Most studies of tritrophic interactions focus on the effects of plant chemistry on higher trophic levels (herbivores and their natural enemies). However, if top-down effects of natural enemies are strong and herbivores have selective impact on plant fitness, natural enemies may exert selective pressure on plant investment in defensive chemistry. We examine the effects of plant chemistry in a tritrophic system native to Eurasia consisting of three apiaceous host plants (Pastinaca sativa, Heracleum sphondylium, and H. mantegazzianum), the parsnip webworm (Depressaria pastinacella), and the polyembryonic encyrtid, Copidosoma sosares. Dutch populations of P. sativa produce lower concentrations of furanocoumarins that are negatively associated with several C. sosares fitness correlates than midwestern US populations which harbor webworms but lack C. sosares (as well as any other natural enemies of the webworm). That Dutch parsnip populations produce significantly lower furanocoumarin levels than midwestern US parsnip plants suggests C. sosares reduces webworm herbivore pressure sufficiently to allow Dutch parsnip plants to invest less in physiologically costly furanocoumarins. Lower numbers of webworms on Dutch parsnip plants compared to plants in midwestern US populations supports this contention. Recent discovery of C. sosares in parsnip populations in the western US provides a unique opportunity to explore how tritrophic relationships are altered during the course of introduction in situations where the trophic structure is intact (western US) and where the third trophic level is absent (midwestern US). Here we compare the furanocoumarin chemistry of western US parsnip populations with those from midwestern US and Dutch parsnip populations.