Chemical barriers to adaptation by a specialist herbivore

Depressaria pastinacella, the parsnip webworm (Lepidoptera: Oecophoridae), feeds throughout eastern North America on Pastinaca sativa (wild parsnip) and few other species. The assumption that specialist herbivores such as the parsnip webworm are adapted to hostplant chemistry, and are therefore unaffected by chemical variation in hostplants, was tested. Flower buds from plants grown first in the greenhouse and then in the field were fed to ultimate instar webworms. Plant phenotype had a significant effect on virtually all webworm food utilization parameters. While nutritional factors (i.e., nitrogen content) were correlated with approximate digestibility, two constituents of the flowers - bergapten and xanthotoxin, both linear furanocoumarins - independently accounted for a significant amount of variation in food utilization indicies. The physiological effects of these furanocoumarins were confirmed in artificial diet experiments. Despite the fact that the two most important furanocoumarins in parsnip flowers relative to webworm feeding and growth are isomers, differing only in the positioning of a methoxy substituent, they have different physiological effects; while xanthotoxin in general has no effect on growth, bergapten decreases growth and digestibility of the diet. These results underscore the need in studies of plant-animal interactions to examine individual chemical components rather than classes of compounds.