Abstract. Plant-mediated egg mortality varied greatly for the oligophagous leafhopper, Carneocephala floridana, a common inhabitant of the salt marshes along Floridas Gulf coast. In the laboratory, survivorship from egg to first instar was generally greater on two herbaceous plants, Borrichia frutescens and Salicornia virginica (80 and 81% respectively) than it was on two grasses, Distichlis spicata and Spartina alterniflora (41 and 77%, respectively). Although C. floridana laid significantly more eggs per clutch on S. alterniflora than on the two herbaceous species, 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 the four host species were infested with eggs of C. floridana at natural densities consistently yielded higher parasitism rates by two mymarid wasps on the grasses, regardless of the background plants. C. floridana switches host plants seasonally, using the grass species mainly during the winter months, when the herbaceous species experience a substantial dieback. The herbaceous species are nutritionally superior to the two grass species as a food source for C. floridana. C. floridana reared on the herbs produce larger, more fecund, adults than they do on the grasses (Rossi 1991). However, during cold winter snaps the grasses, while less "attractive", may provide the most abundant nutritional and ovipositional resources available to C. floridana. The results of this study indicate how variable rates of plant- and parasitoid-mediated egg mortality may work synergistically with the interspecific nutritional status of the plants to explain host switching for this insect.