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Effects of water mite parasitism on Cenocorixa spp. (Heteroptera: Corixidae) Bennett, Andrew M. R. 1993

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Effects of Water Mite Parasitism on Cenocorixaspp. (Heteroptera: Corixidae).ByAndrew M. R. BennettB.Sc. (Hons), U.B.C., 1988A THESIS SUBMITTED IN PARTIAL FULFILLMENT OFTHE REQUIREMENTS FOR THE DEGREE OFMASTER OF SCIENCEinTHE FACULTY OF GRADUATE STUDIES(Department of Zoology)We accept this thesis as conformingto the required standardTHE UNIVERSITY OF BRITISH COLUMBIAAugust 1993© Andrew Bennett, 1993In presenting this thesis in partial fulfilment of the requirements for an advanceddegree at the University of British Columbia, I agree that the Library shall make itfreely available for reference and study. I further agree that permission for extensivecopying of this thesis for scholarly purposes may be granted by the head of mydepartment or by his or her representatives. It is understood that copying orpublication of this thesis for financial gain shall not be allowed without my writtenpermission.(Signature)Department of Zoology The University of British ColumbiaVancouver, CanadaDate^September 28, 1993DE-6 (2/88)i iABSTRACTThe purpose of this study was to understand the mechanisms bywhich parasitic water mites exclude one species of sympatricwater boatmen from low salinity, while a sibling speciessurvives. Attachment of the larval water mite Eylaiseuryhalina Smith was studied in the laboratory on two species offlight polymorphic water boatmen, Cenocorixa bifida hungerfordiLansbury and C. expleta (Uhler). After 24 hours of exposure,prevalence and abundance of mites did not differ significantlybetween host species or host morph (sclerotized versus non-sclerotized). From this, it was concluded that mites recruitedto all host types at the same rate. By measuring prevalence andabundance of attached mites only, it was determined that thenumber of mites initially able to attach also did not differsignificantly between hosts.In analyzing the initial location of attachment of E.euryhalina on the four host types, no significant difference wasfound between species, but a significant difference wasdiscovered between sclerotized and unsclerotized morphs. Thiseffect was evident as a shift of mite attachment from the centreof abdominal segments 2, 3, and 4 on the non-sclerotized hosts,to the thoracico-abdominal membranes (T.A.M.) on the sclerotizedhosts. It is speculated that the thickness of the flying hosts'sclerotized integument forces this change in location of miteattachment.A six- to eight-day study of the morphs of each species thatare predominant in the field found significant differences inmite prevalence between hosts. Non-flying C. expleta hadsignificantly greater prevalence of mites than flying C. bifida.The number of engorging mites was also significantly greater onnon-flying C. expleta. Location of attached and engorging mitesfollowed the same trends as seen in one day experiments. Basedon these findings and initial studies, it is argued that it isthe sclerotization of C. bifida that causes a reduction in theiiiprevalence of mites over time, rather than a host species effectper se. Because on sclerotized hosts, mites can only engorge onthe T.A.M., the number of engorging mites on these hosts islimited to 2 or less, whereas greater number of mites can feedon non-sclerotized hosts. As C. expleta is normally non-flyingin the field, whereas C. bifida is predominantly flying, C.bifida has a competitive advantage where mites are present inabundances of greater than 2 mites per host.Field collections of parasitized hosts showed the samepatterns of spatial mite attachment as in the laboratory, exceptthat sclerotized hosts often had mites attached directly throughthe abdominal terga. This must have been the result of miteattachment prior to host sclerotization. Abundances of mites inthe field were greater than 2 mites per host in somecollections. The predominance of the sclerotized, flying morphof C. bifida appears to allow this species to survive at lowsalinity where mites abound. C. expleta is excluded from thesewaters, but its predominantly non-sclerotized, non-flyingcondition allows better reproduction at moderate to highsalinities in the absence of mites. The alternative methods bywhich these two closely related species of water boatmen havedealt with parasite pressure implicates mite parasitism as apossible impetus in their speciation process.ivTABLE OF CONTENTSSection^ PageAbstractTable of Contents^ ivList of Tables viiList of Figures^ ixAcknowledgements xiIntroduction^ 1A. Description of Thesis^ 1B. Overview of Water Mite Parasitism^ 11C. Overview of the Corixidae^ 22Materials and Methods^ 28A. Study Site 28B. General Techniques^ 341. Corixid Collection 342. Mite Collection^ 353. Infection Experiments^ 394. Analysis of Infection Experiments^ 405. Long-term Engorgement Experiments 426. Lake Sampling of Parasitism^ 43C. Parasitism Parameters^ 441. Measures of Quantity of Mites^ 442. Measures of Location of Mites 46V3. Statistical Testing^ 50D . Experiments and Field Samples^ 501. Initial Mite Recruitment and Attachment (1 day) ^ 502. Effect of Salinity^ 563. Mite Engorgement Experiment^ 604. Field Studies^ 63Results^ 68A. Initial Mite Recruitment and Attachment (Moderatesalinity) ^ 681. Effect of Host Species^ 682. Effect of Host Sclerotization^ 70B . Initial Mite Recruitment and Attachment (Low and Highsalinity) ^ 771. Low Salinity^ 772. High Salinity 78C. Long-term Attachment and Engorgement^ 831. Attachment^ 832. Engorgement 84D . Field Studies^ 891. Comparison of Field Parasitism and LaboratoryParasitism^ 892. Proportions of C. bifida compared to C. expleta^ 953. Proportions of Flying and Non-flying Cenocorixa spp ^ 97viDiscussion^ 99A. Initial Mite Recruitment and Attachment (ModerateSalinity) ^ 991. Effect of Host Species^ 992. Effect of Host Sclerotization^ 102B. Initial Mite Recruitment and Attachment (Low andHigh Salinity) ^ 1061. Low Salinity^ 1062. High Salinity 108C. Long-term Attachment and Engorgement^ 1101. Attachment^ 1102. Engorgement 111D. Field Studies^ 1151. Comparison of Field Parasitism and LaboratoryParasitism^ 1152. Proportions of C. bifida compared to C. expleta^ 1193. Proportions of Flying and Non-flying Cenocorixaspp^ 120E. General Discussion^ 121References^ 126Appendix 1. Mite Life History^ 133viiList of TablesTable PageTable 1 Mites recorded from Becher's Prairie plottedwith respect to salinity^ 13Table 2 Corixids recorded from Becher's Prairie plottedwith respect to salinity 23Table 3 Percentages of flying and non-flying C. bifidain studied lakes from 1962 to 1969^ 26Table 4 Dimensions and salinities of lakes studied atBecher's Prairie^1959 - 1969,^1988,^1990,^1991. 33Table 5 Quantity of parasitism by E. euryhalina onall four host types in Experiment 1^ 72Table 6 Quantity of parasitism by E. euryhalina onnon-flying C. bifida and non-flying C.^expleta(Experiment 2) ^ 74Table 7 Quantity of parasitism by E. euryhalina onflying C. bifida and flying C. expleta(Experiment 3) ^ 75Table 8 Quantity of parasitism by E.^euryhalina onnon-flying C. expleta and flying C.^expleta(Experiment 4) ^ 76Table 9 Quantity of parasitism by E.^euryhalina onall four host types at low salinity(Experiment 5) ^ 80Table 10 Quantity of parasitism by E.^euryhalina onnon-flying C. bifida and non-flying C.^expletaat high salinity^(Experiment 6) 82Table 11 Mortality of hosts and mites collected fromdead hosts over time from engorgement study onflying C. bifida and non-flying C. expleta(Experiment 7) ^ 86Table 12 Quantity of E. euryhalina attaching over6 to 8 days on flying C. bifida andnon-flying C.^expleta^(Experiment 7) ^ 87viiiTable 13 Quantity of E. euryhalina engorging over6 to 8 days on flying C. bifida andnon-flying C.^expleta^(Experiment 7) ^ 88Table 14 Quantity of parasitism by E. euryhalinaattaching on flying C. bifida in fieldcollections for the summers of 1990 and 1991... 92Table 15 Quantity of parasitism by E. euryhalinaattaching on non-flying C.^expleta in fieldcollections for the summers of 1990 and 1991... 93Table 16 Quantity of parasitism by E.^euryhalinaattaching on flying C. expleta in fieldcollections for the summer of 1991^ 94Table 17 Percent composition of Cenocorixa spp.collected in lakes for summers of 1990and 1991^ 96Table 18 Percentages of flying and non-flying Cenocorixaspp.^in lakes for the summer of 1991^ 98FigureFigure 1Figure 2aFigure 2bFigure 3Figure 4Figure 5Figure 6Figure 7Figure 8Figure 9Figure 10Figure 11ixList of FiguresPageTypical water mite life cycle^ 15Map showing location of Cariboo-Chilcotinregion of British Columbia 29Map showing location of Becher's Prairieand Kamloops in Cariboo-Chilcotin^ 29Map showing the water bodies studied onBecher's Prairie^ 30Dorsal plates of the larval mites of thegenus Eylais found in British Columbia^ 38Recorded areas of attachment of E. euryhalinaon Cenocorixa spp^ 47Divisions used to plot attachment of E.euryhalina on the terga of Cenocorixa spp^ 48Location of attachment of E. euryhalinaon all 4 host types at moderate salinityas shown by percent of total mites perhost region (Experiment 1) ^ 73Location of attachment of E. euryhalina onnon-flying C. bifida and non-flying C. expletaas shown by percent of total mites per hostregion (Experiment 2) ^ 74Location of attachment of E. euryhalina onflying C. bifida and flying C. expletaas shown by percent of total mites per hostregion (Experiment 3) ^ 75Location of attachment of E. euryhalina onnon-flying C. expleta and flying C. expletaas shown by percent of total mites per hostregion (Experiment 4)  76Location of attachment of E. euryhalina onall 4 host types at low salinity as shownshown by percent of total mites per hostregion (Experiment 5) ^ 81xFigure 12 Location of attachment of E. euryhalina onflying C. bifida and non-flying C. expletaover 6 to 8 days as shown by percent oftotal mites per host region (Experiment 7)....87Figure 13 Location of engorgement of E. euryhalina onflying C. bifida and non-flying C. expletaover 6 to 8 days as shown by percent oftotal mites per host region (Experiment 7)....88Figure 14Figure 15Figure 16Figure 17Location of attachment of E. euryhalina onflying C. bifida in field collections^ 92Location of attachment of E. euryhalina onnon-flying C. expleta in field collections....93Location of attachment of E. euryhalina onflying C. expleta in field collections^ 94Life cycle of E. euryhalina at Becher'sPrairie compared to life cycle of Cenocorixaspp. based on field data^ 134xiACKNOWLEDGEMENTSI would like to thank most of all, my parents for theircontinued support during this thesis, and my grandfather forsetting an example for me as a teacher and biologist. A greatdeal of thanks also goes to my supervisor, Dr. Geoff Scudder forfinancial support and guidance. Suggestions and reading of thisthesis were done by Dr. Martin Adamson and Dr. Murray Ismanwithout whom, the direction of the thesis would not have beenacheived. Also, I am endebted to Dr. Bruce Smith who did theoriginal research on the ecology and descriptions of the watermites. Without his work, my thesis would never have beenpossible.Other thanks goes to Gareth "Oscar Wilde" Williams for companyduring my first summer at Riske Creek. Also to Louie the cook,and Dr. Locke "Statsboy" Rowe. (Who could mention one of themwithout thinking of the other?) While at U.B.C., various oddassorted entoweenies kept me giggling such as Karen Needham, Dr.Doug "I always loved Trichoptera" Currie, Dean "Collecting inEdmonton" Mulyk, Charlene Higgins, Jayne Yack, Kathy "LabMonitor" Craig.People who helped me procrastinate include (but are notlimited to) Gregor "18 beers" Reid, Kathy Shimizu, the rest ofthe Biotechy weenies, the occasionally friendly staff of the PitBurger Bar and the S.U.B. Arcade, the Columbia Brewing Company,Twevy, Cliffy, Pavy, and the rest of the boys, and of course,Wavy Gravy. (No Art, you're not in here.) Finally, a specialthanks to Christina Canil for all her support and attempts totone down the colour of my t-shirt collection. P.S. It hasn'tworked.1INTRODUCTIONA. Description of ThesisWaterboatmen^(Heteroptera:^Corixidae)^are a commonconstituent of the insect fauna of ponds, lakes, and slow-moving rivers. The family is notable for its wide-rangingsalinity tolerance: from fresh to highly saline waters(Scudder, 1976). Two morphologically similar species of thegenus Cenocorixa are abundant in all but the most salinelakes of the British Columbia Interior, occurring in sympatrythroughout much of their range. C. bifida hungerfordiLansbury (hereafter C. bifida) occurs in lakes withconductivities of 20 to 20 000 pS cm -1 , whereas C. expleta(Uhler) normally lives in water with conductivities of 13 000to 30 000 pS cm -1 . Scudder et al. (1972) found that theexclusion was not caused by C. expleta's inability toosmoregulate at low salinity, in fact Cannings (1978)demonstrated that C. expleta could breed in freshwater in thelab. Nor was the exclusion attributable to differences inlife cycle phenologies (Jansson and Scudder, 1974), orbecause C. bifida outcompeted C. expleta when in sympatry(Reynolds, 1974) (These studies are all reviewed by Scudder,1983). The most plausible explanation so far forwarded isthat of Smith (1977) concerning the effects of parasiticwater mites (Acari: Hydracarina) on water boatmen. My thesis2is a continuation of this study.Smith (1977), studying the field distributions of C.bifida, C. expleta, and the water mites found that watermites occurred in all lakes below 13 000 pS cm -1 , but C.expleta was only found in abundance in lakes above 13 000 pScm 1 . C. bifida, however, was abundant above and below 13 000pS cm-1 . It was proposed that mite parasitism had a greatereffect on C. expleta compared to C. bifida, resulting in theexclusion of C. expleta in waters below 13 000 pS cm -1 (Smith,1977).Prevalence of mites in lakes where C. bifida and C.expleta co-occurred with the mites supported this theory. Inthe pond LE 3 (13 026 pS cm -1 on Sept. 15-20, 1977) parasitismon C. expleta was greater than 40 per cent for 6 samples fromJuly to October. Parasitism of C. bifida in these sampleswas consistently below 20 per cent. Round-up Lake (10 700 pS-1cm on Sept. 15-20, 1977) was similar, with more than 25 %parasitism of C. expleta, while less than 15 per cent of C.bifida were parasitized. Parasitism had a direct effect onhost fecundity as dissections of field collected parasitizedand unparasitized C. bifida showed significant reductions inegg production for the parasitized bugs.With respect to the size of engorging mites on C. expletaand C. bifida, C. expleta was rarely found with fully3engorged mites, whereas fully engorged mites were often foundon C. bifida. Smith (1977) concluded that the absence of C.expleta with fully engorged mites was because of highmortality of C. expleta prior to full mite engorgement,whereas C. bifida could survive even with fully engorgedmites.Laboratory studies by Smith (1977) corroborated the fieldobservations. Larval Eylais euryhalina Smith were used toinfect C. bifida from Long Lake (Chilcotin) (8 064 pS cm -1 onSept. 15-20, 1977) and C. expleta from Barnes Lake (15 197 pScm-1 ) in fresh, dechlorinated water. When equally exposed toE. euryhalina, over 90 per cent of the C. expleta wereparasitized, whereas less than 25 per cent of the C. bifidawere parasitized.While there was good evidence that mites had a greaterimpact on C. expleta than on C. bifida, the actual mechanismbehind this effect was not clear. More study was necessaryon the corixid-mite interaction in the Chilcotin region tounderstand how and why C. expleta was more susceptible. Themain objective of my study was to determine the exactmechanisms by which water mites limit C. expleta in lowsalinity lakes. In addition, the life history of the miteEylais euryhalina Smith was examined in detail. (Data andobservations about mite life history are given in Appendix4To fulfill my main objective, 5 questions were askedregarding the attachment of the mite Eylais euryhalina Smith(formerly E. infundibulifera # 1) on C. bifida and C.expleta. To answer these questions, hypotheses were formed(see below) and experiments performed (see Materials andMethods). From these experiments, a greater effect ofparasitism on one host type was concluded ifa.) more mites attached to one host type and/orb.) a greater susceptible area of mite attachment wasfound on one host (explained below: hypothesis 2b).1.) Does species of host affect a.) quantity of mitesinitially attaching and b.) where mites initially attach?la.) Hypothesis (quantity of mites)There are significantly more mites attaching initially toC. expleta than to C. bifida.This would corroborate the findings of Smith (1977) whoreported more parasitism on C. expleta compared to C. bifidawhen equally exposed to four species of mites including E.euryhalina. More mites attaching to C. expleta initially,was predicted to lead to more engorging mites later, causingreduced fecundity and survival.5lb.) Hypothesis (location of mites)There is no significant difference between the initialpattern of mite attachment on C. bifida and C. expleta (ifhost sclerotization is held constant).No difference was expected because the exact position thatthe mites choose is governed only by the morphology of theair spaces surrounding the hosts' dorsal surfaces, and theseregions are similar in both host species (Jansson, 1972).2.) Does host sclerotization (wing morph) affect a.) quantityof mites initially attaching and b.) where mites initiallyattach?2a.) Hypothesis (quantity of mites)There are significantly more mites attaching tounsclerotized (non-flying) hosts than to sclerotized (flying)hosts.The rationale for proposing this hypothesis is that mitesare able to pierce the integument of unsclerotized hosts moreeasily than sclerotized hosts, making prevalence higher onunsclerotized hosts. Since field populations of C. expletaare predominantly non-flying (unsclerotized), whereas C.bifida populations are predominantly flying (sclerotized),sclerotization could be a factor in the exclusion of C.expleta in areas where mites occur.62b.) Hypothesis (location of mites)Mites on unsclerotized hosts are uniformly spread over theentire dorsum, whereas mites on sclerotized hosts congregateon areas that remain unsclerotized throughout the host'sadult life span.This was proposed because mites are predicted to be unableto pierce the hardened terga of sclerotized hosts, therefore,they will only attach on the small, permanently unsclerotizedmembranes of these hosts. A smaller susceptible area forattachment on sclerotized C. bifida was predicted to resultin reduced overall susceptibility to mite parasitism comparedto unsclerotized C. expleta. The rationale for thisprediction is that crowded mites compete for space and do notextract as much energy from their hosts as a similar numberof evenly spaced mites. Mitchell (1968) reported up to 50 %mortality of Arrenurus spp. mites on the damselfly Cercionhieroglyphicum Brauer when mite crowding occurred.73.) Does salinity affect a.) quantity of mites initiallyattaching and b.) where mites initially attach?3a. Hypotheses (quantity of mites)Low salinity (less than 300 pS cm') does not significantlyalter the quantity of mites initially attaching, on any hosttype, compared to attachment on the same hosts at moderatesalinity. High salinity (above 18 000 pS cm) preventsattachment on all hosts.This question was asked to ensure that the effects ofmites are the same throughout the salinity regime from whichC. expleta is excluded, but significantly less above thesalinity at which E. euryhalina is found (and C. expletaabounds). The rationale for proposing the first hypothesisis that both C. bifida and C. expleta were found to be ableto regulate their body fluids at low salinity (Scudder etal., 1972). From this study, both species appear to have nophysiological disadvantage at low salinity, so there is noreason to believe that low salinity would affect one speciesdifferently than the other with respect to mite parasitism.The hypothesis regarding high salinity is proposed because E.euryhalina is not found breeding in the field at this highsalinity, and it may be because of inability to attach totheir hosts.83b. Hypotheses (Spatial Attachment)Low salinity (less than 300 pS cm") does not significantlyalter the location of mite attachment from what would occurat moderate salinity on all host types. High salinity (above18 000 pS cm') also does not alter the location of miteattachment.These hypotheses were proposed for the same reasons asabove.4.) When does the effect of mites occur, or more specificallydoes the effect occur initially and/or during miteengorgement for a.) quantity of mites and b.) location ofmites?4a.) Hypotheses (quantity of mites)More mites attach and engorge on the predominant non-sclerotized morph of C. expleta than on the predominantsclerotized morph of C. bifida over a 6 to 8 day period.In questions 1 and 2, the quantity and location of initialmite attachment was measured on different host types. Bycomparing the answers to these questions with that ofquestion 4 (long term engorgement), the duration of theeffect of mites could be determined. I formed the hypothesisof question 4a because I believed that the lack of9sclerotization in non-flying C. expleta would lead to ahigher number of mites being able to attach and engorgecompared to the flying C. bifida.4b.) Hypotheses (location of mites)Mites attaching and engorging on unsclerotized C. expletaare uniformly spread over the entire dorsum, whereas mitesattaching and engorging on sclerotized C. bifida congregateon areas that remain unsclerotized throughout the host'sadult life span.The rationale for proposing this hypothesis is as above(hypothesis 2b). Sclerotization gives C. bifida someprotection from mite parasitism whereas C. expleta has noprotection because it is predominantly unsclerotized.5.) Are the data collected from one- and six-day laboratoryexperiments representative of mite parasitism in populationsof field collected corixids for a.) quantity of mites and b.)location of mites?5a.) Hypotheses (quantity of mites)In field collections, more mites are found on C. expletathan on C. bifida. More mites are found on unsclerotizedhosts than sclerotized hosts.10This question was asked to verify that the findings in mylaboratory studies had some bearing on the exclusion of C.expleta in the field. Overall, I predicted the quantity ofmites on field-collected host types to follow the same trendsas seen in the one- and six-day laboratory experiments. Thehypothesis regarding species of host is based on the study ofSmith (1977). He found more mites attaching to C. expletathan C. bifida in several lake samples in which the two hostspecies occur in sympatry with E. euryhalina. I predictedthat more mites would be found on unsclerotized hosts in thefield because of their greater surface area that issusceptible to attachment (see hypothesis 2a).5b.) Hypotheses (location of mites)In field collections, there is no difference in thespatial attachment patterns of mites on C. expleta and C.bifida (field collections are similar to laboratory data,hypothesis lb). Mites found on unsclerotized hosts areuniformly spread over the entire dorsum (field collectionsare similar to laboratory data, hypothesis 2b). Mites foundon sclerotized hosts are also spread over the entire dorsum(field collections differ from laboratory data, hypothesis2b).11The hypothesis concerning species of host was predictedbecause the interspecific differences are not great enough tocause differences in spatial mite attachment (see hypothesislb). Field-collected unsclerotized hosts were predicted tohave the same mite attachment as in the laboratory becausethey remain unsclerotized their entire adult life, thereforepresenting the same attachment area to mites as theunsclerotized hosts in one and six day experiments. Thesclerotized field-collected hosts were predicted to bedifferent from laboratory experiments because in thelaboratory, the attachment of mites occurs aftersclerotization (by design), whereas in the field, attachmentmay occur prior, during or after sclerotization. Miteattachment prior to sclerotization could occur anywhere onthe host's dorsum, causing potential differences in predictedattachment patterns between field and laboratory infectedsclerotized hosts.B. Overview of Water Mite ParasitismLarval water mites form an ecological group known as theHydracarina (= Hydrachnellae = Hydrachnidia). According tothe classification of Prasad and Cook (1972) , they belong tothe Subclass Acari, Order Acariformes, and Suborder Parasit-engona although the higher classification of the Acari is12much debated (O'Connor, 1984). I. M. Smith and Oliver (1986)have reviewed the literature on larval parasitic water mitesand their hosts. At Becher's Prairie, 6 species of mitesfrom 2 genera have been recorded parasitizing water boatmen(Smith, 1977) (See Table 1). Of these, only Eylais discretaKoenike (Acari: Eylaidae) and Hydrachna cruenta Muller(Acari: Hydrachnidae) are Old World species. The Eylaisspecies found in North America are described in Smith (1986),and the Hydrachna species are described in Smith (1987). Myinvestigation is the first since the initial description ofthe ecology of one of these species: Eylais euryhalina Smith.The distribution of water mites at Becher's Prairieextends from the freshest ponds to waters of about 13 000 pScm' (moderate salinity) (Smith, 1977) although only twospecies, E. euryhalina and H. barri Smith are present inmoderate salinity. E. euryhalina is the only species of mitethat occurs in both fresh and moderately saline water,therefore making it the best candidate for a study across arange of salinities.Members of the genus Eylais parasitize long-lived adultaquatic insects such as those of the family Corixidae. Theyalso parasitize other Hemiptera including giant water bugs(Belostomatidae) (Lanciani, 1969) and backswimmers(Notonectidae) (Stout, 1953), as well as aquatic beetles of13MitesEylais euryhalina B BEylais lancianii ? BEylais discreta B BHydrachna davidsi B B BHydrachna barriHydrachna cruenta ? BCorixidaeCenocorixa bifida B B BCenocorixa expleta ? RB B BB bBB BB B B B bR b B B B.--1-,-1.c^a,c._)En^10^'0of^c cc a)^.-4o^>1 M^0'4^t-4^CO^C4Bodies of WaterIncreasing Salinity ^ >Table 1. Mites recorded from Becher's Prairie plotted withrespect to salinity (extracted from Smith, 1977).B = breeding in abundanceb = breeding, but not in abundanceR = recorded, but not necessarily breedingCenocorixa spp. of corixids shown for reference to salinity.• Round-up Lake was lower in salinity than Barnes Lake before1979, but has increased in salinity more than the other lakes inthe area, and now has no breeding mites and is home to almostexclusively C. expleta.14the families Dytiscidae (Aiken, 1985), Gyrinidae andNoteridae (Piatakov, 1916 quoted from Smith and Oliver,1986), Hydrophilidae and Hydraenidae (Lanciani, 1970b), andHaliplidae (Nielsen and Davids, 1975).The general parasitic water mite life cycle is depicted inFigure 1 (adapted from Harris and Harrison, 1974). Intemperate zones, parasitic water mites overwinter as larvaeon their hosts. In the spring, the partially engorged larvaecontinue their engorgement until they reach a critical sizeat which time they cease engorgement and pupate into asessile nymphocrysalis (protonymph) on the back of theirhost. After a brief period (relative to the larval stage),the developed protonymph breaks through the larval cuticle inwhich it pupated becoming a free-living deutonymph. Theimmature nymph actively feeds on ostracods and cladocerans(corixid eggs for the genus Hydrachna) for a short time untilit once more pupates, this time on a submerged substrate,forming a teleiochrysalis (tritonymph). The fully developedtritonymph breaks through its nymphal cuticle emerging as adioecious, octopod adult.Nymphs and adults of the genus Eylais can be identified bytheir bright red colour, relatively large size (up to 16 mmin length), and their habit of trailing the fourth pair oflegs behind them when they swim (Lanciani, 1969). CopulationLarvaTeleiochrysalisNymphNymphochrysalisEggPre-larvaFree-living C:3 Parasitic 16MFigure 1. Typical Water Mite Life Cycle. The length of arrowsapproximates the duration of the respective life stages based onthe life cycle of E. euryhalina during the spring and summer of1991. For comparison, the adult stage was approximately 1 monthin duration during June and July, 1991.16takes place shortly following emergence after which, themales die and the females begin to oviposit. The bright, redeggs are laid in masses on submerged substrates (SeeMaterials and Methods: Section A. 2.). A single female maylay over 10 000 eggs in this manner over a number of weeks(Davids, 1973 for E. discreta).The eggs develop to active prelarvae within their shellsand hatch as hexapod larvae after 24-38 days at roomtemperature (Nielsen and Davids, 1975 on E. infundibuliferaKoenike). The positively phototactic larvae swim to thewater surface and skate across the top surface in search ofan appropriate host. (Hydrachna spp. mites hunt actively fortheir hosts or cling to the underneath of the water surface.)Hosts coming to the surface for air are mounted, and afterfinding a suitable attachment site, the larval mites piercethe insect's integument and begin engorgement. Engorgementcan take weeks or months depending on the temperature, butmost species of Eylais on corixids appear to be bivoltine intemperate regions (Lanciani, 1970a). The summer generationis associated with the host for less than 2 months, whereasthe overwintering generation attaches for much longer.Eylais spp. show the largest increases in size duringengorgement of any water mite (Lanciani, 1971b).17Larvae of the genus Eylais are not truly aquatic,requiring a constant air supply while engorging. They attachonly to areas such as on the thoracic and abdominal dorsumcovered by the forewings (subelytral air space), thehindwings or underside of the forewings (which are also inthe subelytral air space), or occasionally in the air spacebetween the head and prothorax (Davids et a/., 1977). Incontrast, many families of mites are truly aquatic in theirlarval stage (eg. the family Hydrachnidae). Larval Hydrachnaspp. can utilize dissolved oxygen in the water allowingattachment to all surfaces of their host including theexterior of the wings, head and legs (Harris and Harrison,1974 on H. elongata Smith formerly H. cruenta). Attachmentto immature hosts is possible for Hydrachna spp., althoughfull engorgement may not occur before host moulting, causingdeath of the mite. In contrast, some species of mites canattach to an immature host, transfer to the newly moultedadult after host ecdysis, and subsequently begin to engorge(Abro, 1982 for Arrenurus spp. on the damselfly Enallagmacyathigerum Charp.). (Hydrachna spp. and Eylais spp., areincapable of movement once attachment has occurred.)The location of attachment is usually species-specific.Resource partitioning has been demonstrated for the genusEylais such that species that could compete, parasitize18different hosts, different locations on the host, or atdifferent times of the year (Lanciani, 1970a). The exactlocation of attachment on the dorsum or wing by Eylais spp.has been documented by Nielsen and Davids (1975) for E.discreta on Sigara striata (L.) and Cymatia coleoptrata(Fab.). They found that on S. striata, E. discreta preferredtergum 3 followed by terga 2 and 4, while on C. coleoptrata,E. discreta preferred tergum 2, -and then 3. Such exactplotting of mite attachment positions is important inunderstanding the effects of mites on their hosts, and theco-evolution of sympatric host-parasite systems.The effects of water mites on their hosts was reviewed bySmith (1988) and Lanciani (1983). Following Smith (1988),the levels of effects can be classified as follows:1. Effects on individuals.2. Effects on populations.3. Effects on communities.1. Effects on individuals.On an individual basis, the effects range from beingapparently harmless as for Hydrachna conjecta Koenike on thewater boatman Sigara falleni (Fieb.) (Davids, 1973) to beinglethal as demonstrated by Lanciani (1975) on the mite-inducedreduction in survival of marsh treaders (Heteroptera:19Hydrometridae). The causes of mortality have been linked toupset of water balance caused by parasitism (Smith andMcIver, 1984c), and rupturing of the integument in cases ofsuperparasitism of mites on damselflies (Mitchell, 1968).Other important effects of parasitic mites on aquaticinsects include reduction in the fecundity of females (Davidsand Schoots, 1975; Martin, 1975; Smith, 1977), reduction inthe rate of nymphal growth (Lanciani and May, 1982), andreductions in male mating success (Forbes, 1991).2. Effects on Populations.The effects of mortality on individuals can be witnessedat the population level by comparing field samples to anegative binomial distribution (Crofton, 1971). Lanciani andBoyett (1980) demonstrated that on the mosquito Anophelescrucians Wiedemann there was significant mortality of hostsfrom the mite Arrenurus pseudotenuicollis Wilson whenabundance of mites was greater than 11 mites per host. (Thenegative binomial predicted that there would be more hostswith 11 or more mites than were found in field samples.)Direct observations of populations have drawn the sameconclusions. Fernando and Galbraith (1970) reported theabsence of the water strider Gerris comatus Drake and Hottesalmost 2 months earlier than usual, in years when earlycollections detected high levels of parasitism by Limnochares20aquatica L. Similarly, early mortality because of mites hasbeen inferred by the absence of old-aged hosts with highprevalence of mites, despite a high mite prevalence onyounger hosts (McCrae, 1976 on mosquitoes).Mites may also affect other groups of individuals within apopulation. Mitchell (1967) found higher mite parasitism onmale dragonflies compared to females suggesting that miteparasitism can skew the sex ratio of a population. Martin(1975) found that non-flying Sigara falleni had more Eylaisspp. mites on them than flying morphs, implying thatparasitism could alter the frequencies of morphs.Finally, the location of a population in its habitat canbe altered by mite parasitism. Apart from the work of Smith(1977) on the exclusion of C. expleta from low salinity,Wiegert and Mitchell (1973) have shown a similar habitatrestriction with respect to temperature. The brine-flyParacoenia thermalis Viets inhabits thermal pools. It canonly optimize its fitness in areas that are not too hot toallow reproduction, but not too cold or stable to allowparasitic water mites to reach high abundances.3. Effects on Communities.Because parasitic water mites often attach to a wide rangeof hosts, they can affect the community composition of theirhosts through differential effects. Minchella and Scott21(1991) review the importance of parasites (including watermites) in determining community structure. Gledhill et a/.(1982) found differential effects of mites on blackflies(Simulium spp.). They determined that one species of host isless affected by mite parasitism than two other speciesbecause of a defensive covering on the pupa. Other studiesfound that the pupae of the mosquito Aedes cinerus Meigendecrease parasitism by Arrenurus spp. mites through violentshaking behaviour, whereas Aedes communis (Degreer) and Aedespunctor (Kirby) do not shake and are more susceptible tomites in laboratory experiments (Smith and McIver, 1984a). Afield study, however, demonstrated the complexity ofcommunity interactions between these mites and their hosts.In the field, Aedes cinerus is the most parasitized becauseA. communis and A. punctor develop later in the year, and inso doing, avoid mite parasitism almost entirely (Smith andMcIver, 1984b). From these studies, it can be speculatedthat mite parasitism may cause alterations in the seasonalphenology of its hosts.The differential effects of mites not only affect theparasitized generation, but may also affect the offspring.Decreased or delayed fecundity of parasitized females cancause a smaller and/or later following generation which mayaffect competitiveness (Martin, 1975 on water boatmen). In22cases where food or other resources are limiting, such aneffect could be critical to the survival of one host speciesin a community.C. Overview of the CorixidaeThe corixids of Becher's Prairie are the most conspicuousinsects in the lakes. At times, especially near dusk, onesweeping sequence can yield more than 500 corixids, usuallyconstituting more than 90 per cent of the total faunacollected (personal observation in Barnes Lake).In all, 13 species of corixids from 7 genera have beenrecorded at Becher's Prairie (Smith, 1977) (See Table 2).This study deals exclusively with Cenocorixa bifida and C.expleta, but Hesporocorixa laevigata (Uhler), Cymatiaamericana Hussey, and Callicorixa audeni Hung. are alsoabundant and have been studied with respect to parasitism(Smith, 1977). The taxonomic differences between Cenocorixabifida hungerfordi and C. expleta have been determinedmorphologically (Jansson, 1972) and through acousticdifferences^in male^pre-mating^stridulatory patterns(Jansson, 1973).The life cycles of C. bifida and C. expleta are describedby Jansson and Scudder (1974) and are typical of corixids intemperate regions. Adults overwinter with the femalesundergoing ovarian diapause until spring. The males are23Corixidae Bodies of WaterCenocorixa bifida B B B B B B B BCenocorixa expleta ? R R b B B BCymatia americana B B B B R R R RHesperocorixa laevigata B B B BHesperocorixa vulgaris b b R R RHesperocorixa atopodontaHesperocorixa michiganensis ?Callicorixa audeni B b b b R R RArctocorisa sutilisDasycorixa rawsoniSigara bicoloripennis?Rbbb? Rb?Rb?Sigara decoratella R B b b R RSigara penniensis ? R R•••••••■20^(Ti^0,4 cciBodies of WaterIncreasing Salinity ^Table 2. Corixids recorded from Becher's Prairie plotted withrespect to salinity. (Modified after Smith, 1977).B = breeding in numbersb = breeding, but not in numbersR = recorded, but not necessarily breedingN.B. Because the salinities of the lakes have changed since Smith(1977), some of the breeding records may now be in error. I havetried to integrate my findings of 1990 and 1991, with that ofSmith (1977).24sexually mature by late winter and copulation occurs shortlyafter ice break-up.^Eggs are laid on submerged rocks andvegetation (Scudder, 1966). Nymphs hatch in late May(depending on temperature) and progress through five instarsbecoming adults by the middle of June. At the latitudes ofBecher's Prairie only two generations are usual. However,further south a partial third generation may occur (eg. C.expleta in the high salinity lake LB 2 near Kamloops).Water boatmen (including Cenocorixa bifida and C. expleta)often exhibit wing muscle polymorphism. Associated with wingmuscle development is a hardening and darkening process(Young, 1965a). This process is most evident on the notumand abdominal segments, although the head and forewings(hemelytra) are also affected. The hardness of the corixidnotum and tergum is of direct importance to my investigationbecause these are the susceptible areas for attachment ofEylais spp. mites (See Introduction, Question 2).Wing muscle polymorphism is generally believed to be foundin species that have stable habitats for at least part oftheir life cycle (Young, 1961). Those that live in ephemeralhabitats will usually have a flying morph only. The controlof wing muscle development seems to depend on the environmentin which the newly eclosed (teneral) adult develops (Scudderand Meredith, 1972). Factors that may be involved include25temperature, photoperiod, availability of food, and crowding.For Cenocorixa expleta, 1.5 or more days at 15 ° C gives riseto the flying morph, meaning that this morph only occurs inthe late summer when the lakes have warmed.^In the field,populations of C. expleta are predominantly non-flying.^C.bifida, however, is usually predominantly flying.^Scudder(1975) proposed that it is the high percentage of C. bifida'spopulation that develops at warm temperatures that makes itmostly flying. He documented the percentages of flyingversus non-flying C. bifida for 1962-63 and 1966 to 1969 for8 lakes of varying salinities. These data and otherunpublished records are shown in Table 3. These records maybe compared to the data of 1991 (See Results, Table 18).Scudder (1971), following the methods of Young (1965a),classified the development of the wing muscles of Cenocorixaspp. based on the darkening of the mesonotum.^Forconvenience, I used this method in my study.^The teneraladult (freshly eclosed) is designated as stage 0 (uncolourednotum). Non-flying individuals exhibit up to stage 2darkening (partial darkening of mesonotum), while flyingindividuals progress through stages 3 (more area darkeningthan stage 2) to 6 (completely dark mesonotum). Theabdominal segments usually darken in correspondence to thenotum and are completely dark by stage 6. Overwintered non-26Autumn 1962^Autumn 1963Lake^Male (n) Female (n)^Male (n) Female (n)F 80^66^79^78Lye^(50)^(35)^(70)^(40)NF 20^34 21^22F 79^71^89^83Barnes^(34)^(24)^(46)^(59)NF 21^29 11^17Autumn 1966^Spring 1967Male (n) Female (n)^Male (n) Female (n)F 59^58^51^52Lye^(112)^(101)^(126)^(101)NF 41^42 49^48F 54^59^96^92Barnes^(78)^(101)^(69)^(134)NF 46^41 4 8^Autumn 1967^Autumn 1968Male (n) Female (n)^Male (n) Female (n)F 64^52^19^8Lye^(92)^(70) (59)^(51)NF 36^48 81^92F 88^87^34^22Barnes^(25)^(16)^(44)^(45)NF 12^13 66^78Spring 1969^Autumn 1969Male (n) Female (n)^Male (n) Female (n)F 73^61^25^22Lye^(89)^(132)^(71)^(36)NF 27^39 75^78F 52^42^37^17Barnes^(23)^(69)^(8) (36)NF 48^58 63^83Table 3. Percentages of C. bifida flying and non-flying instudied lakes from 1962 to 1969. Extracted from Scudder (1975)and unpublished data.F = FlyingNF = Non-flying27flying Cenocorixa spp. sometimes have fully darkened abdomensdespite mesonotal darkening characteristic of Scudder (1971)stage 2 development.28MATERIALS AND METHODSA. Study SiteResearch was conducted in the Chilcotin region of south-central British Columbia, Canada (Figures 2 and 3). Locally,the region is known as Becher's Prairie and is situated about300 km north of Vancouver and 45 km west of Williams Lake. Thearea is a plateau above the Fraser River at approximately 1 000m elevation, and is typified by small bunchgrass prairies amidlodgepole pine and Douglas fir forests. Because of the rolling,post-glacial topography and solid bedrock formations, manysmall, pothole lakes without inflow or outflow are present, withvarying salinities dependent on the composition of theunderlying strata. In the summers of 1990 and 1991, air andlake temperatures were similar to previous studies (Smith, 1977;Lancaster, 1985) with lake temperatures ranging from 10 to 20 ° Cfor May to September, and air temperature fluctuating from 0 to35 ° C for the same period. Precipitation in the area for 1990was higher than usual with 450.1 mm, while 1991 was slightlyabove average (343.2 mm). The lakes in the area have beenstudied extensively since the 1950's. Chemically, the lowsalinity lakes are magnesium or sodium bicarbonate-carbonate andsodium bicarbonate-type waters, but at higher salinities theyare sodium sulphate and sodium carbonate (Topping and Scudder,1977). The salinities and sizes of the lakes have beendocumented by Scudder (1969a, 1969b). Since the salinities canchange over time, recordings of the studied lakes in 1990 and29Figure 2.Study Site a.) British Columbiab.) Cariboo-Chilcotin Region1. Becher's Prairie2. Kamloops (Lake LB 2)Inset of figure la. depicts Cenocorixa expleta (4X life size).Numbers on thin lines of figure lb. indicate highway numbers.30Figure 3. The studied water bodies of Becher's Prairie1. Box 27 (Pond)2. Barkley Lake3. Near Opposite Crescent (Pond)4. Greer Lake5. Near Pothole Lake (Pond)6. Long Lake (Chilcotin)7. Lake Lye8. Barnes Lake9. Round-up LakeLakes are listed in ascending salinity from 1 through 9.Barkley Lake and the pond Near Pothole Lake were studied by Smith(1977), but were not studied in depth in this study.^Theirrelative salinities are assumed to be similar, (or slightlyhigher) than previous studies (784 to 942 pS cm 1 and 3841 to4987 pS cm-1 respectively in 1976/77).Figure 3.321991 were taken and are presented along with dimensional data inTable 4.^Generally, the salinities of all lakes have beengradually increasing to their highest levels on record.^Thebiota of the lakes is also well studied. Reynolds (1979) givesa general account of the Crustacean zooplankton of the area andScudder (1969b) catalogues some of the more common invertebrates(including Corixidae) with respect to salinity. Reynolds andReynolds (1975) summarize the aquatic angiosperms. Submergentvegetation densely covers most of the fresher lakes, while thoseabove 7 000 pS cm-1 are devoid of such flora. More specificstudies include those of Cannings and Scudder (1978) on theChironomidae; Cannings and Cannings (1987) on the Odonata;Spence (1979) on the Gerridae; and Scudder and Mann (1968) onthe Hirudinea. In general, the diversity of the lakes isinversely proportional to their salinity.AreaDimensionsVolume^Avg. DepthSalinity (p3 cm-1 )1959-1969-1959-1969^1988at Given Dates1990^1990 1991 1991Lake (h) (1000 m 3 ) (m) (Mean) (Range) (August) (May) (August) (June) (Sept)Round-up 30.84 787.6^2.6 7 179 (2 820- 18 742 15 224 22 724 N/A 21 3049 000)Barnes 17.9 348.4^2.0 11^139 (3 370- 17 513 13 687 18 464 N/A 19 52820 000)Lye 46.52 1283.2^2.8 6 383 (4^000- 11 214 9 807 12 072 12 427 13 493Long (similar to Barnes) N/A12 000)N/A N/A N/A 9 232 13 848 12 215(Chilcotin) .Greer 15.17 156.8^1.0 1^848 (1^525- 6 329 3 923 5 681 N/A 5 823Near OppositeCrescent 6.88 99.2^1.4 8352^240)(415^-^973) 2 519 2 108 3 905 N/A 4 261Box 27 4.3 23.0^0.5 40 (26^- 75) 39 49 203 128 227Table 4. Dimensions and salinities of studied lakes at Becher's Prairie for^1959^-^1969(Averages and^ranges),^August^1988,^and 1990/91.34B. GENERAL TECHNIQUES1. Corixid CollectionCorixids for experiments were collected from Barnes Lake(high salinity) to ensure that they were unparasitized. Bugswere collected with an aquatic sweep net using a consistentsweeping technique for each collection. After a sweep wascompleted, the contents of the net were dispensed on to a white,dissecting tray to allow for easier identification. Adults wereseparated from nymphs and the adults that were required for anexperiment were placed in insulated flasks with lake water andaquatic foliage for transportation to the laboratory. Theremaining bugs were returned to the lake except some finalinstar nymphs that were kept and reared in wading pools at thelake edge to observe the last moult and subsequentsclerotization of teneral adults.Identification of adult corixids was done mainly in thefield, although closer scrutiny in the laboratory was necessaryfor some newly eclosed adult bugs. Extensive handling,especially of unsclerotized morphs, increases mortality. Forthis reason, handling was minimized. Identification of specieswas done according to the overall shape, size, and morph. Ifidentification was still equivocal, then the number of hairs onthe pala (distal leg segment) of the forelegs was examined asdescribed in Jansson (1972).Verification of morph and the corresponding wing muscledevelopment was done by examining the darkening of the mesonotum35according to Scudder (1971). For the pale, non-flying morphs,teneral specimens were used, up to early stage 2 of wing muscledevelopment with only a slight beginning of darkening of themetathorax and abdominal segment 1. For the dark, flyingmorphs, only stage 6 individuals were used with entirely darkthoraces, and heavy darkening of at least abdominal segments 1through 5. Non-flying individuals that appeared to have over-wintered were not used for experiments, nor were specimens thatdisplayed full wing muscle development, but had yet to fullydarken their anterior abdominal segments.Over the summer, the two species' abundances changed, as didthe proportions of flying and non-flying morphs. Experimentswere carried out as the numbers of each species and morph becamelarge enough for easy collection. Experiments were started nomore than 12 hours after collection.Samples of corixids from the lake were taken to check forparasitism levels (see Methods Section D. 4 for dates). Thesecorixids were kept alive at 5°C until examination becausepreservation in alcohol whitens the mites, causing them to beharder to see on light-coloured hosts. This makes determinationof parasitism rates more time-consuming.2. Mite CollectionEggs of Eylais spp. were collected from Lakes Greer, Long,and Lye, and identified to species after rearing in thelaboratory (See below). Oviposition occurred earlier in Lake36Greer than in Lake Lye or Long Lake which corresponded withgeneral trends for the development of Cenocorixa spp. in theselakes. In Lake Greer, eggs of the genus Eylais were laid onunanchored strands of Ruppia sp. and Zanichellia sp. (waterweeds) that collected on the surface around the lake edges. Theeggs were found cemented to the long, tubular, grass-likestrands in masses about 1 to 4 cm in length to a maximumdiameter of 0.4 cm. The masses were orange and completelyencircled the thin strands of vegetation. In contrast, the eggsof E. discreta were much brighter red when freshly laid and werefound in masses up to 20 cm in length and 0.5 cm in diameter.In Long Lake and Lye Lake, eggs were found on the submergedstems of Scirpus spp. and Juncus spp.Collection was facilitated by the fact that egg masses werealways clumped on the reeds farthest from the shore, with manyegg masses layered on top of each other on a relatively smallnumber of reeds (5 to 20). The reeds were cut as far down thestem as possible, and then transported to the laboratory in lakewater in insulated flasks. Egg masses were also found onsubmerged logs, rocks and other aquatic vegetation, but were notas easily located and collected.The egg-laden vegetation was taken to a trailer at the RiskeCreek Forestry Station that served as a laboratory. It was thentransferred with the original lake water to transparent, plasticstacking dishes (diameter 25 cm by 10 cm depth). A close-fitting lid was placed on the stacking dishes and the water37level was kept so that the egg masses remained submerged.Through a hole in the lid, a small aeration tube was insertedand kept bubbling very gently throughout the incubation period.A 40 watt lightbulb, illuminated for 16 hours a day, was keptnear the hatching eggs. Because larval mites are phototactic,the light was shone from below the stacking dishes so that onhatching, the larvae would not climb out of the dishes.When hatching occurred, the mites could be seen as a redcloud at the bottom of the dish nearest to the light source.For identification purposes, mites were regularly examined undera compound microscope to ensure that only E. euryhalina was usedfor experiments. The length of the longitudinal furrows on thedorsal plate was used as the criterion for differentiating E.euryhalina from other species of the subgenus Syneylaisfollowing Smith (1986) (See Figure 4). The morphologicallysimilar Eylais (Syneylais) lancianii Smith has been recorded inLake Greer, but was not found in large numbers, and does notoccur in Long Lake or Lake Lye where the majority of the miteswere taken for experiments. Larval Eylais (Eylais) discreta aremuch larger than any of the species of the subgenus Syneylaisand is easily differentiated from these with the naked eye.To ensure that only live and active mites were used in allexperiments, the light source was moved above the container justprior to host innoculation. A pipette was then used to withdrawonly the mites that were gathering close to the water surface.This mite-infected water was placed in small drops on a waxed38 100 pmCFigure 4. Dorsal plates of larval mites of the genus Eylais foundin British Columbia (adapted from Smith, 1986).a. Eylais (Syneylais) euryhalinab. Eylais (Syneylais) lancianiic. Eylais (Syneylais) peutrilli (not recorded at Becher'sPrairie)d. Eylais (Eylais) discreta39dish facilitating counting of the mites under a dissectingmicroscope. The mites were transferred to the experimentalwater by submerging the dish.3. Infection ExperimentsWater for all experiments was gathered from the pond calledBox 27 (< 300 pS cm-1 ), Long Lake (10 000 to 15 000 pS cm -1 ) orBarnes Lake (18 000 to 25 000 pS cm -'). Water samples, takenperiodically, were bottled and later tested for salinity with aBach - Simpson TM conductivity meter. All experimental water washeld in 10 gallon carbuoys for five or more days to kill anylarval water mites that may have been present in the lakesamples. In addition, water was strained just prior to theexperiment using 44 pm Nitex TM netting. (Larval E. euryhalina are50 pm wide.) Two litres of strained water were poured into awhite plastic 4 litre ice cream bucket for each experiment. Theexperiments were run at 20 + 2°C for 24 hours with 16 hours oflight. A 12 cm by 12 cm piece of mosquito netting was placed inthe bottom of the bucket on which the corixids could cling.Once the appropriate number of E. euryhalina had been addedto the water, the corixids were added. Care was taken that thevarious types of bugs were added to the water alternately sothat there was no bias for one species, morph or sex of bugbeing exposed to the parasites for longer than any other. Equalnumbers of males and females were used for all species andmorphs in each experiment unless this was impossible because ofan limited availability of one sex in collection. In these40cases, the nearest equal ratio of males and females was used andwas never greater than a 60 percent bias towards one sex. Afteradding all corixids, a transparent lid was placed over thebucket to prevent exit of bugs or entry of other insects.All experiments were run for 24 hours except the long-termexperiments (See Methods, Section B.5). At the end of eachexperiment dead bugs were removed and placed in labelled, water-filled vials at 5 °C. These bugs were not included in anyanalyses because they were not in contact with the mites in thesame manner for the full 24 hours.New, strained water was placed in a second bucket withnetting in the bottom and the live post-experimental corixidswere then moved to these containers by way of a wide-meshed net.Precautions were taken to prevent water from the experimentbeing transferred to the new water which ensured that only mitesthat were clinging to the corixids would be left in contact withtheir hosts past the experimental period. The corixids werethen left at 20 °C for a further 24 hours after which they wereremoved to 5 °C and total darkness until analysis. Analysis wasalways done within 5 days of the conclusion of the experiment.4. Analysis of Hosts from Infection ExperimentsInfected corixids were examined with a dissecting microscopefor attachment of mites and verification of sex, species, anddegree of sclerotization. Those bugs that had died after the 24hour exposure period, but prior to analysis were examined first.41The hemielytra and wings were lifted to allow examination ofthe wings, thoracic dorsum, and abdominal dorsum. The numberand position of mites were recorded for each corixid.Terminology for corixid morphology was taken from the study ofParsons (1970) on Hesperocorixa. In addition to the position ofmites on hosts, attachment of the mites was also determined.Those mites that were walking around when the wings weremoved, were recorded as unattached. Those that were not movingwere examined more closely by lifting the abdomen of the mite toensure that the mouthparts were inserted. If the abdomen couldbe lifted, while the cephalothorax remained in contact with thehost, then the mite was considered attached. If the entire mitecame free when the abdomen was moved, then the mite wasconsidered unattached. In addition, mites that were obviouslydessicated Were considered unattached (for all practicalpurposes) as were ones in which the mouthparts could be seen asunattached.For the 24 hour experiments, if there was any atypical degreeof engorgement (i.e. more than a slight swelling), then the bugwas discarded because of the suspicion that the attachment ofthe mite occurred in the field before collection. Additionally,if any E. discreta or Hydrachna spp. were present, then thatparticular bug was excluded from analyses. In Barnes Lake(where experimental bugs were collected), this occurred only onflying individuals that had flown in from lower salinity lakesand was very rare (less than 0.1 percent). Any time the42identification of a mite was at all uncertain, it was removedand examined under a compound microscope. Those corixids thatwere still alive at the time of analysis were killed prior toanalysis. All bugs were then placed in vials of 95 percentalcohol, or for purposes of photography or mite identification,in Koenike's solution (5 parts glycerine, 2 parts glacial aceticacid, 3 parts water).5. Long-term Engorgement ExperimentsTo ensure that initial attachment of E. euryhalina led to theonset of engorgement, long-term experiments were conducted.Corixids were infected with E. euryhalina as in the 24 hourinfection experiments. Immediately after 24 hour exposure, allbugs were taken out to small enclosures kept in Long Lake. Itwas decided to do all growth experiments in the lake instead ofin the laboratory because the constant movement of water wasbetter for corixid survival, and because lake experiments wouldbe more indicative of actual growth in the field. In addition,all bugs were used, rather than only parasitized ones, becauseexamination for parasites caused significant mortality of thecorixids.The enclosures were plastic basins of dimensions 30 cm by 35cm and 12 cm deep.^Lake water could pass into these basinsthrough windows cut in opposite ends of the basins: 44 pmNitex TM netting over these windows allowed water to enter, butkept mites and additional food out. A close-fitting lid ofmosquito netting was kept over the enclosures to prohibit entry43or exit of bugs.^Each enclosure was held in a wooden baseweighted to the lake bottom and positioned at the lake edge sothat the water level of the basin was half fullOn each day of the experiment, corixids were fed withcopepods (Diaptomus spp.) from Barnes Lake. Individuals thathad died in the previous 24 hours were removed and taken to thelaboratory for analysis. After the allotted experimental period(6 or 8 days), all corixids were taken from the field andanalysed immediately in the laboratory. The duration of theexperiment was chosen because mortality proved to be too high ifdone for 10 or more days (based on preliminary experiments in1990). Also, after 6 to 8 days, a measurable level ofengorgement had already occurred. Experimental bugs were killedat two different times so that sample sizes of infected,engorging mites would be large enough to render average sizes ofengorgement for both 6 and 8 days post-infection.6. Lake Sampling for ParasitismSamples of both flying and non-flying C. bifida and C.expleta were collected from Long Lake and Lye Lake (See MethodsSection D.4 for dates). As well, collections from Round-upLake and Barnes Lake were done to ensure that no parasites werepresent, while Lake Greer and Near Opposite Crescent Pond weresampled to determine if other parasites apart from E. euryhalinawere present at lower salinities. All sampling was done in thesame manner as described in Methods Section B.1 (General44Techniques:^Corixid^Collection)^and^whenever^possible,collections were made at the same location every time. Analysisof parasitism was also similar to the infection experiments, sothat lake sample data could be compared to the experiments.C. Parasitism Parameters1. Measures of quantity of mites.The rates used to quantify differences in the numbers ofmites between host types were chosen to correspond with previousstudies of mites on water boatmen (Harris and Harrison, 1974;Smith, 1977; Reilly and McCarthy, 1991), and to follow therecommendations of Margolis et al. (1982) regarding usage ofterms in parasitological studies. Prevalence is the proportionof hosts in any given population that are parasitized.Abundance is the average number of mites on each host. Theywere calculated as follows:Prevalence = Bugs Parasitized X 100Total BugsAbundance^Total Mites Total BugsIn each experiment, replicates were performed and the averagevalues of prevalence and abundance for the replicates were usedfor statistical analysis.Prevalence and abundance are measures that include all of themites found on the hosts, regardless of whether they areunattached, attached, or engorging. These two rates are,therefore, the measures of recruitment of mites to their hosts,45but do not take into account the potential effect that a mitemay later have on its host. Nevertheless, I deemed it importantto measure mite recruitment between host types, because I wantedto know if the cause of C. expleta's exclusion was because itinitially attracted more mites than C. bifida.I also wished to measure initial mite attachment on each hosttype. By excluding mites that were obviously unattached, Icalculated prevalence (attached only) and abundance (attachedonly). These parameters were calculated as follows:Prevalence = Bugs with Attached Mites X 100(Attached only)^Total BugsAbundance =^Attached Mites (Attached only) Total BugsBy analyzing these parameters, I sought to determine whether C.expleta's exclusion was a result of mites being able to attachto it more easily than to C. bifida.Finally, one of my experiments allowed the mites to engorgeover 6 days (see Material and Methods: Part D.3). For theengorgement data, additional rates were calculated: prevalence(engorging only) and abundance (engorging only) as follows:Prevalence = ^Bugs with Engorging Mites * X 100(Engorging only) Total Bugs alive after 3 daysAbundance = ^Engorging Mites (Engorging only)^Total Bugs alive after 3 days ** Because some bugs died before engorgement could commence (i.e.3 days), it was not valid to include all hosts in calculationsof prevalence (engorging only) and abundance (engorging only).(Inclusion of all hosts in calculations would class the hosts46that died prior to 3 days as negative with repect to miteengorgement, despite not knowing if these mites would haveengorged or not.)Analysis of prevalence (engorged only) and abundance(engorged only) allowed me to determine whether C. expleta'sexclusion was related to the ability of mites to engorge moreeasily on it than on C. bifida.2. Measures of Location of MitesRecall from the Introduction (hypothesis 2b), that thequantity of mites is not the only factor involved in determiningif mites are having a greater effect on one host type. Only inuncrowded situations will all mites be able to engorge, so thatsize of a host's susceptible area is important. Comparing thelocation of mite attachment between host types allowed me todetermine if one host type presented a larger area forattachment.The areas of the hosts' body where attachment occurred areshown in Figure 5.^The suceptible areas were then subdividedinto regions (see Figure 6). Numbering of abdominal segmentsfollowed Parsons (1970). Both the thoracico-abdominal membranes(T.A.M.) and the anterior edge of each abdominal tergum wereapparently used for attachment more often than would have beenrandomly expected. For this reason, the T.A.M. (both right andleft) was considered as a separate area for analysis, as werethe anterior edges of each abdominal tergum. I named theanterior edges with respect to the two adjoining terga, thus1 THORACICO-ABDOMINAL MEMBRANE(T. A. M.)TERGA2 (MIDDLE AND POSTERIOR EDGE)3 (ANTERIOR EDGE)4 HIND WING (VEIN)5 HIND WING (MEMBRANE)LJAREAS WITH NO RECORDED ATTACHMENT^III47Figure 5. Recorded areas of attachment by E. euryhalina onCenocorixa spp.48NOTUM-ABDOMINAL SEGMENT 1 (NOT-A.S. 1)ABDOMINAL SEGMENT 1 (A.S. 1)ANTERIOR EDGE OF A.S. 2 (A.S. 1-2)ABDOMINAL SEGMENT 2 (A.S. 2)ANTERIOR EDGE OF A.S. 3 (A.S. 2-3)ABDOMINAL SEGMENT 3 (A.S. 3)ANTERIOR EDGE OF A.S. 4 (A.S. 3-4)ABDOMINAL SEGMENT 4 (A.S. 4)ANTERIOR EDGE OF A.S. 5 (A.S. 4-5)ABDOMINAL. SEGMENT 5 (A.S. 5)ANTERIOR EDGE OF A.S. 6 (A.S. 5-6)ABDOMINAL SEGMENT 6 (A.S. 6)ANTERIOR EDGE OF A.S. 7 (A.S. 6-7)Figure 6. Divisions used to plot attachment of E. euryhalina onthe terga of Cenocorixa spp.49A.S. 1-2 is the anterior edge of abdominal tergum 2, situatedjust posterior and partially underneath the posterior edge ofabdominal tergum 1. To calculate which regions had the greatestattachment, I analyzed each bug separately and then summed allthe attachment records in the replicate. I then knew the totalnumber of mites that had attached to each host region for eachreplicate. To standardize these data, I then converted thenumbers of mites in each region to a percentage of total mitesin the replicate as follows:Percent of Mites in Area = Mites Attached in Area X 100Total Mites AttachedThe results of all replicates were then averaged.To statistically test differences in mite attachment betweenhost treatments, I decided to study attachment to the T.A.M.because it is the only morphologically conservative region inall 4 host types.^(It is constant in terms of size andsclerotization.)^Differences in attachment to the T.A.M.should, therefore, be a good indicator of differences in overallattachment patterns between host treatments. While comparingonly one area does not account for differences in all regions,it approximates attachment differences overall because most ofthe variability in attachment between host types was found inthe T.A.M. (See Results). A further X 2 analysis was performed onthe 6 to 8 day attachment and engorgement data to determine ifattachment of mites was more on the left, centre or rightportions of the hosts' body.503. Statistical TestingAll statistical tests were done using Systat TM . For all datain percentage form, percentages were changed to proportions andarcsine transformed before analysis. Paired t-tests were usedwhenever comparing experiments in which two host types wereinnoculated with mites together (i.e. in the same bucket).Other tests used are as stated in the Results. For graphicaland statistical analysis of the location of attachment,unnatached mites were not included.D. EXPERIMENTS AND FIELD SAMPLES1. Initial Mite Recruitment and Attachment (1 day)Of the 5 questions asked in the Introduction, the first 3pertained to the initial stages of the mite-corixid interaction.Questions 1 and 2 were answered by Experiment 1 (moderatesalinity) while question 3, regarding the role of salinity inthe exclusion of C. expleta, was answered by a similarexperiment done at low salinity (Experiment 5), and another testat high salinity (Experiment 6).In Experiment 1, larval E. euryhalina were offerered equalnumbers of each of the four host types in moderate salinitywater. It was then possible to study both the effects of hostspecies (Question 1) and host sclerotization (Question 2) oninitial mite attachment. By using lake water from Long Lake (10000 to 15 000 pS cm -4 ), I was able to test the attachment ofmites at the highest salinity from which C. expleta is excluded.51This could then be compared with a similar experiment at lowsalinity to ensure that the effects of mites are the samethroughout the regime of C. expleta's exclusion (Question 3).Recall from the Introduction:Question 1.) Does species of host affect a.) quantity of initialmite attachment and b.) location of initial mite attachment?Question 2.) Does host sclerotization (wing morph) affect a.)quantity of initial mite attachment and b.) location of initialmite attachment?The design of Experiment 1 was as follows:Experiment 1: (2 replicates)Corixids^ Mites^Salinity30 C. bifida teneral non-flying30 C. bifida flying30 C. expleta teneral non-flying30 C. expleta flying600 mitesModerate(10 000to15 000pS cm- ')From Experiment 1, six sets of data were collected to answerthe various parts of question 1 and 2. The six questions wereas follows:la. Does host species affect quantity of initial miterecruitment?Does host species affect quantity of initial miteattachment?lb. Does host species affect location of initial miteattachment?522a.^Does^hostrecruitment?sclerotization affect quantity of initial miteDoes^hostattachment?2b.^Does^hostsclerotizationsclerotizationaffectaffectquantitylocationofofinitialinitialmitemiteattachment?Note that questions la and 2a have two parts, firstlycomparing mite recruitment and then mite attachment. (Forsimplicity, the hypotheses and questions in the Introductionwere stated in terms of attachment only.)For each of the questions above, a data set was collected andcompared to the expected results (based on the hypotheses in theIntroduction).la. Does host species affect quantity of initial miterecruitment?Data collected:Prevalence and abundance of mites were compared between C.bifida and C. expleta (regardless of host sclerotization).Expected results:Prevalence and abundance of E. euryhalina on C. expleta isgreater than on C. bifida.Analysis:Greater initial recruitment of mites to C. expleta is atleast partially responsible for C. expleta's exclusion from lowsalinity.53Does host species affect quantity of initial mite attachment?Data collected:Prevalence (attached only) and abundance (attached only) werecompared between C. bifida and C. expleta (regardless of hostsclerotization).Expected results:Prevalence (attached only) and abundance (attached only) ofE. euryhalina on C. expleta is greater than on C. bifida.Analysis:Greater initial attachment of mites on C. expleta is at leastpartially responsible for C. expleta's exclusion from lowsalinity.Question lb: Does host species affect location of initialmite attachment?Data collected:Percentage of total mites collected that attach on the T.A.M.was compared between C. bifida and C. expleta (regardless ofhost sclerotization).Expected result:There is no significant difference in the percentages ofattached mites on the T.A.M. between C. expleta and C. bifida.Analysis:Since there was no difference in the location of miteattachment between host species, this factor is not important inthe exclusion of C. expleta from low salinity.54Question 2a: Does host sclerotization affect quantity ofinitial mite recruitment?Data collected:Prevalence and abundance were compared between non-flying(non-sclerotized) and flying (sclerotized) hosts (regardless ofhost species).Expected result:Prevalence and abundance of mites is greater on non-flyinghosts than flying hosts.Analysis:Lack of sclerotization causes higher recruitment of mitescompared to sclerotized hosts. Since C. expleta ispredominantly unsclerotized in the field, this condition isimportant in C. expleta's exclusion from low salinity.Does host sclerotization affect quantity of initial miteattachment?Data collected:Prevalence (attached only) and abundance (attached only) werecompared between non-flying hosts and flying hosts (regardlessof host species).Expected results:As for the effect of sclerotization on initial miterecruitment.Analysis:As above.55Question 2b: Does host sclerotization affect location of miteattachment?Data collected:Percentage of total mites collected that attach on the T.A.Mwas compared between non-flying hosts and flying hosts(regardless of host species).Expected results:On flying (sclerotized) hosts, all mites attach to theT.A.M., whereas on non-flying (unsclerotized) hosts, attachmentis possible over the entire area of the host dorsum.Analysis:Sclerotization of the host reduces the surface area availablefor mite attachment. Since C. bifida is predominantlysclerotized in the field, it is less susceptible to miteparasitism than C. expleta which is mainly unsclerotized.To replicate the results of Experiment 1, three furtherexperiments were run using only 2 of the potential hosts.While it would have been ideal to perform Experiments 2, 3, and4 with a total of 120 corixids and 600 mites per replicate (asin Experiment 1), the numbers of E. euryhalina hatching at anyone time constrained this.^Each replicate was accordinglyhalved in size with 60 bugs total and only 300 mites.Experiments 2 and 3 held host sclerotization constant, andwere thus only concerned with testing for the effect of hostspecies on quantity and location of mites (Question 1). The56designs were as follows:Experiment 2: (2 replicates)30 C. bifida teneral non-flying30 C. expleta teneral non-flyingExperiment 3: (2 replicates)30 C. bifida flying30 C. expleta flying300 mites Moderate(10 000to15 000pS cm - ')300 mites^ModerateExperiment 4 held host species constant, and was onlyconcerned with testing for the effect of host sclerotization(Question 2).The design was as follows:Experiment 4: (2 replicates)30 C. expleta teneral non-flying^300 mites^Moderate30 C. expleta flyingThe data collected from these experiments were tested inexactly the same manner against the hypotheses of question 1 and2.2. Effect of SalinityQuestion 3 of the Introduction dealt with the effect ofsalinity on the exclusion of C. expleta.3.) Does salinity affect a.) quantity of mites initiallyattaching and b.) where mites initially attach?The first part of hypothesis 3a. dealt exclusively with lowsalinity and this factor was studied in Experiment 5. Theexperiment was the same as Experiment 1 except for salinity.57Experiment 5: (2 replicates)Corixids^ Mites^Salinity30 C. bifida teneral30 C. bifida flying30 C. expleta teneral30 C. expleta flyingnon-flying^600 mites^Low(< 300non-flying pS cm-1 )The following data were collected to answer the various parts ofquestion 3 at low salinity.Question 3a (low salinity).^Does low salinity affectquantity of initial mite recruitment?Data collected:Prevalence and abundance were compared between host types andbetween salinities (for similar host types).Expected Result:Prevalence and abundance show the same differences betweenhost types at moderate and low salinity. From hypothesis la, C.expleta will have more mites recruiting to it than C. bifida atboth moderate and low salinities. From hypothesis 2a, non-flying hosts will have more mites recruiting to them than flyinghosts at both salinities.Analysis:Low salinity does not affect the quantity of mites recruitingto different hosts compared to moderate salinity.58Does low salinity affect quantity of initial mite attachment?Data collected:Prevalence (attached only) and abundance (attached only) werecompared between hosts and between moderate and low salinities.Expected result:As above for initial mite recruitment.Analysis:Low salinity does not affect the quantity of mites attachingto different hosts compared to moderate salinity.Question 3b (low salinity). Does low salinity affect locationof initial mite attachment?Data collected:Percentage of total mites attaching on the T.A.M. wascompared between host species and between moderate and lowsalinity.Expected Result:Percentage of total mites attaching on the T.A.M. shows thesame^differences between host types at moderate and lowsalinity.^From hypothesis lb, there is no difference inlocation of mite attachment between C. bifida and C. expleta atboth moderate and low salinity. From hypothesis 2b,significantly more mites attach to the T.A.M. of the flyinghosts than the non-flying at both salinities.Analysis:Low salinity does not affect the location of mite attachment,with respect to moderate salinity.59High salinity:The same questions were asked of high salinity, but becausethe expected result from hypothesis 3 was that no mites wouldattach, this experiment was done with only 2 hosts and half thenumber of mites.^(It was much harder to set up an experimentwith 4 hosts.)^The results were then compared to the similarexperiment performed at moderate salinity (Experiment 2).The design was as follows:Experiment 6: (2 replicates)Corixids^ Mites^Salinity30 C. bifida non-flying^300 mites^High30 C. expleta non-flying 18 000to25 000pS cm-1 )Question 3a (high salinity).^Does high salinity affectquantity of initial mite recruitment?Expected Result:Prevalence and abundance are significantly lower (approaching0 %) at high salinity compared to moderate.Analysis:High salinity decreases the quantity of initial miterecruitment (compared to moderate salinity).60Does high salinity affect quantity of initial miteattachment?Expected Result:Prevalence (attached only) and abundance (attached only) aresignificantly lower (approaching 0 %) at high salinity comparedto moderate.Analysis:High salinity decreases the quantity of initial miteattachment (compared to moderate salinity).Question 3b (high salinity).^Does high salinity affectlocation of initial mite attachment?Since it is predicted that prevalence of mites at highsalinity approaches 0 %, a comparison of location of miteattachment is not necesssary.3. Mite Engorgement Study (6 to 8 days)Question 4 of the Introduction was concerned with miteengorgement.4.) When does the effect of mites occur, or more specificallydoes the effect occur initially and/or during mite engorgementfor a.) quantity of mites and b.) location of mites?Experiments 1 through 4 dealt with the initial effect ofmites on different host types at moderate salinity. Experiment7 studied the effects of mite engorgement at moderate salinityover 6 to 8 days. While it would have been ideal to doExperiment 7 with all four host types, time constraints did not61allow this.^In addition, teneral, non-flying C. bifida hardenvery quickly creating problems in the analysis of engorgement ofmites on flying versus non-flying morphs. I chose, therefore,to use flying C. bifida, and non-flying C. expleta, becausethese two morphs were most abundant in their natural populationsand consequently, most important to study for the effects ofmite parasitism on the exclusion of C. expleta. The design wasas follows:Experiment 7: (4 replicates)Corixids60 C. bifida flying60 C. expleta non-flyingMites^Salinity600 mites^Moderate(10 000to15 000pS cm')Two of the replicates were kept in Long Lake for 6 days afterthe initial 24 hours, while the other two replicates were keptfor 8 days so that size of engorged mites could be compared atthese times.The following data were collected to answer the various partsof question 4.Question 4a: Does host affect the quantity of mite attachmentover 6 to 8 days?Data collected:Prevalence (attached only) and abundance (attached only) ofmites were compared between flying (sclerotized) C. bifida andnon-flying (non-sclerotized) C. expleta.62Expected results:More mites are able to attach on non-flying C. expleta thanon flying C. bifida over 6 to 8 days.Analysis:The predominant morph of C. expleta is more susceptible tomite attachment than the predominant morph of C. bifida over 6to 8 days.Does host affect quantity of mite engorgement over 6 to 8days?Data collected:Prevalence (engorged only) and abundance (engorged only) werecompared between flying C. bifida and non-flying C. expleta.Expected results:As for the effect of host on quantity of mite attachment.Analysis:As above.Question 4b: Does host affect location of mite attachmentover 6 to 8 days?Data collected:Percentage of total mites collected that attach on the T.A.M.was compared between flying C. bifida and non-flying C. expletaover 6 to 8 days.Expected results:On flying C. bifida all mites attach on the T.A.M., whereason non-flying C. expleta, mites attach over the entire dorsum.63Analysis:Sclerotization of the host reduces the surface area availablefor mite attachment. Since C. bifida is predominantlysclerotized in the field, it is less susceptible to miteparasitism over 6 to 8 days than C. expleta which is mainlyunsclerotized.Does host affect location of mite engorgement over 6 to 8days?Data collected:Percentage of total mites collected that engorge on theT.A.M. was compared between flying C. bifida and non-flying C.expleta.Expected results:As for the effect of host on the location of mite attachment.Analysis:As above.4. Field Studies.Corixids were sampled throughout the summer at lakes ofvarying salinity. Lakes to be sampled were chosen based on theparasitological data of Smith (1977) to give an overview of themite-corixid interaction over a wide salinity range.Corixids were collected for 3 reasons:1.) To ensure that laboratory experiments were representativeof field infections with respect to the quantity and location ofE. euryhalina on its hosts (question 5).642.) To determine the relative proportions of C. bifida and C.expleta in lakes (demonstrating that mites are excluding C.expleta where they are present).3.) To determine the relative proportions of flying and non-flying Cenocorixa spp. (demonstrating that the sclerotization ofthe host is related to the presence of mites).Samples were made in the following lakes and on the followingdates:Water body^Date^ SalinityLow moderateNear OppositeCrescent Pond September 14, 1991^4 261 pS cm - 'ModerateLong Lake^June 2, 1991^13 848 pS cm -1July 17, 1991 11 220 to 13848 pS cm-1*September 13, 1991^12 215 pS cm -1ModerateLake Lye^October 21, 1990^12 072 pS cm -1June 2, 1991^11 788 to 13 493 pS cm-1*August 17, 1991^11 788 pS cm -1September 13, 1991^13 493 pS cm -1HighBarnes Lake^August 14, 1991^18 464 to 19 528 pS cm-1*Round-up Lake August 14, 1991^18 820 to 21 304 pS cm -1**^Water samples were not taken on these days of corixidsampling, so the ranges found in that year are given instead.651.) Comparison of field parasitism and laboratory parasitism.Question 5.) Are the data collected from one- and six-daylaboratory experiments representative of mite parasitism inpopulations of field collected corixids for a.) quantity ofmites and b.) location of mites?Of all the field samples, only July 17 (Long Lake), October21 (Lake Lye), August 17 (Lake Lye), and September 13 (LakeLye), yielded enough parasitological data for analysis. Smith(1977) gives a much more thorough account of the overallparasitism rates for C. bifida and C. expleta in a number oflakes.The following data were collected to answer the various partsof question 5.Question 5a: Are laboratory experiments representative of thequantity of mites attaching on field collected corixids?Data collected:Prevalence (attached only) and abundance (attached only) werecompared between field collected host types and then compared tosimilar values for initial (1 day) and long-term (6 day)laboratory experiments.Expected results:Relative quantities of mites attaching on hosts are the samein the field and the laboratory.Analysis:One- and 6-day mite attachment studies in the laboratory are66representative of mite attachment occurring in the field withrespect to quantities of mites.Question 5b: Are laboratory experiments representative of thelocation of mites attaching on field collected corixids?Data collected:Percentage of total mites attached on the T.A.M. was comparedbetween field collected host types and then compared to similarvalues for initial (1-day) and long-term (6-day) laboratoryexperiments.Expected results:In field collections, location of mite attachment is the sameas in laboratory experiments on non-sclerotized C. bifida andnon-sclerotized C. expleta (mites attach all over the dorsum).Location of mite attachment on sclerotized C. bifida and C.expleta differs between field collections and laboratoryexperiments.^In the laboratory experiments, these hosts arepredicted to have attachment only on the T.A.M.^In the field,they are predicted to have attachment all over the dorsum.Analysis:Location of mite attachment on the non-sclerotized C. bifidaand C. expleta is the same in the field and laboratory becausethese hosts offer an entire unsclerotized dorsum to the miteswhether they are in a laboratory experiment or collected fromthe field. The sclerotized field-collected hosts exhibitdifferent mite attachment from laboratory experiments because inthe laboratory, the attachment of mites occurs after67sclerotization (by design) causing all mites to attach on theT.A.M.^In the field, however, attachment may occur prior,during or after sclerotization.^Mite attachment prior tosclerotization could occur anywhere on the host's dorsum causingthe difference in predicted attachment patterns between fieldand laboratory infected sclerotized hosts. Laboratoryexperiments are not always representative of location of miteattachment in the field.2.) Proportions of C. bifida compared to C. expleta.Since the salinities of the lakes have changed since thestudy of Smith (1977), the relationship between salinity, mites,and corixids was re-examined. This was done by noting thesalinities at which mites are present (See Appendix 1), and therelative percentages of C. bifida and C. expleta in the studiedlakes (see Results). C. bifida should be predominant wheremites are present, whereas C. expleta should predominate abovethe salinity at which mites exist.3.) Proportions of flying and non-flying Cenocorixa spp.Since the proportions of flying and non-flying morphs ofCenocorixa spp. change over time (Scudder, 1975), it wasnecessary to determine what forms were predominant in 1990 and1991 (See Results). Based on previous collections, it waspredicted that C. bifida should be predominantly flying, whereasC. expleta should be predominantly non-flying.68RESULTSA. Initial Mite Recruitment and Attachment (Moderate Salinity).1. Effect of Host Species.Question la. Does host species affect quantity of initialmite recruitment?Data collected:In Experiment 1 (Table 5) with all four host types,prevalence of E. euryhalina was not significantly differentbetween C. bifida and C. expleta (Two-way ANOVA, grouped by hostspecies: F = 0.200, P = 0.678). In Experiment 2 with non-flyinghosts only (Table 6) there was also no significant difference, t= -1.230, (P = 0.435), and neither was there in Experiment 3with flying hosts only (Table 7) (t = -1.849, P = 0.316).Abundance was also not significantly different between hostspecies in Experiments 1, 2, and 3.Summary:In 24 hour laboratory experiments, host species did notaffect quantity of mite recruitment.Does host species affect quantity of initial mite attachment?Data collected:For Experiment 1 (Table 5), prevalence (attached only) of E.euryhalina was not significantly different between C. bifida andC. expleta (Two-way ANOVA, grouped by host species: F = 0.001, P= 0.976). Experiment 2 (non-flying hosts only) followed thesame trend (t = -1.760, P = 0.329) (Table 6), as did experiment3 with flying hosts only (t = 0.909, P = 0.530) (Table 7).69Abundance (attached only) followed the same trend.Summary:In 24 hour laboratory experiments, host species did notaffect the quantity of mites attachment.Question lb: Does host species affect location of initialmite attachment?Data collected:In experiment 1, the percentage of total mites that attachedon the T.A.M. was not significantly different between C. expletaand C. bifida (Two-way ANOVA, grouped by host species: F =0.174, P = 0.698) (see Figure 7). Neither was Experiment 3 forflying hosts only (t = -5.798, P =0.109) (see Figure 9).Experiment 2, however, had a significantly higher percentage ofthe collected mites attached on the T.A.M. of non-flying C.bifida compared to non-flying C. expleta (t = 145.4, P = 0.004)(see Figure 8). An overview of the host types shows that flyingC. bifida and flying C. expleta had a consistently highproportion of mites initially attaching on the T.A.M., whereasC. expleta non-flying had a consistently low proportion. C.bifida non-flying had attachment proportions on the T.A.M. thatwere variable and intermediate between the flying hosts and non-flying C. expleta. This variance in the attachment patterns ofmites on non-flying C. bifida was consistent throughout allexperiments.Summary:70On the flying hosts, species of host did not appear to affectthe location of attachment of mites (Experiments 1 and 3). Onthe non-flying hosts, the variance in the attachment patterns ofmites on non-flying C. bifida precluded a definite answer ofthis question (Experiments 1 and 2).2. Effect of Host Scierotization.Question 2a: Does host sclerotization affect quantity ofinitial mite recruitment?Data collected:In Experiment 1 (Table 5), prevalence of E. euryhalina wasnot significantly different between non-flying (non-sclerotized)and flying (sclerotized) hosts (Two-way ANOVA, grouped by hostsclerotization, F = 0.010, P = 0.925). The same was true forExperiment 4 (Table 8) with only C. expleta (t = 0.326, P =0.799). Abundance was also not significantly different betweennon-flying and flying hosts in Experiments 1 and 4.Summary:In 24 hour laboratory experiments, host sclerotization didnot affect quantity of mite recruitment.Does host sclerotization affect quantity of initial miteattachment?Data collected:In Experiment 1 (Table 5), prevalence (attached only) of E.euryhalina was not significantly different between non-flyingand flying hosts (Two-way ANOVA, grouped by host sclerotization,71F = 0.617, P = 0.476). The same trend was found in Experiment 4with only C. expleta (t = 1.049, P = 0.485) (Table 8).Abundance (attached only) was also not significantly differentbetween host species in Experiments 1 and 4.Summary:In 24 hour laboratory experiments, host sclerotization didnot affect quantity of mite attachment.Question 2b: Does host sclerotization affect location of initialmite attachment?Data collected:In experiment 1, the percentage of total mites that attachedon the T.A.M. was significantly different between non-flying andflying hosts (Two-way ANOVA, grouped by host sclerotization: F =34.001, P = 0.004) (see Figure 7). So was Experiment 4 withonly C. expleta (t = - 57.079, P = 0.011) (see Figure 10).Flying hosts had a high percentage of mites attached on theT.A.M., whereas non-flying hosts had a low percentage, with mostmites attaching on the centres (laterally) of abdominal terga 2,2-3, and 3.Summary:Host sclerotization affected the location of initial miteattachment. Mites on sclerotized hosts attached almostexclusively on the T.A.M., whereas mites on unsclerotized hostswere all over the dorsum.72PARASITISM^ HOSTRATES C. bifida^C. bifida^C. expleta^C. expleta(AVG. OF 2 REPS)^non-flying flying non - flying flyingPREVALENCE ( + /-s . E . ) 66.8 (+/ - 9.8) 34.7 (+/ - 3.43) 42.1 (+/ - 0.0) 80.0 (+/-2.8)PREVALENCE(ATTACHED ONLY)^(+/-S.E.) 62.3 (+1-9.8) 19.5 (+1-0.95) 34.2 (+/-2.8) 55.4 (+/-1.2)ABUNDANCE ( + /-S . e . ) 3.31 (+1-0.59) 0.64 (+/-0.03) 1.00 (+/-0.15) 3.90 (+1-0.15)ABUNDANCE(ATTACHED ONLY) (+/-s .E. ) 2.37 (+/-0.62) 0.35 (+/ - 0.01) 0. 63 (+1-0.09) 1.70 (+/-0.18)Table 5. Experiment 1. Parasitism rates for quantity of mitesassociated with all 4 host types over 24 hours at moderatesalinity (10 000 to 15 000 pS cm -1 ).^All parasitism ratesshow no significant difference between hosts.^Forstatistical analyses, see text.T.A.M.NDIUMNOTUM,AS ,AS 1AS 1-2AS 2AS 2-3AS 3AS 3-4AS 4AS 4-5AS 5.Wing (vein)Q Wing (memb.)CDLilCO-^T.A.M.C)MDTUMNDTW-A.S . 1AS 1AS 1-2AS 2AS 2-3AS 3AS 3-4AS 4AS 4-5AS 5.Wing (vein)Wing (memb.)C. bifidanon-flyingMites^72,21Bugs 18,28C. expletanon-flyingMites^17,7Bugs 19,19C. expletaflyingMitesBugs9,924,2754,2125,173,C. bifidaflyingMitesBugs0^20^40^60^80^100^20^40^60^80^100AVG MITES ATTACHED PER REGION (PERCENT OF TOTAL)73Figure 7. Experiment 1. Location of mite attachment on all hosttypes at moderate salinity (10 000 to 15 000 pS cm'), plotted asa percent of total mites collected on each host type. Numbers ofmites in boxes are attached mites only (per replicate) associatedwith number of bugs directly below. Error bars are standarderror.LOCATION OF ATTACHMENT as measured by percent of total miteson T.A.M.Two-way ANOVA, grouped by host species: F = 0.174, P = 0.698Two-way ANOVA, grouped by host sclerotization: F = 34.001,P = 0.004AVG. MITES ATTACHED PER REGION (PERCENT OF TOTAL)MitesBugs13,3314,21C. bifidanon-flying60^ 100^200 806040 100T.A.M.NOTUMNOTUM-AS.1AS 1AS 1-2AS 2AS 2-3AS 3AS 3-4AS 4AS 4-6AS 6.Wing (vein)Wing (memb.)74Figure 8. Experiment 2. Location of mite attachment on non-flyinghosts at moderate salinity (10 000 to 15 000 )IS cm-') plotted asa percent of total mites collected on each host type. Numbers ofmites in boxes are attached mites only (per replicate) associatedwith number of bugs directly below. Error bars are standarderror.PARASITISMRATES(AVG. OF 2 REPS)HOSTC. bifida^C. expletanon-flying^non-flyingStatistical Analysisbetween Hostst^PPREVALENCE^(+/-S.E.) 55.9^(+1 -2.90) 69.3^(+1 -0.94) -1.230 0.435PREVALENCE(ATTACHED ONLY)^(+/ -S.E.) 46.4^(+/-1.22) 62.6^(+/ - 1.73) -1.760 0.329ABUNDANCE^(+/ -S.E.) 2.10^(+/-0.24) 2.2^(+/-0.07) -0.250 0.844ABUNDANCE(ATTACHED ONLY) (+/-s.E.) 1.26^(+/-0.12) 1.4^(+/-0.14) -2.077 0.286% OF MITES ATTACHEDON T.A.M.^(+/-s.E.) 50.3^(+/-1.48) 13.6^(+/-1.6) 145.4 0.004Table 6. Experiment 2. Parasitism rates and statisticalanalyses for quantity and location of mites associated withnon-flying hosts over 24 hours.Mites 13,17Bugs 26,26Mites 13,18Bugs 33,2940 60 80^100AVG MITES ATTACHED PER REGION (PERCENT OF TOTAL)-^I.C. bifidaflying11 60^80 20^10040T.A.M.NO7UM(CT UM-AS . 1AsAS 14AS 2AS 2-3AS 3AS 3-4AS 4AS 4-5AS 5•Wing (vein)Wing (memb.) a..0^20C. expletaflying^ —75Figure 9. Experiment 3. Location of mite attachment on flyinghost types at moderate salinity (10 000 to 15 000 iS cm -1 ),plotted as a percent of total mites collected on each host type.Numbers of mites in boxes are attached mites only (per replicate)associated with number of bugs directly below. Error bars arestandard error.PARASITISMRATESAVG. OF 2 REPS)HOSTC. bifida^C. expletaflying flyingStatistical Analysisbetween Hostst^PPREVALENCE (+/ -S.E.) 21.2^(+/ -3.40) 32.8^(+/ - 5.5) -1.849 0.316PREVALENCE(ATTACHED ONLY)^(+/ -S.E.) 17.3^(+/ - 3.40) 22.0^(+/ - 2.91) 0.909 0.530ABUNDANCE^(+/-s.E.) 0.69^(+/-0.00) 0.79^(+/ -0.02) -1.214 0.439ABUNDANCE(ATTACHED ONLY) (+/ -S.E.) 0.57^(+/ - 0.02) 0.51^(+/ - 0.04) 5.000 0.126% OF MITES ATTACHEDON T.A.M.^(+/-s.E.) 78.5^(+/ - 4.85) 52.3^(+/ - 8.7) 5.798 0.109Table 7. Experiment 3. Parasitism rates and statisticalanalyses for quantity and location of mites associated withflying hosts over 24 hours.T.A.M.NOT UMNOTUM-AS1AS. 1AS 1 -2AS.2AS. 2-3AS 3AS 3-4AS 4AS 4-6 C. expleta C. expletaAS. 6• non-flying flyingWing^(vein)Mites 41,23 Mites 19,11Wing^(nemb.) Bugs^1 26,22 Bugs 28,220^20^40^60^80^100^20^40^60^80^100AVG. MITES ATTACHED PER REGION (PERCENT OF TOTAL)76Figure 10. Experiment 4. Location of mite attachment on C.expleta (non-flying and flying) at moderate salinity (10 000 to15 000 pS cm-1 ), plotted as a percent of total mites collected oneach host type. Numbers of mites in boxes are attached mitesonly (per replicate) associated with number of bugs showndirectly below. Error bars are standard error.PARASITISMRATES(AVG. OF 2 REPS)HOSTC. expleta^C. expletanon-flying flyingStatistical Analysisbetween Hostst^PPREVALENCE (+/-s.E. ) 48.9^(+/-4.43) 40.5^(+/-1.70) 0.326 0.799PREVALENCE(ATTACHED ONLY)^(+/ -S.E.) 44.8^(+1 - 4.61).27.1^(+/-3.27) 1.049 0.485ABUNDANCE^(+/-s.E. ) 1.55^(+/ - 0.08 1.48^(+/ - 0.05) -0.109 0.931ABUNDANCE(ATTACHED ONLY) (+/ - S.E.) 1.31^(+/ - 0.09 0.59^(+/ - 0.03) 3.944 0.227% OF MITES ATTACHEDON T.A.M.^(+/ - 8 E.) 18.8^(+/ - 2.05) 75.6^(+1-1.1) -57.08 0.011Table 8. Experiment 4. Parasitism rates and statisticalanalyses for quantity and location of mites associated withC. expleta (non-flying and flying) over 24 hours.77B. Initial Mite Recruitment and Attachment: Low and HighSalinity1. Low salinity.Question 3a (low salinity).^Does low salinity affect quantityof initial mite recruitment?Data collected:High mortality of flying C. expleta in Experiment 5 did notpermit analysis of this host type, but prevalence of mites atlow salinity did not differ between the remaining three hosttypes (F = 5.704, P = 0.095) (Table 9). Abundance followed thesame trend. Comparing prevalences of these hosts at lowsalinity with the same hosts at moderate salinity (Experiment 1)revealed no effect attributable to salinity (Two-way ANOVA,grouped by salinity: F = 1.177, P = 0.320).Summary:Low salinity does not affect the quantity of initial miterecruitment (compared to moderate salinity).Does low salinity affect quantity of initial mite attachment?Data collected:Prevalence (attached only) in Experiment 5 (Table 9) was notsignificantly different between host types at low salinity (F =1.706, P = 0.320). Comparing the same host types at moderatesalinity showed that there was no effect because of low salinityon prevalence (attached only) (Two-way ANOVA, grouped bysalinity:^F = 1.320, P = 0.294).78Summary:Low salinity does not affect quantity of initial miteattachment (relative to moderate salinity).Question 3b (low salinity).^Does low salinity affect locationof initial mite attachment?Data collected:Percentage of total mites attaching on the T.A.M. was greateron flying C. bifida compared to non-flying C. bifida and non-flying C. expleta (F = 110.65, P < 0.001) (Figure 11). 1 Ananalysis of the location of mite attachment on the three hosttypes at moderate and low salinity showed no salinity effect(Two-way ANOVA, grouped by salinity: F = 3.701, P = 0.103).Summary:Low salinity did not affect the location of initial miteattachment (relative to moderate salinity).2. High salinity:Question 3a (high salinity). Does high salinity affect quantityof initial mite recruitment?Data collected:Prevalence was significantly lower on non-flying C. bifidaand non - flying C. expleta at high salinity (Experiment 6, Table'The attachment of mites on one replicate of flying C.expleta is shown in Figure 11 for graphical comparison only.These data were not used for any statistical analysis.7910) compared to moderate salinity (Experiment 2, Table 6) (Two-way ANOVA, grouped by salinity: F = 94.794, P = 0.001). Athoughmite recruitment was minimal at high salinity, it did occur.Abundance was also significantly lower at high salinity comparedto moderate salinity.Summary:High salinity decreased the quantity of initial miterecruitment (with respect to moderate salinity).Does high salinity affect the quantity of initial miteattachment?Prevalence (attached only) was significantly less on hosts athigh salinity (Table 10) than at low salinity (Table 6) (Two-wayANOVA, grouped by salinity: F = 87.199, P = 0.001). Abundance(attached only) followed the same trend.Summary:High salinity decreased the quantity of initial miteattachment (with respect to moderate salinity).Question 3b (high salinity).^Does high salinity affectlocation of initial mite attachment?Very low attachment of mites at high salinity made thisquestion unnanswerable.80PARASITISM^HOSTRATES C. bifida^C. bifida^C. expleta^C. expleta(AVG. OF 2 REPS)^non-flying flying non-flying flyingPREVALENCE (+/-s.E.) 73.3 (+/ - 3.55) 37.1 (+/ - 0.38) 78.0 (+1 - 2.33) # of bugsPREVALENCE(ATTACHED ONLY) (+/-s.E.) 67.8 (+/-^5.4) 37.1 (+/ - 0.38) 59.6 (+/-3.38) aliveABUNDANCE (+/-s.E. ) 2.00 (+/ - 0.16) 1.50 (+/ - 0.18) 2.30 (+/ - 0.28) is tooABUNDANCE(ATTACHED ONLY) (+/-s.E.) 1.45 (+/ - 0.12) 0.84 (+/ - 0.12) 1.76 (+/-0.21) low.Table 9. Experiment 5. Parasitism rates for quantity of mitesassociated with all 4 host types over 24 hours at lowsalinity (< 300 pS cm-1 ).^All parasitism rates show nosignificant difference between hosts.^For statisticalanalyses with moderate salinity (Experiment 1), see text.C. expletanon-flyingC. expletaflyingMites 2334,13Bugs13,14 4I^MitesMites 30,10Bugs 25,21 T.A.M.NOTUMMOTUV-ASIAS 1AS 1-2AS 2AS 2-3AS. 3AS 3-4AS 4AS. 4-6AS 6•Wing (vein)Wing (memb.)I V^^IC. bifidanon-flyingMites^22,21Bugs 12,19C. bifidaflyingT.A.M.NOTUMNOTUNFASIAS. 1AS 1 -2AS. 2AS. 2-3AS 3AS 3-4AS 4AS. 4-6AS. 6.Wing (vein)Wing (memb . )0^20^40^60^80^100^20^40^60^80^100AVG MITES ATTACHED PER REGION (PERCENT OF TOTAL)81Experiment 5. (Low salinity).Figure 11. Experiment 5. Location of mite attachment on all hosttypes at low salinity (< 300 pS cm -1 ), plotted as a percent oftotal mites collected on each host type. Numbers of mites inboxes are attached mites only (per replicate) associated withnumber of bugs shown directly below. Error bars are standarderror.LOCATION OF ATTACHMENT as measured by percent of total miteson T.A.M.One-way ANOVA: F = 110.65, P = 0.001Analysis of location of mite attachment between moderatesalinity (Experiment 1) and low salinity (Experiment 5).Two-way ANOVA, grouped by salinity: F = 3.701, P = 0.103.82PARASITISM^ HOST^Statistical AnalysisRATES C. bifida^C. expleta^between Hosts(AVG. OF 2 REPS)^non-flying^non-flying t^PPREVALENCE^(%)(4/-s.E.) 13.9^(+/ - 0.25) 10.9^(+/ - 2.72) -0.713 0.616PREVALENCE^(%)(ATTACHED ONLY) (+/ - S.E.) 9.4^(+/ - 3.50) 10.9^(+/ - 2.72) -0.177 0.899ABUNDANCE^(+/ - S.EJ 0.14^(+/ - 0.01 0.14^(+/ - 0.05) -0.104 0.934ABUNDANCE(ATTACHED ONLY) (+/ - S.E.) 0.10^(+/ - 0.03 0.13^(+/-0.04) -0.298 0.816Table 10. Experiment 6. Parasitism rates and statisticalanalyses for quantity of mites associated with non-flyinghosts over 24 hours at high salinity (> 18 000 pS cm -'). Forstatistical comparison with moderate salinity (Experiment 2),see text.83C. Long-term Attachment and Engorgement.1. AttachmentData for this section were collected from Experiment 7 andwere the summation of 8 days of data. The mortality data forthe two host types, while not of direct importance to thequestions asked below, are displayed in Table 11. This showsthe dynamics of the mite-corixid interaction over 6 to 8 days onthe predominant morphs of both host species. It was found inthis study and in a similar study in 1990, that the mortalityrate of non-flying C. expleta was higher in the first 3 daysthan flying C. bifida, but that this was true of unparasitizedcontrols as well. Non-flying C. expleta was dying more quicklythan flying C. bifida even without parasitism by E. euryhalina.As with questions 1, 2, and 3, recruitment followed the sametrends as attachment. For brevity's sake, I do not mentionrecruitment in the Results of Questions 4 and 5, but consider itfully in the Discussion.Question 4a: Does host affect the quantity of mite attachmentover 8 days?Data collected:In Experiment 7 (Table 12), prevalence (attached only) of E.euryhalina was significantly greater on non-flying (non-sclerotized) C. expleta than flying (sclerotized) C. bifida (t =-3.996, P = 0.028). Abundance (attached only) was alsosignificantly different.84Summary:Over 6 to 8 days, non-flying C. expleta had significantlymore mites attaching to it than flying C. bifida. Host typeaffected mite attachment.Question 4b: Does host affect location of mite attachmentover 8 days?Data collected:The percentage of total mites attached on the T.A.M. inExperiment 7 was significantly greater on flying C. bifida thannon-flying C. expleta (t = 6.319, P = 0.008) (See Figure 12).Mites on flying C. bifida were exclusively attached to theT.A.M. and wings, whereas mites on non-flying C. expleta wereattached all over the dorsum, especially terga 2, 2-3, and 3. AX2 analysis supported these data. Mites on flying C. bifida wereattached more than would be randomly expected to the left andright thirds of the host dorsum, compared to the centre third (X 2= 58.3, P < 0.001) Mites on non-flying C. expleta wereattached more on the centre of the dorsum, compared to the leftand right thirds (X 2 = 80.6, P < 0.001).Summary:Host type affected location of mite attachment over 8 days.2. Mite Engorgement.Question 4a. Does host affect quantity of mite engorgementover 6 to 8 days?Data collected:85Prevalence (engorged only) in Experiment 7 was significantlygreater on non-flying C. expleta compared to flying C. bifida inExperiment 7 (t = -3.83, P = 0.031) (see Table 13). Abundance(engorged only) was also significantly different.Summary:Host type affected the quantity of mites able to engorge over6 to 8 days.Does host type affect location of mite engorgement over 6 to8 days?Data collected:In Experiment 7, percentage of total mites collected thatengorged on the T.A.M. was significantly greater on flying C.bifida compared to non-flying C. expleta (t = 64.2, P < 0.001)(Figure 13). All mites that commenced engorgement on flying C.bifida were found on the T.A.M., whereas most of the mitesengorging on non-flying C. expleta were found on the centres ofabdominal segments 2, 2-3, 3, and 3-4. A X 2 analysis of themites on the left, right, and centre thirds of the host showedthe same pattern as for mite attachment (X 2c. b i fida = 37.2, P <0.001, X 2 C. expleta 80.6, P < 0.005). In general, location of miteengorgement was very similar to mite attachment over 8 days.Summary:Host type affected the location of mite engorgement over 6 to8 days.Time (days) after initial infection^1 - 2.5^2.5 - 4.5^4.5 - 6Percentage of C. bifida^3.96^7.76^5.50dead (+/- S.E.)^(+/- 1.04)^(+/- 0.75)^(+/- 1.52)Percentage of total^4.49^26.1^8.92mites collected (+/- S.E.) (+/- 2.50)^(+/- 11.8)^(+/- 3.30)Percentage of C. expleta^42.2^26.9^16.8dead (+/- S.E.)^(+/- 2.74)^(+/- 4.12)^(+/- 3.84)Percentage of total^56.3^23.8^10.4mites collected (+/- S.E.) (+/- 4.57)^(+/- 3.37)^(+/- 3.66)0 - 6^0 - 8 *17.2 23.3(+/- 1.55)^(+/- 3.92)39.6 59.8(+/- 8.13)^(+/- 4.87)86.0^91.8(+/- 4.52)^(+/- 5.76)90.5^89.6(+/- 6.82)^(+/- 7.33)Only 2 replicates were allowed to run for 8 days.Table 11. Experiment 7. Mortality data for flying C. bifidaand non-flying C.expleta over 6 to 8 days of mite growth inLong Lake.T.A.M.NoTumNOVU110-AS1AsAS 1-2AS 2AS 2-3AS 3 =SWAS 3-4 =MCAS 4AS 4-6AS 6•C. bifidaflyingC. expletanon-flyingWing^(vein) ^Mites1 6,17,74,17 Mites 24,160,173,145Wing^(memb.) Bugs 51,59,58,51 Bugs 49,54,52,4520^40^60^80^100 20^40^60^80AVG. MITES ATTACHED PER REGION (PERCENT OF TOTAL)87Figure 12. Experiment 7. Location of mite attachment on flying C.bifida and non-flying C. expleta over 6 to 8 days at moderatesalinity (10 000 to 15 000 pS cm -'). Data are plotted as apercent of total mites collected on each host type. Numbers ofmites in boxes are attached mites only (per replicate) associatedwith number of bugs shown directly below. Error bars are standarderror.PARASITISMRATESAVG. OF 4 REPSHOSTC. bifida^C. expletaflying non-flyingStatistical Analysisbetween HostsPREVALENCE (+/-S.E.) 25.1^(+/ -2.65) 64.6^(+/-5.0) -4.021 0.028PREVALENCE(ATTACHED ONLY)^(+/-S.E.) 22.8^(4 - 2.45) 63.7^(+/ - 5.0) -3.996 0.028ABUNDANCE^(+/-S.E.) 0.71^(4 - 0.21) 2.72^(4 - 0.31) -3.241 0.048ABUNDANCE(ATTACHED ONLY) (+/ -S.E.) 0.51^(+/-0.11) 2.49^(+/-0.29) -3.388 0.043% OF MITES ATTACHEDON T.A.M.^(+/-S.E.) 87.9(+/-3.15) 7.2^(+/-0.32) 6.319 0.008Table 12. Experiment 7. Parasitism rates and statisticalanalyses for quantity and location of attached mitesassociated with flying C. bifida and non-flying C. expletaover 6 to 8 days mite growth in Long Lake.40 60 800 20 40^60^80^100C. bifidaflyingMitesI ^I9 ,16, 38 ,13 Bugs^51,59,58,51  AVG. MITES ENGORGING PER REGION (PERCENT OF TOTAL)T . A .MKA UMKIT UM- A.S.1ASAS 1-2AS.2AS. 2-3AS. 3AS 3-4AS. 4AS. 4-6AS.Wing (vein)Wing (memo.)88Figure 13. Experiment 7. Location of mites engorging on flying C.bifida and non-flying C. expleta over 6 to 8 days at moderatesalinity (10 000 to 15 000 pS cm-1 ). Data are plotted as apercent of total mites collected on each host type. Numbers ofmites in boxes are attached mites only (per replicate) associatedwith number of bugs shown directly below. Data are plotted as apercent of total mites collected on each host type. Error barsare standard error.HOST^Statistical Analysis^C. bifida C. expleta^between Hostsflying^non-flying t^PPREVALENCE(ENGORGING ONLY)(+/-s.E.) 20.1^(+/ - 2.36) 39.6 (+/- 2.66 -3.83 0.031ABUNDANCE(ENGORGING ONLY)(+/-s.E.) 0.33^(+/- o.o5) 0.86 (+/- 0.17) -3.47 0.040% OF MITES ATTACHEDON T.A.M.^(+/ -S.E.) 100^(+/-0.0) 13.3 (+/ - 2.00) 64.2 <0.001Table 13. Experiment 7. Parasitism rates and statisticalanalyses for quantity and location of engorging mitesassociated with flying C. bifida and non-flying C. expletaover 6 to 8 days mite growth in Long Lake.89Unfortunately, low sample sizes prohibited a statisticalanalysis of size of engorgement between host types over 6 and 8days. Some qualitative data however, are given in Appendix 1 onthe engorgement process of E. euryhalina.D. Field Studies.Recall from the Materials and Methods that corixids werecollected for 3 reasons:1) To ensure that the quantity and location of E. euryhalina inlaboratory experiments is representative of field infections(question 5).2) To determine the relative proportions of C. bifida and C.expleta in lakes (i.e. that mites are still excluding C. expletaat low salinity).3) To determine the relative proportions of flying and non-flying Cenocorixa spp. (i.e. that flying hosts predominate wheremites are present).1) Comparison of field parasitism and laboratory parasitism.Question 5a: Are laboratory experiments representative of thequantity of mites attaching on field collected corixids?Data collected:The September 13, 1991 collection had prevalence (attachedonly) on flying C. bifida of 29.4 % (Table 14b), on non-flyingC. expleta of 46.8 % (Table 15b), and on C. expleta flying of37.8 % (Table 16b), but these values were not significantly90different (F = 1.226, P = 0.297). In the collections of August17, 1991 (Table 15a, 16a) and October 21, 1990 (Tables 14c, 15c,16c), prevalence (attached only) was also not significantlydifferent between host types.^Abundance (attached only)followed the same trend.^Collections of non-flying C. bifidawere too low for statistical comparison.Summary:Prevalence of attached mites was not significantly differentbetween host types in field collections. This result was thesame as initial laboratory experiments, but different than the 8day study between flying C. bifida and non-flying C. expletawhich found a significant difference.Question 5b: Are laboratory experiments representative of thelocation of mites attaching on field collected corixids?Data collected:In the September 13, 1991 collection, there was nosignificant difference in the percentage of mites attaching onthe T.A.M. between flying C. bifida (Figure 14), non-flying C.expleta (Figure 15) and flying C. expleta (Figure 16)^(F =0.112, P = 0.894).^This result was different than labexperiments which showed significantly more mites on the T.A.M.of flying hosts compared to non-flying. Other field collectionsalso found no significance in the location of mite attachmentbetween hosts.Comparing individual host types collected in the field to91laboratory experiments, the attachment of mites on non-flying C.expleta was similar in that mites were mainly attached toabdominal segments 2, 2-3 and 3 (Figure 15). Attachment ofmites to flying C. expleta was different in that a much lowerproportion of mites was attaching to the T.A.M. in the field(Figure 16). Attachment in the field occurred through thehardened terga of abdominal segments 2, 2-3 and 3.Attachment of mites on flying C. bifida was similar tolaboratory data in the July 17, 1991 collection with over 70 %attachment to the T.A.M., but different on September 13, 1991and October 21, 1990 with less than 10 % of mites on the T.A.M.in each collection (Figure 14).Summary:Location of attachment was similar in the laboratory and thefield on non-flying C. expleta, but different on flying C.expleta. On flying C. bifida, location of attachment wassimilar in one collection, but different in two others.July 17, 1991Long Lake99C. bifidaflyingMites^26Bugs 14September 13, 1991Lye LakeC. bifidaflying1 MitesBugsPARASITISM^DATERATES July 17, 1991PREVALENCE (%)PREVALENCE (t)(ATTACHED ONLY).ABUNDANCEABUNDANCE(ATTACHED ONLY)Table 14a.PARASITISM^DATERATES Sept. 13 1992PREVALENCE (%) 32.3PREVALENCE (t)(ATTACHED ONLY) 29.4ABUNDANCE 0.32ABUNDANCE(ATTACHED ONLY) 0.29T . A .M.MOTUM6CITUM-A.S.1AS 1AS 1-2AS 2AS 2-3AS. 3AS. 9-4AS. 4AS. 4-6AS 6•Wing (vein)Wing (memb IT.A.M.ocaum143TUA4-AS.1AS 1AS 1-2AS.2AS2-3AS3AS 3-4AS 4AS 4-6AS 6.Wing (vein)Wing (memb.)103457.157.13.071.86Table 14b.October 21, 1990Lye LakeC. bifidaflyingMites Bugs^15^I60 20^40^60^80PARASITISM^DATERATES Oct. 21,1990PREVALENCE (%) 25.0PREVALENCE (%)(ATTACHED ONLY) 25.0ABUNDANCE 0.25ABUNDANCE(ATTACHED ONLY) 0.25Table 14c.TAMNOTUMNOT UM-AS 1AS.1AS 1 -2 /2AS2-3/3• AS.3-4/4• AS4-5/5+WINGHEAD SPACE0 100MITES ATTACHED PER REC*ON EFERCENTT OF TOTAL)92Field SamplesFigure 14. Field studies. Location of mite attachment on flyingC. bifida by collection date, plotted as a percent of total mitescollected.Table 14. Field studies. Quantity of mite attachment on flying C.bifida a.) July 17, 1991: Long Lake b.) September 13, 1991: LakeLye c.) October 21, 1990: Lake Lye.PARASITISM DATEPARASITISM DATE46.8PREVALENCE (%) 48.9ABUNDANCE 0.66PREVALENCE (%)(ATTACHED ONLY)DATEOct. 21, 1990PARASITISMRATEST.A.M.MEnU1.4KrL1.4-As1AS.1AS 1-2AS 2AS 2-3AS9AS 3-4AS 4-6AS 6.Wing^(vein)Wing^(memb. 1T.A.M.ktrumKraol-AalAS 1AS1-2§^AB2 ^LU AS2-31- ^AS. 3AS 34AS4AS4-6AS 6•Wing^(vein)Wing^(mernb. ITAMNCnliMWJM-ASIAS16 AS.1 -2/2IT AS2-3/3r- AS.3-4/4ASA-5/54WINGFEAR SPACE-^ _- --^August 17,^1991^_Lye Lake- --^ -- --^ I^ -- I -=^ _-^ C. expleta^-- non-flying _--Mites I 14- Bugs 18 -1^1^I-2^ --September 13, 1991^_Lye Lake -----2 ---C. expletanon-flying^--Mites 30 -Bugs 47I 1^i^1--^October 21, 1990Lye Lake-==----------^ IC. expletanon-flying-^I---^I Mites 14-Bugs 41^(i^I^t^I0^20^40^60^80^100mus ATTACHED PER REGION PERCENT OF TOTAL)RATES^Aug. 17, 1991PREVALENCE (%) 38.9PREVALENCE (%)(ATTACHED ONLY) 38.9ABUNDANCE 0.77ABUNDANCE(ATTACHED ONLY) 0.77Table 15a.Table 15b.Table 15c.RATES^Sept. 13,1991ABUNDANCE(ATTACHED ONLY) 0.64PREVALENCE (%) 29.3PREVALENCE (%)(ATTACHED ONLY)ABUNDANCE ABUNDANCE(ATTACHED ONLY)29.30.340.34Field Samples93Figure 15. Field studies. Location of mite attachment on non-flying C. expleta by collection date, plotted as a percent oftotal mites collected.Table 15. Field studies. Quantity of mite attachment on non-flying C. expleta a.) August 17, 1991: Lake Lye b.) September 13,1991: Lake Lye c.) October 21, 1990: Lake Lye.T . A . MPCTLA4PCTLeA-AS 1ASAS 1-26^AS 2W^AS 2-3AS. 3AS 9-4AS. 4AS 4-6ASWing (vein)Wing (memb.)EJSeptember 13, 1991Lye LakeT . A .M.OCTLA4►D71.114-AS.1AS 1AS 1-2AS. 2AS 2-3AS. 3ti) AS 9-4AS. •AS. 4-6As b•Wing (vein)Wing (memb.)August 17, 1991Lye Lake20^i22^I -1 MitesBugsC. expl etaflying0C. expletaflying24Mite s45Bugs20 ^ 60^8040 1000MITES ATTACHED PEP REGION (PERCENT OF TOTAL)PARASITISM^DATERATES Aug. 17, 1991PREVALENCE (%) 45.5PREVALENCE (%)(ATTACHED ONLY) 31.8ABUNDANCE 1.09ABUNDANCE(ATTACHED ONLY) 0.91Table 16a.PARASITISM^DATERATES^Se t. 13, 1991PREVALENCE (%) 37.8PREVALENCE (%)(ATTACHED ONLY) 37.8ABUNDANCE 0.58ABUNDANCE(ATTACHED ONLY) 0.53Table 16b.Field Samples94Figure 16. Field studies. Location of mite attachment on flyingC. expleta by collection date, plotted as a percent of totalmites collected.Table 16. Field studies. Quantity of mite attachment on flying C.expleta a.) August 17, 1991: Lake Lye b.) September 13, 1991:Lake Lye.952) Proportions of C. bifida compared to C. expleta.Percentages of C. bifida versus C. expleta in the variouslakes over time are given in Table 17. C. bifida comprised over90 % of Cenocorixa spp. in all collections in Long Lake (9 232to 13 484 pS cm -1). Collections below this salinity in NearOpposite Crescent Pond, Greer Lake, and Box 27 never recorded C.expleta during 1990 or 1991, whereas C. bifida was present inall 3 bodies of water. In Lake Lye (9 807 to 13 493 pS cm -1 ),the percentage of C. expleta fluctuated from 57.2 % to 66.9 %.In the 2 high salinity lakes (Round-up and Barnes) (> 13 000 pScm-1), C. expleta dominated comprising over 90 % of the totalCenocorixa spp.96Date of CollectionBody of Water(salinity)Long (Chilcotin)July 20/90bif 94.6 %June 2/91bif 100 %August 12/91bif 100 %Sept.^14/91bif 96.2 %(9 232 to exp^5.4 % exp^3.8 %13 484 pS cm -1 ) (n = 56) (n = 32) (n = 26) (n = 26)Lye n/a bif 42.8 % bif 33.8 % bif 33.1 %(9 807 to exp 57.2 % exp 66.2 % exp 66.9 %13 493 pS cm') (n = 28) (n = 68) (n = 148)Barnes(13 687 ton/a bif^4.2 %exp 95.8 %n/a n/a19 528 pS cm 1 ) (n = 73)Round-up(15 224 ton/a bif^9.3 %exp 90.7 %n/a n/a22 724 pS cm-1 ) (n = 54)bif = C. bifida exp = C. expletaTable 17. Field data. Percent composition of Cenocorixa spp.water boatmen collected in lakes by date. n/a = notavailable.973) Proportions of flying and non-flying Cenocorixa spp.Percentages of non-flying (sclerotized and non -sclerotized)and flying morphs are presented in Table 18. In Long Lake, theover-wintering population of C. bifida for 1990 was 100 % flying(based on June 2, 1991 sample: n = 32). The following over-wintering generation (based on September 14th sample of 25) was80 % flying.Lake Lye followed a similar trend with a June 2nd collectionrevealing 100 % flying (n = 12) and a September 14th collectionof 77.3 % flying (n = 44). The few C. bifida that were foundover-wintering at higher salinities (Barnes and Round-up) werealso predominantly of the flying morph.C. expleta on the other hand was more often found as the non-flying morph. Lake Lye had 75.0 % non-flying (n = 16) on June2nd and 69.6 % non-flying on September 13th (n = 92). TheSeptember collection had 18.5 % of the total C. expleta withentirely darkened dorsa, but only stage 2 wing muscledevelopment (non-flying). (In June, 37.5 % of the C. expletawere non-flying, but completely dark.) Higher salinities hadmostly non-flying C. expleta. Round-up had 77.6 % non-flying onJune 2, 1991.98Date of CollectionBody of Water^Corixid^June 2/91 August 12/91 Sept. 14/91(salinity)^TypeLong (Chilcotin) bif nf^0 %^0 %^20.0 %9 232 to^bif fly 100 % 100 % 80.0 %13 484 pS crri l )^ (n = 32)^(n = 26)^(n = 25)exp nf (light)^n/a^n/a^0 %exp nf (dark) 0 %exp fly^ 100 %(n = 1)Lye(9 807 to13 493 pS cm -1 )bif nfbif fly0 %100 %(n = 12)13 %87 %(n= 23)22.777.3(n =%%44)exp nf (light) 37.5 % 51.1^% 51.1 %exp nf (dark) 37.5^% 24.5 % 18.5 %exp fly 25.0 % 24.4^% 30.4 %(n = 16) (n = 45) (n = 92)Barnes bif nf 0 % n/a n/a(13^687 to bif fly 100 %19 528 pS cm -') (n = 3)exp nf (light) 62.9 %exp nf (dark) 17.1^%exp fly 20.0 %(n = 70)Round-up^bif nf^0 %^n/a^n/a(15 224 to bif fly 100 %22 724 pS (n = 5)exp nf (light) 38.8 %exp nf (dark) 38.8 %exp fly 22.4 %(n = 49)bif nf = C. bifida non-flyingbif fly = C. bifida flyingexp nf = C. expleta non-flyingexp fly = C. expleta flyingTable 18. Field data. Percentages of flying and non-flyingCenocorixa spp. in lakes at given times. n/a = not available.99DISCUSSIONA. Initial Mite Recruitment and Attachment (Moderate Salinity).1. Effect of Host Species.Question la. Does host species affect quantity of initialmite recruitment?Host species does not affect quantity of initial miterecruitment. Recruitment, as measured by prevalence andabundance, was not significantly different between host species(Tables 5, 6, and 7). This contradicts Hypothesis la. It wasexpected that there would be a difference in recruitment basedon the studies of Smith (1977). The lack of a significantdifference means that the exclusion of C. expleta is not becausemore mites are initially finding C. expleta compared to C.bifida.Does host species affect quantity of initial mite attachment?Host species does not affect the quantity of initial miteattachment. Attachment, as measured by prevalence (attachedonly) and abundance (attached only) did not differ significantlybetween host species (Tables 5, 6, and 7). This alsocontradicts Hypothesis la. The exclusion of C. expleta is notbecause more mites are initially able to attach to C. expletacompared to C. bifida.100Question lb. Does host species affect location of initialmite attachment?Host species does not affect the location of initial miteattachment. Experiments with all hosts (Figure 7) and flyinghosts (Figure 9) showed no significant difference in attachmentbetween species when sclerotization was held constant.^Thissupports Hypothesis lb.^In Experiment 2 (Figure 8) with non-flying hosts only, there appeared to be a species effect, assignificantly more mites attached to the T.A.M. of non-flying C.bifida compared to non-flying C. expleta. Post-experimentalexamination determined however, that some of these non-flying C.bifida had become partially sclerotized, resulting in inabilityof the mites to attach to the abdominal segments. Since thelocation of mites does not differ between C. bifida and C.expleta (when sclerotization remains constant), a species-specific effect on mite location is not the cause of C.expleta's exclusion.Overview of Initial Host Species Differences.I predicted Hypothesis la on mite recruitment and attachmentbecause of the results of Smith (1977), who found significantlymore mites attaching to C. expleta compared to C. bifida.Discrepancies between my results and his must have arisenbecause his experiments may have used both sclerotized andunsclerotized hosts, and were of variable time. Also, his lowsample sizes and lack of replicates did not allow for the fact101that the attachment of E. euryhalina is largely dependent onchance, and variability within replicates and between host typesis bound to occur.Acceptance of Hypothesis lb implies that there is nomorphological or behavioural difference between C. bifida and C.expleta that would cause mites to attach in different locationsbetween species. Since the location of attachment is similarbetween species, energy loss because of parasitism should be thesame. (Differences in energy loss are possible when mitesbecome crowded on one host type: see Discussion, Question 2b.)The implication that morphological differences betweenspecies are not great enough to cause differences in miteparasitism is confirmed by Jansson (1972). His descriptionsshow that morphological differences between Cenocorixa bifidaand C. expleta are primarily restricted to the hairs on thedistal segment of the forelegs (Jansson, 1972). Since this areais not used for attachment by water mites, the lack of hostdifferences in mite location and quantity is understandable.The only noticeable morphological difference that could causedifferences in the initial mite attachment and/or recruitment isthe predominance of the sclerotized morph of C. bifida comparedto the normally unsclerotized C. expleta (see Question 2).Behavioural differences between species were similarly notgreat enough to cause differences in mite attachment. Davids(1973) speculated that differences in the grooming behaviour ofcorixids may account for differences in the quantity and102location of the mite Hydrachna conjecta. I saw both C. bifidaand C. expleta groom against mites but could discern nodifference between species. Another behaviour that might havecaused higher quantity of mites on C. expleta, is if C. expletaspent more time near the water surface where mites collect. Iobserved that both species spent approximately the same amountof time near the water surface, and Reynolds (1974) concludedthat habitat utilization in the field was not significantlydifferent between species.2. Effect of Host Sclerotization.Question 2a. Does host sclerotization affect quantity ofinitial mite recruitment?Host sclerotization does not affect quantity of initial miterecruitment. C. expleta's predominant unsclerotized morph didnot have more mites recruiting to it than the predominant flyingmorph of C. bifida (Tables 5 and 8). This contradictsHypothesis 2a. Greater recruitment to the predominantunsclerotized morph of C. expleta, compared to the predominantsclerotized morph of C. bifida, is not the cause of C. expleta'sexclusion.Does host sclerotization affect quantity of initial miteattachment?Host sclerotization does not affect quantity of initial miteattachment. Initial attachment of E. euryhalina was notsignificantly different between sclerotized and unsclerotized103hosts (Tables 5 and 8). This also contradicted Hypothesis 2a.The exclusion of C. expleta is not because more mites areinitially attaching to the predominant unsclerotized morph of C.expleta compared to the predominant sclerotized morph of C.bifida.Question 2b. Does host sclerotization affect location ofinitial mite attachment?Host sclerotization does affect location of initial miteattachment (Figures 7 and 10). Most mites on sclerotized morphsattached to the T.A.M., whereas mites on non-sclerotized morphswere attached mostly on abdominal segments 2 and 3. Thissupported Hypothesis 2b. The crowding of mites on the T.A.M. ofsclerotized C. bifida should offer partial protection from theeffects of mite parasitism compared to non-sclerotized C.expleta. This is part of the cause of the exclusion of C.expleta (see below).Overview of Initial Host Sclerotization Differences.I predicted Hypothesis 2a on the recruitment and attachmentof mites because I believed that differences in hostsclerotization would cause differences in the quantity of miteattachment between host types, even after only 24 hours.reject Hypothesis 2a because I conclude that these differencesare not evident in the early stages of parasitism (but seeQuestion 4).Acceptance of Hypothesis 2b on the location of mites implies104that differences in the location of mite attachment betweensclerotized and non-sclerotized hosts could be part of the causeof C. expleta's exclusion. In the first 24 hours of the mite-corixid interaction, it is the only factor that is important tothe exclusion of C. expleta. The cause is that mites are morecrowded on the T.A.M. of sclerotized hosts, compared to theabdominal segments of non-sclerotized hosts.There are many examples in which mites that are crowdedtogether do not affect their hosts as much as non-crowded mites.Reilly and McCarthy (1991) found that Hydrachna conjecta weresignificantly smaller when 2 mites attached to the samehemielytron of their corixid hosts compared to when 1 miteattached to each hemielytra. (Smaller mite size is anindication of less host energy loss because of mite parasitism;Davids, 1973). Blockage of a mite's feeding tube or stylostomemay be one cause of the lessening of energy drain from a host.This can occur through host defenses or through competition bythe stylostomes of other mites (Abro, 1982).In interactions with relatively small hosts andproportionately large mites, impedance of mite growth can occurwith even 2 mites per host. Aiken (1985) found significantlysmaller Eylais sp. on the beetle Dytiscus alaskanus J. Balfour-Browne when 2 mites were present compared to single infections.Similar results were found by Lanciani (1971b) on beetles andcorixids, and Davies (1959) on black flies.In cases of extreme crowding, mite mortality can occur, which105must cause less energy loss to the host than if evenly spacedmites are engorging. Mitchell (1968) found up to 50 % mortalityof the mite Arrenurus mitoensis Imamura and Mitchell when mitedensity approached 30 mites per segment of the host damselflyCercion hieroglyphicum. Observations of mites attached to theT.A.M. of Cenocorixa spp. indicate that in the later stages ofengorgement, mite mortality occurs such that only one mite perT.A.M. can proceed past the initial stages of engorgement.Lanciani (1971b) suggested that the size of mites on aquaticinsects is limited by the size of the subelytral space anddemonstrated this through a study of the mite Hydrachna stipataLundblad on a backswimmer of the genus Notonecta. When attachedon the outside of the hemielytra, H. stipata was significantlylarger than when attached on the underside of the hemielytra.Based on the size of a fully engorged E. euryhalina (less than 2mm dorsal diameter = 6.28 mm 2), full engorgement of even one mitecould not occur if attached on one of the T.A.M (a triangulararea of 0.5 mm by 0.25 mm = 0.06 mm2 ) . I have witnessed theeffects of attempted engorgement on the T.A.M. from mitescollected in the field on non-flying C. expleta. The mitesbecome very elongated, with the anterior portion of theirabdomens stretched, while engorgement proceeds only in theposterior of the abdomen, where the depth of the subelytralspace is greater. Such deformity would most probably slow mitegrowth and certainly preclude metamorphosis to anymphochrysalid.^Thus the maximum energy drain on flying C.106bifida would be less than the equivalent of 2 fully engorgedmites (1 per side), and energy may even be drained at a slowerrate than from a normally engorging mite. Fully sclerotizedhosts would, therefore, have an advantage over their non-sclerotized conspecifics if the abundance of mites in thepopulation was over 2.The parasitism rates in the field often exceed this level asseen in my study and the more extensive field studies of Smith(1977). For example, my July 17th collection in Long Lake,taken at peak time for larval E. euryhalina, showed an abundanceof 3.07 mites per host on flying C. bifida, but an abundance(engorged only) of only 0.5 mites per host. The sclerotizationappeared to be providing the flying hosts with some protectionfrom super-parasitism by limiting the number of mites that couldcommence engorgment to 2 or less. This number is within therange that flying C. bifida can withstand, as shown by thepresence of 2 nymphochrysalid shells on some sclerotized hostsin the spring.B. Initial Mite Recruitment and Attachment: Low and HighSalinity1. Low salinity.Question 3a (low salinity).^Does low salinity affect quantityof initial mite recruitment?Low salinity does not affect quantity of initial miterecruitment compared to moderate salinity. Mite recruitment was107the same at low salinity (Table 9) and moderate salinity (Table5) for three of the host types. (High mortality of flying C.expleta at low salinity precluded an analysis of this hosttype.) This supports Hypothesis 3a. Based on this conclusion,Experiments 1, 2, 3, and 4 should be indicative of miterecruitment throughout the salinity at which C. expleta isexcluded.Does low salinity affect quantity of initial mite attachment?Similarly, low salinity does not affect quantity of initialmite attachment compared to moderate salinity. Mite attachmentwas the same at low salinity (Table 9) compared to moderatesalinity (Table 5) for three host types. This also supportsHypothesis 3a. Experiments 1 through 4 are indicative of miteattachment at the salinity at which C. expleta is excluded.Question 3b (low salinity).^Does low salinity affectlocation of initial mite attachment?Low salinity does not affect location of initial miteattachment (Figure 11) relative to moderate salinity (Figure 7).Location of mite attachment on three host types was notsignificantly different between moderate and low salinity. Thissupports Hypothesis 3b. Experiments 1 through 4 are indicativeof the location of mite attachment throughout the salinity atwhich C. expleta is excluded.108Overview on the Effects of Low Salinity on Mite Parasitism.Hypothesis 3a and 3b were predicted because I believed thatthe effects of mites would be the same from low salinity all theway through to the highest salinity from which C. expleta isexcluded.^Evidence for this is provided by Scudder et al.(1972).^They found no difference between C. bifida and C.expleta in the long-term ability to osmoregulate in lake waterfrom 730 to 12 200 pS cm -1, From this, they concluded that bothwere physiologically freshwater insects. In terms of theexclusion of C. expleta, the verification of Hypothesis 3 showsthat low salinity causes no change in C. expleta (i.e. inbehaviour) that would cause more mites to attach to it comparedto C. bifida. The increased mortality of flying C. expleta mayindicate however, that this host type is more susceptible toinitial osmoregulatory shock than other host types.2. High salinity:Question 3a (high salinity).^Does high salinity affectquantity of initial mite recruitment?High salinity does affect the quantity of intial miterecruitment compared to moderate salinity. Initial miterecruitment was significantly lower at high salinity (Table 10)than at moderate salinity (Table 6). My result contradictsHypothesis 3a, which predicted no mites would recruit, but sincethere is a significant decrease in recruitment, I conclude thathigh salinity does affect recruitment of E. euryhalina.109Does high salinity affect quantity of initial miteattachment?See conclusions above for mite recruitment.Question 3b (high salinity).^Does high salinity affectlocation of initial mite attachment?Owing to very low attachment of mites at high salinity, acomparison of the location of mite attachment at moderatesalinity was not possible.Overview on the Effects of High Salinity on Mite Parasitism.Hypothesis 3a regarding high salinity was predicted because Ibelieved that the factor that limited mites at high salinity wastheir inability to recruit and/or attach. With respect to theexclusion of C. expleta, the significant difference inattachment between moderate salinity and high salinityexperiments shows that there is a salinity between 10 000 and 18000 pS cm-1 that mites lose their ability to attach at the samerate. At this salinity, mites cease to have a substantialeffect on C. expleta, and consequently, C. expleta ceases to beexcluded.110C. Long-term Attachment and Engorgement.1. AttachmentQuestion 4a: Does host type affect the quantity of miteattachment over 6 to 8 days?Host type affected the quantity of mite attachment over 6 to8 days. Attachment was significantly greater on the predominantnon-sclerotized morph of C. expleta compared to the predominantsclerotized morph of C. bifida (Table 12). This supportedHypothesis 4a. The exclusion of C. expleta from low salinity iscaused, at least partially, by the higher number of mites thatare able to attach to the predominant morph of C. expleta over 6to 8 days of mite exposure.Question 4b: Does host type affect location of miteattachment over 6 to 8 days?Host type affected the location of mite attachment over 6 to8 days. Mites attached more on the T.A.M. of sclerotized C.bifida than the T.A.M. of non-sclerotized C. expleta (Figure12). This supported Hypothesis 2b. The decreased area ofsusceptibility of sclerotized C. bifida gives them a competitiveadvantage over non-sclerotized C. expleta, when in the presenceof mites.1112. Mite Engorgement.Question 4a. Does host type affect quantity of miteengorgement over 6 to 8 days?Host type affected the quantity of mite engorgement over 6 to8 days. Significantly more mites were able to engorge on non-sclerotized C. expleta than on sclerotized C. bifida (Table 13).This supported Hypothesis 4a. After 6 to 8 days, the greaternumber of engorging mites on the predominant non-sclerotizedmorph of C. expleta is at least partially the cause of C.expleta's exclusion.Question 4b. Does host type affect location of miteengorgement over 6 to 8 days?Host type affected the location of mite engorgement over 6 to8 days. Significantly more mites engorged on the T.A.M. offlying C. bifida than on the T.A.M. of non-flying C. expleta(Figure 13). This supported Hypothesis 4b. After 6 to 8 days,the crowded nature of the engorging mites on flying C. bifidaimpedes some of the mites' growth. This effect gives partialprotection to flying C. bifida, whereas non-flying C. expleta isfully affected by mites, and is excluded from areas where mitesare prevalent.112Overview of Long-term Mite Attachment and Engorgement.Acceptance of Hypotheses 4a on mite attachment andengorgement is the first evidence I found that more mites areassociated with C. expleta than C. bifida. Since I used thepredominant field morphs in my long-term experiment, theattachment differences witnessed should be most representativeof what is occurring to the two host species in the field.Higher prevalence on one host type has been shown to correlatewith lower host fecundity (Davids and Schoots, 1975; Smith,1977) and higher host mortality (Lanciani 1975, 1986). Thiscould be the most important factor in the exclusion of C.expleta from lakes of low and moderate salinity.The question of whether there is a long-term species effector an effect of sclerotization was not fully determined becauseof the use of only the predominant morphs of each species. Asalready stated (pp. 60-61), I would have liked to have used all4 hosts types as I did for Questions 1, 2, and 3, but use ofteneral non-flying C. bifida over 6 to 8 days would havepresented problems in analysis because some of them harden inthis time. Nevertheless, the use of the predominant morphs ofeach species assured that the results in my experiments arerepresentative of what is occurring in the field. From myfinding that species has no effect initially, whereassclerotization affects the location of initial mite attachment,I would speculate that the effects of sclerotization are moreresponsible for the results of Experiment 7.113I found that over 6 to 8 days, recruitment of mites (asmeasured by prevalence and abundance) differed between hosts.Since 24 hour studies (Experiments 1 through 4) indicated thatmites recruit to hosts equally, differences in recruitment over6 to 8 days must be because mites are leaving the hosts after 24hours, or they are being removed by the hosts. I observed thatdisturbed E. euryhalina are capable of disattaching within threedays of initial infection. These mites, perhaps failing tolocate the T.A.M. or finding this location unacceptable, mightleave the host and could conceivably even find and attach to anon-flying C. expleta. Mites were observed under laboratoryconditions entering and exiting the air space of hosts after afew minutes of exposure.With respect to mites being removed, Harris and Harrison(1974) stated that Hydrachna sp. (H. barri by Smith, 1987) maybe knocked off the legs of their corixid hosts, and Davids(1973) speculated that H. conjecta may be brushed of theunderside of the hemielytra. Mites could be dislodged duringflight and since the lake temperature during Experiment 7 wasabove the temperature at which flight is initiated (15 °Caccording to Scudder, 1969a), flight in the covered enclosureswas possible. Whatever the cause, over 6 to 8 days, more miteswere found on non-flying C. expleta compared to flying C.bifida, despite similar initial recruitment. Mite attachment,as measured by prevalence (attached only) was also significantlylower on flying C. bifida (Table 12). Mite engorgement, as114measured by prevalence (engorging only), followed the same trend(Table 13). The process by which C. bifida lowers the number ofpotential attaching and engorging mites is an important factorin its ability to withstand mite parasitism. Non-flying C.expleta, failing to lower mite engorgement is excluded fromareas with mites.Acceptance of Hypothesis 4b, regarding location of mites over6 to 8 days, causes me to reach the same conclusions as for theeffects of sclerotization on initial mite attachment (Hypothesis2b). Since I was using only two host types, I cannot, withcertainty, attribute the difference in location of miteattachment to a host sclerotization effect rather than a specieseffect. It does, however, seem most plausible thatsclerotization is causing the effect, because of the initialdifferences caused by sclerotization. The fact that 100 % ofthe mites engorging on flying C. bifida are on the T.A.M. showsthat this is truly the only susceptible area on this host type.Based on previous arguments (see Discussion, Hypothesis 2),flying C. bifida is protected from mite parasitism significantlymore than non-flying C. expleta.115D. Field Studies.1) Comparison of field parasitism and laboratory parasitism.Question 5a: Are laboratory experiments representative of thequantity of mites attaching on field collected corixids?Initial laboratory experiments were representative of thequantity of mites attaching on field collected corixids, butlaboratory experiments over 6 to 8 days differed from fieldcollections. In both initial laboratory experiments and fieldcollections, I found no significant difference between hosttypes. In 6 to 8 day laboratory experiments I did find asignificant difference between non-flying C. expleta and flyingC. bifida. Since it was found that attachment of mites in thefield did not differ between host types, there is no evidencefrom these field collections that mites are affecting C. expletamore than C. bifida (but see Overview).Question 5b: Are laboratory experiments representative of thelocation of mites attaching on field collected corixids?Laboratory experiments were representative of location ofmite attachment on non-flying C. expleta, but not representativeon either flying host. This supported Hypothesis 5b. Field-collected non-flying hosts are always non-sclerotized, providingmites with their preferred attachment sites on abdominalsegments 2 and 3. Field-collected flying hosts however, mayhave had mite attachment prior, during, or after sclerotization,116creating high variability in the location of mite attachment.Considering this in relation to how sclerotization protectsflying hosts (Question 2b), mites will have a much longer periodof time to find, attach and engorge on non-flying C. expletacompared to flying C. bifida. Location of mite attachment inthe field supports the theory that water mites have a greatereffect on C. expleta than on C. bifida.Overview of Field CollectionsFrom field collections, there was not a significantly greaterquantity of mites attaching to C. expleta compared to C. bifida.My 6 to 8 day laboratory experiment would indicate, however,that more mites do attach to non-flying C. expleta compared toflying C. bifida over 6 to 8 days. This contradiction requiresan explanation.The field collections of Smith (1977), Reilly and McCarthy(1991), and Aiken (1985) show great variability in the quantityof Eylais spp. mites attaching to their hosts over time. Theyall show a relatively low overwintering prevalence, a higherpeak in early summer when the summer generation of larval mitesis present, and a decrease in prevalence shortly after;accounted for by completion of larval mite development, and/ordeath of infected hosts.My collections showed no difference in prevalence, but thisdoes not preclude the fact that E. euryhalina is excluding C.expleta. If mites are recruiting to their hosts at the same117rate (Experiment 1 through 4), then no difference would be seenin field collections at this time. Also, if C. expleta aredying more quickly because of the later stages of engorgement,then a collection at this time might actually show lowerparasitism on C. expleta, because only uninfected hosts are leftin the population. My collections must have been at times inthe mite-corixid interaction when prevalences did not differbetween hosts.Both Smith (1977) and I found no nymphochrysalids on C.expleta. This would suggest that C. expleta has a higher long-term mite-induced mortality which is supported by my findings ofExperiment 7. This finding may be because of an effect of hostspecies and/or host sclerotization.Acceptance of Hypothesis 2b on the location of mites in thefield supports the theory that mites are excluding C. expletabecause of the predominance of its non-flying morph. From myfindings in the laboratory and the field, the sequence of eventsthat creates a differential effect of parasitism on C. expletais as follows:Larval E. euryhalina recruit to all hosts at the same rate(Table 5). If the potential host is non-sclerotized (non-flying), mites attach to the preferred sites in the middle ofthe abdominal dorsum (Figures 7, 13: non-flying hosts).Presumably, as sclerotization continues, there becomes a pointat which the integument of the abdomen is too hard or thick forthe mite's mouthparts to pierce, at which time attachment to the118T.A.M. becomes the mites' only option apart from leaving thehost (Figures 7, 10, and 13: flying hosts). This exodus ofmites from sclerotized hosts is reflected in significantly lessattached and engorging mites on flying C. bifida compared tonon-flying C. expleta over 6 to 8 days (Tables 12 and 13). Aspostulated in the discussion of question 2b, attachment to theT.A.M. probably does not cause as much energy loss to the host.Therefore, in terms of parasitism, it is in the host's bestinterest to become sclerotized as quickly as possible.Support for the hypothesis that host sclerotization controlsmite location is derived from a comparison of studies of mitesthat commence engorgement on teneral hosts during eclosion, withstudies of mites that begin engorging on sexually mature(sclerotized) insects. Mites on teneral hosts are foundengorging directly through the sclerites. Examples areHydrachna virella Lanciani on the dorsal pronotum of thebackswimmer Buenoa scimitra Bare (Lanciani, 1980); Arrenurusagrionicolus Uchida on the ventrum of the 7th abdominal segmentof the damselfly Cercion hieroglyphicum (Mitchell, 1968); andHydryphantes tenuabilis Marshall on the marsh treader Hydrometraaustralis (= myrae) Drake and Hottes that attaches through theabdominal segments directly after host eclosion (Lanciani,1971a).In contrast, the studies of mites that attach to hardenedhosts show these hosts engorging exclusively through membranousregions. Limnochares americana Lundblad attaches to various119damselflies, but always at the bases of the legs (Conroy andKuhn, 1977; life cycle from Smith and Cook, 1991). Partuniellathermalis usually attaches to the alary membranes at the base ofthe wings of the brine fly Paracoenia sp. (Wiegert and Mitchell,1973). Thyas barbigera Viets attaches to the posterior face ofthe thorax of Aedes spp. mosquitoes 72.3 % of the time, and 9.3% of the time on the cervical membrane between the head andthorax (Mullen, 1977). 2 Arrenurus spp. attach at the base of thelegs of the damselfly Lestes sponsa (Hansemann) (Abro, 1982).2. Proportions of C. bifida compared to C. expleta.Based on the field data from the summers of 1990 and 1991(Table 17), it would appear that mite parasitism is stillaffecting C. expleta in the same way it did in the study ofSmith (1977). In the summers of 1990 and 1991, E. euryhalinawas breeding in all lakes up to and including the salinity ofLake Lye (9 807 to 13 493 pS cm -'). Below this salinity, C.bifida comprised over 90 % of the Cenocorixa fauna in all lakes,while above this salinity, C. expleta comprised over 90 % of theCenocorixa fauna (Table 17). In Lake Lye, where mite prevalancewas moderate (not as high as in Long Lake), C. expleta comprised57.2 % to 66.7 % of the Cenocorixa spp. This lake is at theupper limit of E. euryhalina's salinity tolerance, and from myfindings on the effects of high salinity on E. euryhalina (Table2The posterior face of the thorax of mosquitoes is one ofthe softer parts of the body (Corbet, 1963).1206 compared to Table 10), the salinity may be lessening theeffects of mite parasitism.^This would allow C. expleta toestablish itself. The field data of 1990 and 1991 on theoccurrence of C. expleta, C. bifida and E. euryhalina supportthe hypothesis of Smith (1977) that parasitic water mitesexclude C. expleta from lakes of low salinity.3. Proportions of flying and non-flying Cenocorixa spp.The proportions of flying and non-flying Cenocorixa spp.during the summers of 1990 and 1991 were similar to thosereported in previous studies (Table 3). C. bifida waspredominantly flying, whereas C. expleta was predominantly non-flying (Table 18). Smith (1977) reports that in theoverwintering populations of 1976 and 1977, the non-flying morphaccounted for no more than 1 % of the population. In summary,the overwintering populations of C. bifida were predominantlyflying in 1962, 1963, 1967, 1976, 1977, 1990, and 1991, or 7 outof 10 years studied. Only in 1968 and 1969 did the non-flyingmorph predominate, while in 1966, the flying and non-flyingmorphs were in equal proportions. In contrast, C. expleta waspredominantly non-flying in the two years of my investigation,and all previous studies have found that C. expleta is mostlynon-flying (Scudder, 1975).As previously stated, the difference in the percentage offlying morphs between C. bifida and C. expleta must affect thesurvivorship of the two species when field abundance levels are1212 mites per host or higher (Question 2b). Although both speciesbegin their adult lives unsclerotized, C. bifida usually becomesflying and sclerotized, making it vulnerable only directly aftereclosion to adult, a period which takes no more than 8 days at20 ° C (Scudder, 1971). C. expleta, however, will usually remainnon-flying for its entire adult life, meaning that it must avoidthe gauntlet of parasites for its entire adult life. Thisdifference in the temporal "window of opportunity" for watermites, is the basis of the exclusion of C. expleta at lowsalinity.E. General DiscussionMy study of E. euryhalina has shown a greater effect ofparasitism on C. expleta compared to C. bifida. The questionremains, however, does a study of only E. euryhalina adequatelyaccount for the effects of all mites on Cenocorixa spp.?Other mites are present at Becher's Prairie, especially E.discreta and H. davidsi Smith, which are sometimes moreprevalent than E. euryhalina. especially at low salinity. Withrespect to E. discreta, there appears to be no reason whysclerotization of the host dorsum would not affect attachment inthe same manner as for E. euryhalina. Davids et al. (1977)described the preferred attachment sites in the field for E.discreta on some corixids of the. genera Sigara and Cymatia.They found that larval E. discreta preferred abdominal segments2, 3 and 4 on all hosts and my observations on E. discreta122concur with these findings. Lanciani (1969) claims that E.discreta prefers the tergum of abdominal segment 3, followed by2 and 4, with attachment to abdominal terga 1 and 5 only onheavily parasitized hosts. It appears that at low salinity, E.discreta and E. euryhalina occupy a similar niche, sometimescompete for the same attachment sites, and must be affected inthe same way by host sclerotization.I expect that a fully sclerotized host would preventattachment of E. discreta in its preferred site because of fieldobservations of E. discreta attaching to the T.A.M. of flyinghosts as well as dead, dessicated E. discreta on the hardenedterga of flying hosts. Preliminary measurements of thethickness of a sclerotized abdominal tergum show that it remainsvery thin, even when sclerotized, indicating that it is not thethickness of the sclerite that protects the host from miteengorgement. Longer chelicerae, therefore, would not help E.discreta to engorge because the host defense is more likelybased on the hardness of the integument rather than itsthickness.Hydrachna davidsi poses another challenge to the exclusiontheory based on sclerotization.^Their attachment is to theunderside of the hemelytra. As with Eylais spp. larvae, theparasitism only occurs on adult (winged) hosts, so in thisrespect, the effects of parasitism are acting on the samemembers of the population. I am not sure, however, that thedegree of sclerotization of the hemelytra differs between flying123and non-flying hosts. If this were true, attachment of H.davidsi should be possible on both C. bifida and C. expleta forthe entirety of their adult lives, irrespective of the degree ofsclerotization. From this assumption, H. davidsi could not belimiting C. expleta by the same mechanisms as Eylais spp., if atall.Attachment of H. davidsi on C. expleta and C. bifida wasstudied by Smith (1977). He found that mites attachedsignificantly more often (P < 0.05) on C. expleta than on C.bifida. The mechanism responsible is not known, and a fullstudy on H. davidsi and the sclerotization of the hemelytra ofCenocorixa spp. would be required to determine this.Nevertheless, the absence of C. expleta in low salinity lakescan be explained without an understanding of the effects of H.davidsi. The high prevalence of E. discreta and presence of E.euryhalina at low salinity means that there is no water bodybelow the salinity of Barnes Lake (13 687 to 19 528 pS cm -1 frommy study), in which parasitism will not occur on C. expleta.Combining the mite parasitism data from the lab and fieldwith the historical percentages of flying and non-flyingCenocorixa spp., one can make speculate on the co-evolution ofmites and water boatmen.The long-term fluctuations in flying morphs of C. bifida areexplained by differences in the temperature of the lake duringdevelopment of the last larval instar (Scudder and Meredith,1972). Above 15 ° C, development to the flying morph occurs in124both species in the laboratory and interspecific differences inthe percentage of flying morphs are thought to arise from slightdifferences in the timing of their life cycles (Jansson andScudder, 1974). It is possible, however, that abiotic factorsare not the only forces that control the proportions of flyingmorphs found in the field. The hypothesis is forwarded here,that parasitism by water mites is one factor accounting fordifferences in the composition of corixid populations.The findings of this work show a greater effect of parasitismon non-flying individuals compared to flying forms. Increasedmortality of non-flying individuals in regions of parasitism(low to moderate salinities) would cause the percentage of non-flying individuals to be lower than in areas with no parasites(high salinity). This helps explain the finding of Scudder(1975) that at the lower salinity range of each species, thereare greater percentages of flying morphs than at the higherlimits of the salinity range.In addition, parasitism may act as a selection agent favouringflying morphs. One would expect that in areas of highparasitism, both species would favour the production of theflying morph at lower and lower temperatures towards aphysiological minimum. This minimum may be 15 ° C as reported byScudder and Meredith (1972). In contrast, in permanent lakeswhere there are no parasites, there may be selection pressureagainst production of the flying morph (with its associatedovarian diapause), because it is associated with reduced125reproductive fitness (Young, 1965b). There would be no reasonfor an individual in a permanent, productive lake to foregoreproduction so that it could disperse. In these lakes, thepercentage of flying morphs should be lower, as reported inScudder (1975), and the temperature of inducement of wing muscledevelopment should be higher.From this reasoning, it can be further hypothesized that miteparasitism could have played a role in the speciation process ofC. expleta and C. bifida. The ancestor of C. bifida and C.expleta would have had two different selection forces on it whenmite parasitism evolved. At low salinities, where mites werepresent, the population would have evolved into a sclerotized,migratory form. At high salinity, where mites were not present,the population would have evolved into a non-flying, non-sclerotized, but more fecund form. Then through reproductiveisolation, these two subpopulations may have speciated into thepresent-day C. bifida and C. expleta.126REFERENCESAbro, A. 1982. The effects of parasitic water mite larvae(Arrenurus spp.) on Zygopteran imagoes (Odonata) J.Invert. Pathol. 39: 373-381.Aiken, R. B. 1985. 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Microhabitat selection and regionalcoexistence in water-striders (Heteroptera: Gerridae).Ph.D. thesis, Universtity of British Columbia, Vancouver,Canada. pp 313.Stout, V. M. 1953. Eylais waikawae n. sp. (Hydracarina) andsome features of its life history and anatomy. Trans. R.Soc. N. Z. 81: 389-416.Topping, M. S. and G. G. E. Scudder. 1977. Some physical andchemical features of saline lakes in central BritishColumbia. Syesis. 10: 145-166.Wiegert, R. G. and R. D. Mitchell. 1973. Ecology ofyellowstone thermal effluent systems: intersects of blue-green algae, grazing flies (Paracoenia, Ephydridae) and watermites (Partuniella, Hydrachnellae). Hydrobiologia 41: 251-271.132Young, E. C. 1961. Degeneration of flight-musculature in theCorixidae and Notonectidae. Nature, Lond. 189: 328-329.Young, E. C. 1965a. Teneral development in British Corixidae.Proc. Roy. Entomol. Soc. London Ser. A. Gen. Entomol. 40:159-168.Young, E. C. 1965b. Flight muscle polymorphism in BritishCorixidae: ecological observations. J. Anim. Ecol. 34:353-389.133Appendix 1.A. Life History Study of Eylais euryhalina Smith1. Life cycleFigure 17 shows the life cycle of E. euryhalina throughoutthe summer at Becher's Prairie based on field collections andobservations in 1990 and 1991. Dates are only approximationsand some information is not known because the study was onlycarried out from May to September.- Letters below correspondto points on the life cycle diagram.A. (May 4th)^Mites were almost entirely in parasiticstages (larva or nymphochrysalis) although rare free-livingnymphs were collected (1990). Larvae were either completelyunengorged (and looked dessicated) or were approaching fullengorgement. Nymphochrysalids displayed a wide range ofdevelopment from freshly eclosed to showing full legsegmentation and ocelli development.B. (June 2nd) Many free-living nymphs of E. euryhalinawere present in Long Lake and Lake Lye.^Eggs of E.euryhalina were found on Scirpus spp. in Lake Lye (1991) (12427 pS cm'), but did not hatch after several weeks in the lab(I suspect that they may have been from the previous summer).C. (June 9th) First E. euryhalina adult (female) wascollected (1991) from Long Lake (13 848 pS cm -1 ), andFigure 17. Life cycle of E. euryhalina at Becher'sPrairie compared to the life cycle of Cenocorixa spp.Letters correspond to approximate dates as describedin text pp. 133 - 136.135identified as such by the presence of an ovipositor (notvisible in nymphs).D. (June 19th) Many adult E. euryhalina were seen inlakes. More eggs from Long Lake were collected, but theyagain do not hatch in the lab (1991). I was unable to findteleiochrysalids in field.E. (June 27th) The first presence of Eylais spp. free-living larvae on the surface of Lake greer (5 823 pS cm') wasobserved (1991). In the laboratory, they were identified asboth E. euryhalina and E. discreta.^Egg masses of bothspecies were found on Ruppia sp. and Zanischiella sp.floating vegetation and successfully incubated in the lab.F. (July 6th) Free-living E. euryhalina larvae were seenon Long Lake (1991).G. (August 1st - 5th)^Freshly laid E. euryhalina eggmasses were no longer found in Lake Greer (E. discretapersisted for a week or two later), suggesting death of E.euryhalina adults that had over-wintered as parasitic larvae.Free-living larvae were relatively scarce in Long Lakecompared to mid-July.^A sentinel^study^(puttingunparasitized hosts in a lake with parasites) in Long Lakeshowed only 10.3 % parasitism on C. expleta non-flying (N29) and 0 % on C. bifida non-flying (N = 11) (August 5th to12th, 1991).136H. (September 13)^Second summer generation adult andfree-living E. euryhalina were present in Near OppositeCrescent Pond (4 261 pS cm -1 ) as well as Lake Lye.I. (October 21) Eggs collected from Lake Lye still hatchafter incubation at 20 ° C for 24 hours.^Adults cannot befound in Lake Lye or Long Lake.2. Salinity rangeThe presence of free-living larvae was used to determinewhether E. euryhalina was breeding in a certain body ofwater. As shown in Table 1 (p. 13), E. euryhalina was foundbreeding from low salinity (Box 27: < 300 pS cm -1 ), throughmoderate salinity (Near Opposite Crescent Pond, Lake Greer,Near Pothole Lake, Long lake (Chilcotin), and Lake Lye (9 807to 13 493 pS -1 ). Free-living larvae were not found in BarkleyLake (784 to 942 pS cm -1), but extensive collections were nottaken.Field collections and observations of other life stages ofE. euryhalina were also recorded. Engorging larvae andnymphochrysalids were found on flying hosts in all lakesincluding high salinity lakes such as Barnes, Round-up, andeven LB2 (20 639 to 22 724 pS cm -'). Free-living nymphs werealso recorded in small numbers from these lakes, but adultsand egg masses were never found.1373. Mite engorgement processFollowing Lanciani (1971) and Reilly and McCarthy (1991),increase in area of the dorsal region was used to measuremite engorgement by the formula for area of an ellipse = 1/2V (base X height).^The unengorged mites were 0.024 mm 2(0.15mm long by 0.1 mm wide).^Measurable engorgement wasfirst witnessed 3.5 days after initial infection (2.5 daysafter hosts were removed from mite-infected water) with aslight increase in width. This was followed by an increasein length as the intersegmental membranes swelled beyond theposterior end of the dorsal shield. After engorgement beganat 3.5 days, the mites' legs were entirely immobile, and by 8days some of the mites had already attained a roughlyspherical shape, unless in a site where full engorgement wasinhibited. Some mites, however, remained unengorged after 6to 8 days and closer examination revealed that they were dryand almost certainly dead. Under some of the unengorgedmites were black, necrotic spots similar to that describedfor Hydrachna conjecta on Sigara falleni (Davids, 1973). Thenecrotic spot usually, but not always correlated with theunengorged state of the associated mite. Necrotic spots werenot found on laboratory infected flying hosts as the reactiondid not occur on the T.A.M. which was the only place wheremites were able to commence engorgement. After 6 days, the138average size of mites for 2 replicates was 0.037 mm 2 (n = 49)and 0.038 mm 2 (n = 14) on C. bifida flying compared to 0.037mm2 (n = 4) and 0.025 mm 2 (n = 7) on C. expleta non-flying.After 8 days, C. bifida flying had mites of an average sizeof 0.031 mm 2 (n=2) and 0.045 mm2 (n= 12) and C. expleta non-flying had mites of 0.047 mm2 (n = 7) and 0.033 mm2 (n= 4).Once again, these data were not statistically tested and arepresented only to show that while engorgement was occuring,after 8 days the percentage of the total engorgement was verysmall. (A fully engorged E. euryhalina is nearly 2mm indiameter which equals 6.28 mm 2 .)B. Discussion of E. euryhalina Life History1. Life cycleFrom observations in the field and laboratory, someimportant facts about E. euryhalina have been discovered.Eggs are present all year, but do not remain viablethroughout the winter as evidenced by early collections ofeggs which appeared unembryonated and failed to hatch in thelaboratory. I found degradation of overwintered eggs andprelarvae, such that larval features evident in the fall(i.e. eyespots and vitelline) were not apparent in thespring. Lanciani (1970a) states that only 3 of 20 Eylaisspp. studied in North America overwinter exclusively in egg139stage, and E. euryhalina does not appear to be in thiscategory. From early spring observations, E. euryhalinaoverwinters exclusively as a parasitic larvae.The potential infection period by E. euryhalina isestablished at 4 months: from late June until the lakesfreeze in October with a decrease (or absence) of free-livinglarvae in August and September; the period of non-infectivitybeing dependent on the lake. It appeared that the lesssaline, usually smaller lakes (Near Opposite Crescent andLake Greer) had earlier appearance of free-living larvae thanthe larger, moderate salinity lakes (Long Lake and Lake Lye).Shallow surface temperatures in these lakes are similar andshow a small degree of variation between years (Scudder,1975), so differences in mite life cycles may have beenrelated to salinity, with lower salinity being quicker.(Free-living larvae were witnessed on June 27th in Lake Greercompared to July 6th for Long Lake.) Correspondingly, theabsence of first generation free-living larvae in summer alsooccurred earlier in the smaller, less saline lakes as well.The actual dates of the appearance of the second generationfree-living larval mites was not determined. By September13, 1991, second generation free-living larvae were presenton both Near Opposite Crescent Pond and Lake Lye.140From the presence of new, viable eggs in the field, it canbe estimated that E. euryhalina laid eggs for about 35 daysin the summer of 1991. With respect to the duration of theparasitic stage, an approximation of the period ofoverwintering for E. euryhalina can be judged by determiningthe parasitic duration of the latest possible attaching mitein the fall. Ice formed on the lakes at the end of October1990, and assuming that the last attaching mite is the lastone to leave the host in the spring (the end of May), themaximum possible time of the parasitic interaction (larvaeand nymphochrysalis) is 7 months at this latitude (52 ° N).Lanciani (1969) states that larvae of the genus Eylais may beparasitic for a maximum of 11 months. The duration of theparasitic phase in the summer was not definitivelydetermined, as laboratory rearing of mites on corixids inthis study and others (Davids, 1973) has proven difficult.2. Salinity rangeIt was found that the upper salinity limit of E.euryhalina was not based on failure of the larval form to beable to recruit or attach at high salinities. Experiment 6demonstrated ability of the larvae to attach at salinitiesabove the natural range of the species (albeit at low rates).Collections of nymphs in high salinity lakes suggests thatinability to engorge is not the limiting factor, although141perhaps commencement of engorgement must occur at lowersalinities before migration of the host to high salinity.The absence of adult mites in collections at these same highsalinities implicates the development from nymph toteleiochrysalis, or teleiochrysalis to adult as the limitinglife stage for E. euryhalina at high salinity.3. Mite engorgementBecause of the short duration of laboratory induced miteengorgement, no conclusions are drawn regarding the effectsof host type on mite growth rate.

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