Most therories of chemical coevolution between plants and insects assume reciprocal genetic changes effected by the interacting species. Definitive genetic changes, however, have rarely been examined in natural systems. In the interaction between Depressaria pastinacella, the parsnip webworm, and Pastinaca sativa, the wild parsnip, herbivory varies according to plant furanocoumarin content and webworms can exert sufficient selection pressure to cause changes in the distribution of these genetically controlled chemical traits in plant populations. In this project, the extent to which physiological and behavioral responses to parsnip furanocoumarins are under genetic control in parsnip worms and whether these chemicals can exert sufficient selection pressure to change the distribution of physiological and behavioral traits in webworm populations will be determined. This study will therefore provide quantitative data for evaluating not only concepts of chemically mediated coevolution but also more general theories of host adaptation and speciation in herbivorous insects.
