Effects of water mite parasitism on Cenocorixa spp. (Heteroptera: Corixidae)

The purpose of this study was to understand the mechanisms by which parasitic water mites exclude one species of sympatric water boatmen from low salinity, while a sibling species survives. Attachment of the larval water mite Eylaiseuryhalina Smith was studied in the laboratory on two species off light polymorphic water boatmen, Cenocorixa bifida hungerfordi Lansbury and C. expleta (Uhler). After 24 hours of exposure, prevalence and abundance of mites did not differ significantly between host species or host morph (sclerotized versus non-sclerotized). From this, it was concluded that mites recruited to all host types at the same rate. By measuring prevalence and abundance of attached mites only, it was determined that the number of mites initially able to attach also did not differ significantly between hosts. In analyzing the initial location of attachment of E. euryhalina on the four host types, no significant difference was found between species, but a significant difference was discovered between sclerotized and unsclerotized morphs. This effect was evident as a shift of mite attachment from the centre of 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 mite attachment. A six- to eight-day study of the morphs of each species that are predominant in the field found significant differences inmite prevalence between hosts. Non-flying C. expleta had significantly greater prevalence of mites than flying C. bifida. The number of engorging mites was also significantly greater on non-flying C. expleta. Location of attached and engorging mites followed the same trends as seen in one day experiments. Based on these findings and initial studies, it is argued that it is the sclerotization of C. bifida that causes a reduction in the prevalence of mites over time, rather than a host species effect per se. Because on sclerotized hosts, mites can only engorge on the T.A.M., the number of engorging mites on these hosts is limited to 2 or less, whereas greater number of mites can feed on non-sclerotized hosts. As C. expleta is normally non-flying in the field, whereas C. bifida is predominantly flying, C. bifida has a competitive advantage where mites are present in abundances of greater than 2 mites per host. Field collections of parasitized hosts showed the same patterns of spatial mite attachment as in the laboratory, except that sclerotized hosts often had mites attached directly through the abdominal terga. This must have been the result of mite attachment prior to host sclerotization. Abundances of mites in the field were greater than 2 mites per host in some collections. The predominance of the sclerotized, flying morph of C. bifida appears to allow this species to survive at low salinity where mites abound. C. expleta is excluded from these waters, but its predominantly non-sclerotized, non-flying condition allows better reproduction at moderate to high salinities in the absence of mites. The alternative methods by which these two closely related species of water boatmen have dealt with parasite pressure implicates mite parasitism as a possible impetus in their speciation process.Science, Faculty ofZoology, Department ofGraduat