Enzymatic Adaption in a Specialist Herbivore for Feeding on Furanocoumarin‐ Containing Plants

Furanocoumarins are plant secondary compounds toxic or deterrent to a number of herbivores. The parsnip webworm, Depressaria pastinacella (Lepidoptera: Oecophoridae), however, feeds only on three genera of Umbelliferae, all of which contain furanocoumarins. The purpose of this study was to determine the tolerance of D. pastinacella to furanocoumarins occurring in its hosts, and to evaluate the adaptive significance of polysubstrate mono—oxygenases (PSMOs) in enabling this specialist to feed on plants chemically inaccessible to most other herbivores. The D. pastinacella larvae were offspring of adults collected as pupae on parsnips in the wild. The larvae were reared to pupation on a semidefined artificial diet with and without the furanocoumarins xanthotoxin and angelicin, under conditions of UV light appropriate to photoactivate xanthotoxin. Bioassays were performed on pupal mass for xanthotoxin and angelicin, and tested using one—way and two—way ANOVAs, respectively. No effect on larval growth and survivorship by xanthotoxin, a linear furanocoumarin toxic to polyphagous herbivores, was detected. PSMOs in the microsomal fraction of the midgut possess a high specific activity for this furanocoumarin. The importance of these enzymes in the detoxification of xanthotoxin was further suggested by the observation that xanthotoxin induces microsomal enzymes that in turn metabolize this allelochemical. Enzymatic adaption to xanthotoxin allows D. pastionacella to feed on plants unavailable to many generalist herbivores. Moreover, no effect of angelicin, an angular furanocoumarin, was found on parsnip webworm growth. Angelicin is toxic to at least one other herbivore adapted to feeding on xanthotoxin. Although the mechanism of the parsnip webworm's adaption to angelicin was not determined, the ability to tolerate this compound further suggests that D. pastinacell is a specialist on furanocoumarin—containing hosts.