THE EFFECT OF PLANT-INDUCED AND PARASITOID-INDUCED EGG MORTALITY ON THE INTERSPECIFIC DISTRIBUTION OF AN OLIGOPHAGOUS HERBIVORE

Plant-mediated egg mortality varied greatly for the oligophagous leafh opper, Carneocephala floridana, a common inhabitant of the salt marshe s along Florida's Gulf coast. In the laboratory, survivorship from egg to first instar was generally greater on two herbaceous plants, Borri chia frutescens and Salicornia virginica (80 and 81% respectively) tha n it was on two grasses, Distichlis spicata and Spartina alterniflora (41 and 77% respectively). Although C. floridana laid significantly mo re eggs per clutch on Sp. alterniflora than on the two herbaceous spec ies, this increase in egg number was offset by the higher mortality of eggs laid on this species. The greatest source of identifiable plant- mediated egg mortality was death due to desiccation of the leaf tissue surrounding the egg clutch. In addition, field experiments in which t he four host species were infested with eggs of C. floridana at natura l densities consistently yielded higher parasitism rates by two mymari d wasps on the grasses, regardless of the background plants. C. florid ana switches host plants seasonally, using the grass species mainly du ring the winter months, when the herbaceous species experience a subst antial dieback. The herbaceous species are nutritionally superior to t he two grass species as a food source for C. floridana. C. floridana r eared on the herbs produce larger, more fecund, adults than they do on the grasses (Rossi 1991). However, during cold winter snaps the grass es, while less ''attractive'', may provide the most abundant nutrition al and ovipositional resources available to C. floridana. The results of this study indicate how variable rates of plant-and parasitoid-medi ated egg mortality may work synergistically with the interspecific nut ritional status of the plants to explain host switching for this insec t.