ACQUISITION OF A NATIVE HOSTPLANT BY AN INTRODUCED OLIGOPHAGOUS HERBIVORE

An insect species that has been associated with a particular host may be expected to possess preadaptations that allow it to utilize related host species as it encounters them. If the related host has been available for only a short time, it is reasonable to expect that the new host is less suitable for the colonizing insect. Introduced from Europe, Depressaria pastinacella (Lepidoptera: Oecophoridae), the parsnip webworm, is found throughout eastern North America not only on the introduced European hostplant Pastinaca sativa (wild parsnip) but also on the native confamilial host plant Heracleum lanatum (cowparsnip). Full-sib families of webworms founded by parents from both hostplants were raised on diets containing fruits of either parsnip or cowparsnip. Parental hostplant had a significant effect on both pupal weight and development time of progeny; identity of the sire in each cross accounted for a significant amount of variance in pupal weight of progeny, indicating the presence of additive genetic variance for this trait. Overall survival is highest for individuals on diets containing parsnip fruits; mortality of female offspring of adults collected from H. lanatum was markedly higher than mortality of male caterpillars from either host population. This sexual dimorphism in survivorship resulted in sex ratios as high as 3.4: 1 in treatments consisting of offspring of H. lanatum adults on diets containing H. lanatum. Preference tests conducted on larvae collected in two different localities failed to demonstrate any preference for H. lanatum fruits, even for larvae collected from H. lanatum; larvae from one locality exhibited a significant preference for wild parsnip. Larvae fed fruits of cowparsnip exhibited reduced consumption rates and growth rates compared with larvae fed fruits of wild parsnip. Thus, individual webworms associated with cowparsnip may experience lower fitness than conspecifics associated with parsnip, the presumed original European hostplant.