . The number of mites per p a r a s i t i z e d host a t the end of engorge-ment i s i n v a r i a b l y c l o s e to 1.0, p a r a s i t i z e d hosts c a r r y i n g o n l y one, r a r e l y two mites each. Presumably t h i s i s the maximum l o a d i n g t h a t C. b i f i d a can endure. While i n t h i s study w i n t e r d i e - o f f noted by Davids (1973) d i d not occur, there was i n some cases a sudden drop i n the v a r i o u s parameters i n the A p r i l 22nd to May 4th i n t e r v a l , when E. i n f u n d i -b u l i f era #1 underwent a sudden growth s u r g e , ( i . e . Long Lake, C h i l c o t i n , F i g . 12; Round-up Lake, F i g . 10). Whether t h i s i s an a r t i f a c t of m i g r a t i o n or death due to a sudden energy d r a i n i s not known. I t i s p o s s i b l e t h a t t h i s corresponds to the w i n t e r d i e - o f f r e p o r t e d by Davids (1973) . There was o n l y a p a r t i a l second g e n e r a t i o n of H. c o n j e c t a , E. d i s c r e t a and E. i n f u n d i b u l i -f e r a #1 d u r i n g 1976, w h i l e Davids (1973) with H. c o n j e c t a and 90 Lanciani (1969) with E. discret a and E. i n f u n d i b u l i f e r a #2 reported two generations. I t i s l i k e l y that both differences i n latit u d e and an unseasonably cold spring and summer of 1976 are responsible for the difference. Davids (1973) and Harris (1970) showed that mite developmental time was correlated with tempera-ture. Mites dropped o f f t h e i r hosts a week or two e a r l i e r i n 1977 than 1976, and i n the summer of 1977 there was a large second generation of E. i n f u n d i b u l i f e r a #1 i n the Clinton lakes. I t thus appears that there can be a considerable v a r i a t i o n between years. Onset of ovarian diapause i n C. b i f i d a appeared to correspond with the termination of the second generation i n mites, so i t could be that the host's physiology i s the cue to prepare for winter. Davids (1973) concluded that there was no difference i n attachment rates on the two sexes based on f i e l d r e sults s i m i l a r to those of this study. M i t c h e l l (1967), however, found a most pronounced difference i n Arrenurus parasitism of odonates. One could have predicted a preference for males i n c o r i x i d s , as there i s no apparent decrease i n reproductive c a p a b i l i t y when par a s i t -ized, but the p r o b a b i l i t y of host contact i s probably too low to allow choice. There was no evidence that teneral corixids were preferred over f l i g h t forms (Table IV), although mites may take advantage of hosts immobilized during f i n a l ecdysis. It i s there-fore concluded that there i s no major difference i n attachment rates within a species, and that we can consider a population to be homogeneous i n s u s c e p t i b i l i t y to parasitism. 91 While there were d e f i n i t e upper s a l i n i t y l i m i t s for the v a r i -ous mite species (Fig. 21), the l i f e history stage i n which the s a l i n i t y i s l i m i t i n g i s not evident. If a mite species bred i n a lake, the l a r v a l mites were found to attach to the hosts throughout the summer and f a l l . There was no observed t a i l i n g o f f i n attachment rates as the season progressed and the s a l i n i t y increased. I t was not the adult or nymphal stages that were affected, as they were present even into October i n lakes such as Barnes Lake, where no mites reproduced. For H. conjecta one could suggest that as i t s l i m i t of 5,000 umhos. (Fig. 21) co-incides with the l i m i t of submerged macrophytes with air-chambered stems (in which they lay t h e i r eggs), that i t i s a lack of s u i t -able ov i p o s i t i o n s i t e s that l i m i t them. Davids (1973) stresses the importance of t h i s behaviour to ensure oxygenation of the eggs. In lakes of higher s a l i n i t y H. conjecta were found to lay eggs i n reed and rush stems, but whether these are suitable sub-strates i s not known. The only other Hydrachna species that was dis t r i b u t e d widely enough for consideration was H. skorikowi, which w i l l not lay eggs i n plants even i f they are available. It i s the only Hydrachna species to extend past the s a l i n i t y l i m i t of submergent macrophytes (Fig. 21). There i s , therefore, considerable v a r i a t i o n i n parasitism between habitats. S a l i n i t y has an obvious c o r r e l a t i o n with mite d i s t r i b u t i o n , and we consequently can group the lakes studied as to s a l i n i t y and parasitism. In the s a l i n i t i e s above 13,000 umhos. there were no mites breeding, represented by lakes LB 2, Long Lake (Clinton), and Barnes Lake. Barnes Lake sometimes supports 92 a population of mites, and i s thus a borderline to th i s c l a s s i f i -cation. In the 6,000 to 13,000 ymhos. range there are e s s e n t i a l -l y only two mite species found, namely H. skorikowi and E. infun-d i b u l i f era #1. The former i s c h a r a c t e r i s t i c a l l y only present i n low prevalence rates (0-1%), but the l a t t e r goes through a con-siderable range (5-35%), and this class can be subdivided using prevalence of E. i n f u n d i b u l i f e r a #1 as the c r i t e r i o n . The 8,500 to 13,000 ymhos. range i s characterized by a 10-15% prevalence, while at 6,500 to 8,500 ymhos. there i s a peak prevalence of E. in f u n d i b u l i f e r a #1 at 15-35%. Five thousand to 6,500 ymhos. i s a t r a n s i t i o n state: E. i n f u n d i b u l i f e r a #1 has dropped to a low prevalence (5-10%) c h a r a c t e r i s t i c of waters below 6,500 ymhos., but i t i s s t i l l too saline for the mite species c h a r a c t e r i s t i c of low s a l i n i t y . Waters below 5,000 ymhos. tended to have a low E. infundibul-i f era #1 prevalence rate i n C. b i f i d a populations, but high rates for E. discret a and H. conjecta (up to 25% and 40% resp e c t i v e l y ) . These range from c h a r a c t e r i s t i c a l l y E. discreta lakes (shoreline with open water patches, i . e . Westwick Lake, F i g . 18) to H. con-jecta lakes (densely populated with Myriophylum and other submer-gent plants, i . e . Barkely Lake, F i g . 19). Waters of over 13,000 ymhos. conductivity thus serve as a refuge from mite parasitism, while i n waters of 6,500 to 8,500 and 0 to 4,500 ymhos. there are considerable parasite loads. E. i n f u n d i b u l i f e r a #1 evidently s h i f t s from C. b i f i d a as i t s major host i n high s a l i n i t y waters to Cymatia americana i n low 93 s a l i n i t y (Fig. 21 and Table VI). Considering the choice experi-ment (Fig. 22) a change i n r e l a t i v e abundance of the mite would be s u f f i c i e n t to cause t h i s . As Cymatia americana i s much more susceptible than C. b i f i d a (approximately 4 to 5 f o l d i n the attachment experiment F i g . 22), the abundance of E. infundibu1i-fera #1 i n lower s a l i n i t i e s could be a small f r a c t i o n of i t s abundance i n higher s a l i n i t y , and therefore avoid superparasitiz-ing Cymatia americana. There may be some degree of p a r t i t i o n i n g of the c o r i x i d resource between mites when we consider species associations (Appendix III) . Such corixids as S. bicolor1pennis have t h e i r own species of mites while suffering l i t t l e or no parasitism by other mite species. As well, only r a r e l y would more than one species of mite have a high enough prevalence on the same host i n the same lake to r e s u l t i n strong competition (Fig. 21). If two species of mite were at 20% prevalence on the same host, the expected overlap i s s t i l l only 4%. As mentioned, Hydrachna i s predominantly a mite of thick vegetation, while Ey l a i s does not fare well i n thi s s i t u a t i o n ( i . e . Barkely Lake) and prefers open waters. We therefore would expect habitat preferences to reduce the p o s s i b i l i t y of competition. There was no evidence of the i n t e r s p e c i f i c multiple p a r a s i t -ism rate being less than the expected value i n the f i e l d r e s u l t s . A lower rate would have been an in d i c a t i o n of avoidance of i n t e r -s p e c i f i c competition. Furthermore, i n the laboratory experiment (Table V, page 70) there was no decrease i n the l i k e l i h o o d of attachment to a previously p a r a s i t i z e d host. Considering the li k e l i h o o d of host discovery, i t i s probably a better strategy to 94 attempt to outcompete the previously attached mite. It appears that i n t e r s p e c i f i c competition i s not a strong selective pressure in t h i s system, probably reduced because of host and habitat differences. Lanciani (1970) and Harris and Harrison (1974) discuss p a r t i t i o n i n g of attachment s i t e s on water boatmen by the mites, but I do not f e e l t h i s i s j u s t i f i e d . A c o r i x i d rarely supports more than one mite for f u l l development, and i f a large enough number of mites attack i t to warrant space competition, the c o r i x i d i s not l i k e l y to survive. Such a .discussion should be r e s t r i c t e d to parasites that have a small or imperceptible e f f e c t on the host, and thus can ex i s t i n large numbers. The observed differences i n parasitism of d i f f e r e n t hosts i n the f i e l d (Table VI) i s at least p a r t i a l l y due to d i f f e r e n t i a l attachment rate. Under equivalent exposure, there were d e f i n i t e differences i n s u s c e p t i b i l i t y between host species (Fig. 22). Host size appeared to be i n s i g n i f i c a n t , as the larger H. laevigata was much less p a r a s i t i z e d than were the smaller C. b i f i d a or Cymatia americana (Fig. 22), despite being almost twice the si z e . The exact reason why one host i s preferred over another under equivalent exposure i s not clear. Habitat differences, as pointed out by Harris (1970), are also l i k e l y to be important. For example, Cymatia americana and H. laevigata are predominantly associated with submerged plants, while C. b i f i d a and S. b i c o l o r i -pennis are most abundant i n shallows with a mud substrate. When C. b i f i d a and C. expleta were compared i n terms of para-s i t i s m rates, i t i s evident C. expleta must be under a much heavier attack (Figs. 23 and 24). When we consider the rate of 95 C. b i f i d a parasitism i n the f i e l d by E. 1nfundibulifera #1 (10-35%, F i g . 21) and the strong preference for C. expleta (approximately 7.5:1, F i g . 25), the p r o b a b i l i t y of C. expleta being p a r a s i t i z e d approaches 100% i n lakes of 7,000 to 13,000 ymhos. maximum surface conductivity. This would have disastrous effects on the breeding potential of C. expleta, assuming host effects comparable to C. b i f i d a : i t could cause the elimination of C. expleta. It appears, however, that these two c o r i x i d species are not equally affected. Engorgement begins i n the same manner on both hosts (Fig. 27), but C. expleta with completely engorged mites are rarely found; The vast majority of C. expleta i n the spring only carry mites that f a i l e d to begin engorgement. During August through October many dead C. expleta were found f l o a t i n g with p a r t i a l l y engorged mites attached. It was also noticed that i n a cage located i n Lake Lye used for holding C. expleta for experi-mentation, six out of twenty-two bugs found dead were par a s i t i z e d , while i n a sample of forty-eight l i v e c o r i x i d s , only two were para s i t i z e d (chi-square P<.05). Of forty-one p a r a s i t i z e d C. expleta taken from the cage i n late August, forty had at least one mite with some stage of engorgement, only six carried an unengorged mite. In spring f i e l d samples, only four out of s i x t y -two p a r a s i t i z e d C. expleta bore mites with some stage of engorge-ment (compared to 35 of 41 i n cage, chi-square P<.001). I con-clude that E. i n f u n d i b u l i f e r a #1 can begin engorgement on C. expleta, but the host i s k i l l e d i n the vast majority of cases. It cannot be attributed to the stylosome e f f e c t as found by Davids (1973), for i n that case no mites began engorgement, as was found i n this study with E. discreta on S_. bicoloripennis. As mentioned e a r l i e r , dead mites remain on the host, so that p o s s i b i l i t y that mites die and drop o f f C. expleta i s remote. If parasitism i s f a t a l to the vast majority of C. expleta, coexist-ence with C. b i f i d a i n the presence of mites would decimate a population of C. expleta before i t s impact on the breeding potential could be r e a l i z e d . When we observe the ra t i o s of C. expleta to C. b i f i d a over the s a l i n i t y range they coexist i n , the sudden break i n the d i s -t r i b u t i o n at the upper s a l i n i t y l i m i t for mites i s understandable (Fig. 21). From 7,000 to 8,500 ymhos. (LE 5, LE 4, Long Lake, Chilcotin) C. expleta i s just marginally detectable, despite breeding i n lakes of this range. Considering the parasitism rates of C. b i f i d a i n these waters (Fig. 21), and the host pref-erence (Fig. 25), C. expleta would probably soon be eliminated from these lakes i f i t were not for immigration. In the 8,500 to 13,000 ymhos. waters (Lake Lye, Round-up Lake, LE 3), C. expleta i s present i n moderately low numbers, the lower parasitism rates allowing some of the population to escape parasitism. Once the mites upper s a l i n i t y tolerance has been reached (between 13,000 and 15,000 ymhos.), then the r e l a t i v e abundance of C. expleta soars (Long Lake Clinton, Barnes Lake). This i s well below the maximum s a l i n i t y l e v e l for C. b i f i d a (20,000 ymhos., Scudder, 1969b). The limited coexistence of C. bl f I d a and C. expleta has been recorded previously (Scudder, 1969b). While the upper s a l i n i t y limte for coexistence i s set by C. b i f i d a ' s s a l i n i t y tolerance (20,000 ymhos.. i n C. b i f i d a compared to 29,000 ymhos. 97 in C. expleta, Scudder, 1969b), C. expleta i s p h y s i o l o g i c a l l y a fresh water insect (Scudder, J a r i a l , and Choy, 1972),and can be bred successfully i n fresh water (Cannings, 1978). As they have broad overlaps i n habitat and food requirements, b i o l o g i c a l i n -teractions were suggested to be possible l i m i t i n g factors to coexistence (Reynolds, 1974; Scudder, 1976). Reynolds (1974), however, f a i l e d to demonstrate food resource p a r t i t i o n i n g , but noticed a decrease i n r e l a t i v e abundance of C. expleta, but not C. b i f i d a , when these two species coexist. Since C. b i f i d a would act as a reservoir for mite in f e c t i o n s , attempts at coexistence i n the two Cenocorixa species i n s a l i n i -t i e s under 15,000 ymhos. could mean the extinction of C. expleta. High s a l i n i t y , therefore, serves as a refugium for C. expleta. C. expleta were found i n v i r t u a l l y every lake during the spring, but i n lower s a l i n i t i e s (under 6,000 ymhos.), females did not contain eggs. However, I do not suggest that mite parasitism i s the only factor keeping C. expleta out of fresher waters, but i s i s of fundamental importance from 7,000 to 13,000 ymhos., where the two species and mites coexist. I t i s a pote n t i a l problem to C. expleta should the l a t t e r every reproduce i n fresher:, waters. In the present range of coexistence of these Cenocorixa species, E. i n f u n d i b u l i f e r a #1 i s the only mite present i n large enough numbers to have an e f f e c t . In lower s a l i n i t i e s , i t i s l i k e l y E. discre t a could have the same e f f e c t , based on the mite suscepti-b i l i t y (Fig. 25) and prevalence (Fig. 21). The chances of a dispersing C. expleta landing i n waters under 13,000 ymhos. or over 29,000 ymhos. i s far greater than 98 finding water t h a t ' l i e s between. Few water bodies i n the research area f a l l within this s a l i n i t y range. As well, corixids are ap-parently attracted to any shiny surface, and seem to be unable to discriminate between suitable and unsuitable habitats before land-ing (Popham, 1964; Scudder, 1969b, 1976). Cannings (1977) has shown that i t may be disadvantageous for corixids to f l y from high s a l i n i t y waters to low s a l i n i t y (but not the reverse), for physio-l o g i c a l reasons. With these factors i n mind, i t could well be that the high proportion of non-flying morph i n C. expleta (Scudder, 1975) i s related to i t s slim chance of successful d i s -persal to suitable waters. C. b i f I d a , by comparison, has a wide range of acceptable lakes, very few i n t h i s region exceed i t s tolerances. While at times C. b i f i d a may have large proportions of non-flying individuals (Scudder, 1975), i n the past two springs there have been less than 1% i n every lake studied. No advantage could be established for t h i s morph under parasitism (page 29), and presumably i f i t were at an advantage i t would be i n higher proportion. Presumably f l i g h t muscle polymorphism i n C. b i f i d a must have some other benefit. The concept of parasites mediating the outcome of competition i s not new (Park, 1948). Barbehenn (1969) suggested that para-s i t e s of mammals could be a mechanism for p a r t i t i o n i n g habitats, regardless of the actual competitive advantages of the species involved. By his l o g i c , i f Parasite #1 of Host #1 has a greater detrimental e f f e c t on alternate Host #2, and a r e c i p r o c a l arrange-ment occurs with Parasite #2 of Host #2, then the coexistence of Host #1 and Host #2 would be l i m i t e d . If we designate C. b i f i d a and C. expleta as Host #1 and Host #2 respectively, and replace Parasite #2 with s a l i n i t y and Parasite #1 i s E. i n f u n d i b u l i f e r a #1, then the hypothesis would f i t the proposed s i t u a t i o n i n Cenocorixa. The two Cenocorixa species would not necessarily have to be ecological homologues, as long as t h e i r habitat pref-erence was the same and the reaction of the two host species to the s a l i n i t y and parasitism was as described. I t appears that mite parasitism can be a major impact on c o r i x i d ecology. Parasitism rates i n the f i e l d were regularly over 10% prevalence, occasionally over 40%. Considering egg pro-duction i s reduced approximately 75% for a pa r a s i t i z e d female (Figs. 7 and 8), mite parasitism represents a considerable load on the host population. By the works of Davids (1973) and Davids and Schoots (1975), as well as t h i s study, we can extrapolate t h i s load to most water boatmen. The differences i n loading and host e f f e c t among c o r i x i d species would influence t h e i r r e l a t i v e successes, and a bug under especial pressure would have to either develop defences ( i . e . stylosome e f f e c t , Sigara f a l i e n i ; Davids, 1973), avoid habitats i n which mites were abundant, or suffer the burden of a substantially lowered f i t n e s s . 100 LITERATURE CITED Abdel-Malek, A., 1948. The biology of Aedes t r i v i t a t u s . Econ. Ent. 41, 951-954. 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Trans. Roy. Soc. N.Z. '8_1, 417-466 . M a r t i n , J . , 1969. L i p i d composition of f a t body and i t s c o n t r i -b u t i o n to the maturing o o c i t e s i n P y r r h o c o r i s a p t e r u s . J . I n s e c t P h y s i o l . 15_, 1025-1045. M a r t i n , N., 1975. Observations on the r e l a t i o n s h i p between E y l a i s and Hydrachna ( A c a r i , Hydracarina) and S i g a r a spp. (Insecta:Hemiptera:Corixidae) . N.Z. J . of Z o o l . 2_, 45-50. McCrae, A. W. R., 1976. The a s s o c i a t i o n between l a r v a l p a r a s i t i c water mites (Hydracarina) and Anopheles implexus (Theobald) ( D i p t e r a , C u l i c i d a e ) . B u l l . Ent. Res. 6_6, 633-650. M i t c h e l l , R., 1957. Major e v o l u t i o n a r y l i n e s i n water mites. Syst. Z o o l . 6_, 137-148. M i t c h e l l , R., 1967. Host e x p l o i t a t i o n of two c l o s e l y r e l a t e d water m i t e s . E v o l u t i o n 21, 59-75. 104 Miyazaki, I., 19 36. Uber die schadigung der anophelesmiicken durch eine wassermilbenart. Fukuoka Acta Med. 2_9_, 5-15. Mullen, G. R., 1974. The taxonomy and bionomics of aquatic mites (Acarina:Hydrachnellae) p a r a s i t i c on mosquitoes i n North America. Ph.D. thesis, Cornell University, Ithaca, N.Y. Park, T., 1948. Experimental studies of interspecies competition. 1. Competition between populations of the flour beetles, Tribolium confusurn Duval and Tribolium castaneum Herbst. Ecol. Monogr. 18, 265-308. Popham, E. J., 1952. Notes on c o r i x i d bionomics (Hemiptera, Heteroptera) . Entomologist 8_5, 73-77. Popham, E. J., 1964. The migration of aquatic bugs with special reference to the Corixidae (Hemiptera, Heteroptera). Arch. Hydrobiol. 60_, 450-496. Reynolds, J . D., 1974. Aspects of the ecology of two species of Cehocorixa (Corixidae:Hemiptera) i n allopatry and sympatry. Ph.D. thesis, University of B r i t i s h Columbia, Vancouver, B.C. Reynolds, J . D., and S. C. P. Reynolds, 19 76. Aquatic macro-phytes of some B. C. saline lakes. Syesis 8_, 291-295. Rockstein, M., 1973. The Physiology of the Insecta, Vol. 1, 2nd e d i t . Academic Press, N. Y. 512 pp. Scudder, G. G. E., 1969a. The fauna of saline lakes on the Fraser Plateau i n B r i t i s h Columbia. Verh. Int. Verein. Theor. Angew. Limnol. 17, 430-439. 105 Scudder, G. G. E., 1969b. The d i s t r i b u t i o n of two species of Cenocorixa i n inland saline lakes of B r i t i s h Columbia. J. Entomol. Soc. B.C. 66^ , 32-41. Scudder, G. G. E., 1971. The postembryonic development of the i n d i r e c t f l i g h t muscles i n Cenocorixa b i f i d a (Hung.) (Hemiptera:Corixidae). Can. J. Zool. £9, 1387-1398. Scudder, G. G. E., 1975. F i e l d studies on the f l i g h t muscle polymorphism i n Cenocorixa (Hemiptera:Corixidae). Verh. Int. Verein. Theor. Angew. Limnol. 19, 3064-3072. Scudder, G. G. E., 1976. Water-boatmen of saline waters (Hemip-tera : Corixidae) . Iri Marine Insects, editor L. Cheng, North-Holland Publishing Co., N. Y. pp.263-289. Scudder, G. G. E., M. S. J a r i a l , and J. Choy, 19 72. Osmotic and io n i c balance i n two species of Cenocorixa (Hemiptera). J. Insect Physiol. 18, 883-895. Scudder, G. G. E., and J . Meredith, 1972. Temperature induced development i n the i n d i r e c t f l i g h t muscles of adult Cenocor-ixa (Hemiptera:Corixidae) . Dev. B i o l . 2_9, 330-336. Simpson, J. E., 1968. The f l i g h t muscle polymorphism i n Cenocor-ixa b i f i d a . M.Sc. thesis, University of B r i t i s h Columbia, Vancouver, B. C. Soar, C. D., and W. Williamson, 1925. The B r i t i s h Hydracarina. Ray Society, London 110, 1-216. Thomas, K. K., 1974. L i p i d composition of the f a t body and haemolymph and i t s r e l a t i o n to l i p i d release i n Oncopeltus fasciatus. J . Insect Physiol. 20_, 845-858. 106 Thomas, K. K., and L. I. G i l b e r t , 196 7. In. v i t r o studies on the release and transport of phospholipids. J. Insect Physiol. 13_, 963-980. Topping, M. S., 1969. Giant chromosomes, ecology, and adaptation i n Chironomus tentans. Ph.D. thesis, University of B r i t i s h Columbia, Vancouver, B. C. Topping, M. S., and G. G. E. Scudder, 19 77. Some physical and chemical features of saline lakes i n central B r i t i s h Colum-b i a . Syesis (in press). Vinson, S. B., 1975. Biochemical coevolution between parasit-oids and t h e i r hosts. In Evolutionary Strategies of P a r a s i t i c Insects and Mites, editor P. W. Price. Plenum Press, N. Y. p p.14-48. Young, E. C , 1965a. The incidence of f l i g h t polymorphism i n B r i t i s h Corixidae and description of the morphs. J . Zool. (London) 146, 567-576. Young, E. C , 1965b. Teneral development i n B r i t i s h Corixidae. Proc. Roy. Entomol. Soc. Lond. Ser. A., Gen. Entomol. 40, 159-168. 107 APPENDIX I. A KEY TO THE WATER MITE LARVAE FOUND ON CORIXIDAE IN THE FRASER PLATEAU REGION OF B.C. 1(A). S i x l e g segments (exc l u d i n g c o x a l s c l e r i t e s ) ; gnathosome s m a l l , l e s s than 1/4 of t o t a l body l e n g t h (unengorged l a r v a e ) ; a e r i a l l a r v a e by M i t c h e l l (1957) d e f i n i t i o n , weakly s c l e r o t i z e d , w i t h long l e g s and b e a r i n g many long setae; attaches to the d o r s a l r e g i o n of the host's abdomen, u s u a l l y on the second or t h i r d t e r g i t e s , very r a r e l y to the wings or pronotum i n the a i r -space behind the head 2. E y l a i s spp. 1(B). F i v e l e g segments (exc l u d i n g c o x a l s c l e r i t e s ) ; gnathosome l a r g e , making up 1/3 or more of the body l e n g t h (unengorged l a r -vae) ; a q u a t i c l a r v a e by M i t c h e l l (1957) d e f i n i t i o n , h e a v i l y s c l e r o t i z e d w i t h s h o r t stocky l e g s , w i t h few s h o r t body setae and n a t a t o r y l e g setae; attaches to the host on the i n s i d e and o u t s i d e s u r f a c e s o f the h e m i e l y t r a , to v a r i o u s e x t e r n a l r e g i o n s (head, metaxyphus, legs) but never d o r s a l abdominal regions of C o r i x i d a e 5. Hydrachna spp. 2(A). Second and t h i r d c o x a l s c l e r i t e s separate, not fused; l a r g e r mite w i t h long l e g s ; b e a r i n g a b i f i d t i b i a l claw on the palp; d o r s a l p l a t e b e a r i n g pores j o i n e d by the l o n g i t u d i n a l f u r -row, the t h i r d p a i r of pores j o i n e d by the t r a n s v e r s e furrow; wide range o f ho s t s , i n c l u d i n g most c o r i x i d s p e c i e s p r e s e n t i n 108 Figure 28. Ey l a i s species p a r a s i t i c on Corixidae. A. Ey l a i s discreta (ventral view) palp = p. B. Eylais d i s c r e t a (dorsal shield) longitudinal furrow = If transverse furrow = t f pores on dorsal s h i e l d are numbered. C. Ey l a i s i n f u n d i b u l i f e r a #1 (ventral view) f i r s t coxal s c l e r i t e = e l second coxal s c l e r i t e = e2 t h i r d coxal s c l e r i t e = e3 D. Eylais i n f u n d i b u l i f e r a #2 (dorsal shield) E. E y l a i s infund ibuTifera #1 (dorsal shield) F. Ey l a i s 1nfundibulifera #3 (dorsal shield) Leg setae omitted for c l a r i t y . 110 Figure 29. Hydrachna species p a r a s i t i c on Corixidae. A. Hydrachna skorikowi (ventral view) B. Hydrachna conjecta (coxal s c l e r i t e s ) C. Hydrachna SP#1 (ventral view) gnathosome = g. D. Hydrachna cruenta #1 (ventral view) E. Hydrachna cruenta #2 (coxal s c l e r i t e s ) Legs and swimming setae omitted for c l a r i t y . I l l 112 f r e s h e r waters (under 5,000ymhos. maximum annual c o n d u c t i v i t y ) E. ( E y l a i s ) d i s c r e t a F i g u r e 28 (A and B). 2(B). Second and t h i r d c o x a l s c l e r i t e s fused; s m a l l mite with s h o r t l e g s ; p a l p l a c k i n g t i b i a l claw; d o r s a l p l a t e with the l o n g i -t u d i n a l furrow medial to pore rows, t r a n s v e r s e furrow absent; s p e c i e s w i t h r e l a t i v e l y r e s t r i c t i v e host s p e c i f i c i t y but c o l l e c -t i v e l y e x p l o i t i n g most host s p e c i e s p r e s e n t and over a wide s a l i n i t y range 3. E. (Syneylais) spp. F i g u r e 28 (C) . 3(A). L o n g i t u d i n a l furrow extending from pores 1 or 2 to beyond pore 6, never s t a r t i n g a f t e r pore 2; i n t h i s r e g i o n only r e p o r t e d from Hesperocorixa spp., but r e p o r t e d by L a n c i a n i (1969) from S i g a r a spp. i n New York S t a t e ; found i n f r e s h e r waters (under 4,000ymhos. maximum annual c o n d u c t i v i t y ) . E. i n f u n d i b u l i f e r a #2 F i g u r e 28 fo ) . 3(B). L o n g i t u d i n a l furrow s t a r t i n g between pores 3 and 5, never before pore 3, and extending beyond pore 6; i n t h i s r e g i o n r e c o r d -ed from most c o r i x i d s p e c i e s a v a i l a b l e 4. 4(A). L o n g i t u d i n a l furrow u s u a l l y s t a r t i n g a t pore 3, o c c a s i o n -a l l y mid-way between pores 3 and 4 but always c l o s e r to pore 3 (note: when d i s c r e p a n c y between the two furrows of one specimen, accept the longer one); r e p o r t e d from Cenocorixa spp., Dasycorixa  rawsoni, Hesperocorixa l a e v i g a t a , and Cymatia americana i n t h i s r e g i o n ; found i n a wide s a l i n i t y range, up to 15,000ymhos. maxi-mum annual c o n d u c t i v i t y E. i n f u n d i b u l i f e r a #1. F i g u r e 28(E). NEW SPECIES. 113 4(B). L o n g i t u d i n a l furrow beg i n n i n g a t pore 4 or between pores 4 and 5 i n r e g i o n of 4, extending to beyond pore 6; to date only recorded on S i g a r a spp. i n f r e s h waters (under l,000umhos. maxi-mum annual c o n d u c t i v i t y ) . . . . . E . i n f u n d i b u l i f e r a #3. F i g u r e 28 ( F ) . NEW SPECIES. 5(A). Median margin of E 3 ( t h i r d c o x a l s c l e r i t e ) lh times or more as long as t h a t of E 1 ( f i r s t c o x a l s c l e r i t e ) 6. 5(B). Median margin of E 3 ( t h i r d c o x a l s c l e r i t e ) l e s s than or roughly equal l e n g t h to t h a t of E 1 ( f i r s t c o x a l s c l e r i t e ) 7. 6(A). A n t e r i o r margins of E 1 form an angle of l e s s than 90°; median margin of E 1 c l e a r l y s h o r t e r than l a t e r a l margin; s e t a EB 3 long and h a i r l i k e , o r i g i n a t i n g from the a n t e r i o r margin of E 3; l a r v a e a t t a c h to the i n n e r s u r f a c e of the h e m i e l y t r a , very r a r e l y to the underside of the p r o n o t a l d i s c ; r e p o r t e d from a wide v a r i e t y of hosts; p r e s e n t i n f r e s h e r waters (under 5,000 ymhos. maximum annual c o n d u c t i v i t y ) . . . . H . c o n j e c t a F i g u r e 29 (B). 6(B). A n t e r i o r margins of E 1 form a s t r a i g h t l i n e ; median mar-g i n o f E 1 c l e a r l y longer than l a t e r a l margin; s e t a EB 3 s h o r t and p e g - l i k e , i n s e r t e d b e f o r e the middle o f E 3; l a r v a e a t t a c h t o the e x t e r i o r of the h e m i e l y t r a , t o the eyes, head, and l e g s , r a r e -l y to the v e n t r a l body; r e p o r t e d from Cenocorixa spp., Dasycorixa  rawsoni, and Hesperocorixa l a e v i g a t a ; recorded from high s a l i n i t y waters (8,000 to 15,000ymhos. maximum annual c o n d u c t i v i t y ) but may occur i n f r e s h waters as wel l . . . . H . s k o r i k o w i F i g u r e 29 (A). NEW SPECIES. 114 7(A). Gnathosome long and tapered, approximately twice as long as basal width; posterior margin of E 3 without a thornlike pro-cess; recorded only from Sigara bicoloripennis to date; only attaches to the external surface of the hemielytra; occurs i n fresh water (under l,000umhos. maximum annual conductivity). Hydrachna SP.#1. Figure 29 (C). 7(B). Gnathosome short and stocky, approximately as long as the basal width; prominant thornlike process on posterior margin of E 3; these two species have never been recorded attached to the hemielytra; recorded primarily Cymatia americana but one species occasionally found on other hosts 8. H. cruenta complex. 8(A). Only attaches to the metaxyphus or coxal bases i n close proximity to the metaxyphus; primary host i s Cymatia americana, but has been found on Cenocorixa b i f i d a and Hesperocorixa l a e v i -gata; only recorded from fresh waters (under l,500umhos. maximum surface conductivity) H. cruenta #1. Figure 29 (D). NEW SPECIES t 8(B). Only attaches to the d i s t a l leg segments ( t a r s i ) ; so far only reported from Cymatia americana; only found i n fresh water (under l,000umhos. maximum surface conductivity). H. cruenta #2. Figure 29 (E). NEW SPECIES t T- Note: In thi s work I have considered these to be separate spec-i e s , but th e i r d i s t i n c t i o n i s not clear. Morphologically they 115 are v i r t u a l l y i d e n t i c a l , but there i s a very s t r i k i n g s i t e s p e c i f i c i t y difference, with no overlap whatsoever. As these two groups were i n d i f f e r e n t lakes, t h i s discontinuity was evident. However, u n t i l an accurate morphologic d i s t i n c t i o n can be found, the p o s s i b i l i t y of these being d i f f e r e n t populations of the same species must be considered. tr1 ro CO to tr II II II &1 fa w w H CD hi hj m o n o (D O CD CD CL hi a P J p . Ch P - H -3 ID 3 3 lQ Qg l£) l£) .tr1 CD CD 3 SP 8 Barkley L. Westwick L. Greer Lake Nr.Pothole L j Boitano L. LE 5 LE 4 Long L . , C h i l Lake Lye Round-up L. LE 3 Barnes Lake Long Lake, C l i n t o n LB 2 to to tn P -0) hi P> H O CO CO H -M H -3 CD P> r r P> to tO tO cn P -pi hi P> o o 3 0 o CD TJ 3" P> to cn P -lO P> hi P> O i CD O O hi P> ft CD 63 tr) S3 to cn P -i Q P> hi p) t r P -O 0 H O hi P -TJ CD 3 3 P -CO 03 63 tO tO D p) CO O O hi P -X P) tr •s-hi P -O i P> tO o pi CO o O hi P -X P> hi CO O 3 P -tO til 03 tO tO tO to 03 03 to to to to to to to CD CO TJ CD hi 0 O 0 hi P -X P> 3 P -O t r H -P) 3 CD 3 CO P -CO to to to to cr to to to to 03 03 03 03 03 to to 03 03 to 03 63 to to to to to to to to 03 03 03 03 03 03 03 03 03 03 to to 03 03 03 03 03 pi r t CD hi 03 0 P) CD 3 03 •< O i hi PJ o t f 3 PJ O hi c CD 3 r r P) * to 03 03 03 03 to 63 63 63 63 63 63 63 63 63 63 63 63 03 t r H H PJ P -CO P -3 H i C 3 O i P -t r c H P -H i CD hi PJ # 63 6J 63 63 6J h-1 P> P -CO p -3 H i 63 03 03 03 63 63 63 63 63 63 63 O | j CO h3 s p - H rt to CD ci CO h3 H O O | | h3 M CO > > 'xl a >V a W a s! o > H P3 X M PH H Cd O > h3 3 M a M a o o G a h3 H 117 APPENDIX I I I . HOST-PARASITE ASSOCIATION RECORDS Cenocorixa b i f i d a X X X X X Cenocorixa expleta X X X Cymatia americana X X X X X C a l l i c o r i x a audeni X X X Dasycorixa rawsoni X X Arctocorisa s u t i l i s X X X Sigara decoratella X X X Sigara bicoloripennis X X X X Sigara conocephala X Sigara penniensis X Hesperocorixa laevigata X X X X X X Hesperocorixa vulgaris X X Hesperocorixa atopodonta X Hesperocorixa michiganensis X Legend: X indicates a recorded attach- rH CNJ n ment, but not necessarily successful engorgement. Eylais infundibulifera # Eylais infundibulifera # "Eylais infundibulifera # Eylais discreta Hydrachna conjecta Hydrachna skorikowi Hydrachna cruenta #1 Hydrachna cruenta #2 Hydrachna SP#1 

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