The role of parasitoids in evolution of habitat and larval food plant preference by three Pieris butterflies

This article attempts to explain that parasitoids provide the evolutionary pressure responsible for relationships between habitat use and larval food plant use in herbivorous insects. Three species of butterflies of the genus Pieris, P. rapae, P. melete, and P. napi use different sets of cruciferous plants. They prefer different habitats composed of similar sets of crucife rous plants. In our study, P. rapae used temporary habitats with ephemeral plants, P. melete used permanent habitat with persistent plants, although t hey also used temporary habitats, and P. napi used only permanent habitat. The choice experiment in the field cages indicated that each of the three b utterfly species avoided oviposition on plants usually unused in its own ha bitat, but accepted the unused plants which grew outside its own habitat. T heir habitat use and plant use were not explained by intrinsic plant qualit y examined in terms of larval performance. Pieris larvae collected from per sistent plants or more long lasting habitats were more heavily parasitized by two specialist parasitoids, the braconid wasp Cotesia glomerata and the tachinid fly Epicampocera succincta. The results suggest that Pieris habita t and larval food plant use patterns can be explained by two principles. Th e evolution of habitat preference may have been driven by various factors i ncluding escape from parasitism. Once habitat preference has evolved, selec tion favors the evolution of larval food plant preferences by discriminatin g against unsuitable plants, including those which are associated with high parasitism pressures.