This project will investigate the chemical and sensory foundations of host specificity in herbivorous insects. Differences in the ability of closely related insect species to use a plant host, and discrepancies between responses of adults and larvae of the same species will be examined. Two cabbage butterflies, Pieris rapae and P.napi oleracea (Lepidoptera) and five differentially utilized crucifers will serve as a model system. The major objectives are (1) to determine how behavioral and sensory responses differ between life stages and between species, and (2) to determine the extent to which chemical differences between plants account for their susceptibility to pierid herbivory. Different bioassays will guide the isolation of plant compounds affecting oviposition, feeding and growth of the insects. Electrophysiological recordings will be used to compare sensory and behavioral responses to the compounds. The results will provide an integrated view of the way that interactions between plant chemistry and insect responses at different life stages lead to insect-hostplant specificity.