Water mite parasitism of water boatmen (Hemiptera:Corixidae)

In this study the consequences of water mite parasitism on water boatmen were investigated, concentrating on two host species of the genus Cenocorixa. It was established that mite parasitism severely restricted egg production in Cenocorixa bifida Hung.: whether this should be attributed to a nutritional drain or through hormonal intervention was considered. The possibility of mite interference in flight ability and post-imaginal flight muscle development was also investigated. It was found that mite parasitism of C. bifida in the field varied considerably between habitats, salinity of the lake water influencing both the mite species involved and the prevalence of mite parasitism. When tested both in the field and laboratory, there was no apparent difference in parasitism rates based on the sex, morph or teneral development of the host. It was concluded that individuals of a species were equally susceptible to attack. There was, however, a very definite difference in susceptibility between host species based on equivalent exposure under laboratory conditions. When C. bifida and Cenocorixa expleta Uhler in particular were compared, C. expleta was significantly preferred by the four main mite species infecting C. bifida. This was substantiated in field data. Considering the prevalence of mites on C. bifIda, and the susceptibility of C. expleta to parasitism, the probability of the latter being parasitized approaches 100% in lakes within the salinity tolerance range of mites. When parasitism of these two host species was further investigated, it became apparent that C. expleta cannot sustain mite parasitism and in most cases, died. Past workers have noted the limited coexistence of C. expleta and C. bifida. Despite both species being physiologically fresh water insects, they only cohabit lakes in the upper salinity range of C. bifIda. When the relative abundance of these two species was compared over the salinity range in which they coexist, C. expleta was rare until the upper salinity limit of mites was reached. There was a defined change in their relative abundance at this point, C. expleta being in the majority when salinity was "above this limit. It is evident that water mites severely reduce the reproductive success of C. expleta in low salinities. They are therefore instrumental in influencing the outcome of any biological interactions between C. expleta and C. bifida in these lakes.Science, Faculty ofZoology, Department ofGraduat