Furanocoumarins and their detoxification in a tri-trophic interaction

The parsnip webworm, Depressaria pastinacella, specializes on wild parsnip, Pastinaca sativa, and several species of Heracleum, hostplants rich in toxic furanocoumarins. Rates of furanocoumarin metabolism in this species are among the highest known for any insect. Within its native range in Europe, webworms are heavily parasitized by the polyembryonic parasitoid wasp Copidosoma sosares. In this study, we determined whether these parasitoids are exposed to furanocoumarins in host hemolymph, whether they can metabolize furanocoumarins, and whether parasitism influences the ability of webworms to detoxify furanocoumarins. Hemolymph of webworms fed artificial diet containing 0.3 % fresh weight xanthotoxin, a furanocoumarin prevalent in wild parsnip hosts, contained trace amounts of this toxin; as well, hemolymph of webworms consuming P. sativa flowers and fruits contained trace amounts of six of seven furanocoumarins present in the hostplant. Thus, parasitoids likely encounter furanocoumarins in host hemolymph. Assays of xanthotoxin metabolism in C. sosares failed to show any ability to metabolize this compound. Parasitized webworms, collected from populations of Heracleum sphondylium in the Netherlands in 2004, were on average 55 % larger by weight than unparasitized individuals. This weight is inclusive of host and parasitoid masses. Absolute rates of detoxification (nmoles min⁻¹) of five different furanocoumarins were indistinguishable between parasitized and unparasitized ultimate instars, suggesting that the intrinsic rates of metabolism are fixed. Thus, although parasitized larvae are larger, detoxification rates are not commensurate with size; rates in parasitized larvae expressed per gram of larval mass were 25 % lower than in unparasitized larvae.