Tritrophic effects of furanocoumarins on development and life history of polyembryonic parasitoids

Plants are able to persist in the presence of herbivores due to their abilities to defend themselves with phytochemicals and the presence of natural enemies of those herbivores. Chemical defenses can often harm natural enemies in addition to herbivores. The well-studied ditrophic interactions between larval lepidopteran herbivores and host plants in the family Apiaceae provide a model for the study of tritrophic interactions that include a natural enemy. In this document, I describe a series of laboratory experiments in which xanthotoxin, a linear furanocoumarin produced by apiaceous plants, is added to the diet of parasitized and unparasitized caterpillars. Two species, one (parsnip webworms) specializing on xanthotoxin-containing host plants and another (cabbage loopers) with a highly polyphagous feeding habit, are compared. Both caterpillar species are parasitized by polyembryonic wasps in the genus Copidosoma. Clutch size and survivorship of C. sosares, a specialist of parsnip webworms, are reduced when parsnip webworms are reared on an artificial diet high in xanthotoxin compared to low xanthotoxin and xanthotoxin-free diets. Diet had no effect on unparasitized parsnip webworms. Copidosoma floridanum clutch size, survivorship, and development time, as well as cabbage looper pupa mass and development time, were all negatively affected when host cabbage loopers were reared on artificial diets with moderate levels of xanthotoxin compared to low and xanthotoxin-free diets. High xanthotoxin diets were lethal to cabbage loopers. We detected xanthotoxin in host hemolymph, the tissue in which both Copidosoma species develop. Neither Copidosoma species demonstrated any ability to metabolize xanthotoxin. It is likely that the ability of the host caterpillar to metabolize dietary xanthotoxin is responsible for the observed differences in the two parasitoid species, rather than specialization by the parasitoids themselves.;Parasitized and unparasitized parsnip webworms were collected in 2004-6 from 35 sites in the western United States along with seeds from the host plants upon which they fed. All webworms were collected from wild parsnip and cowparsnip, and any C. sosares developing inside webworms were allowed to emerge. Wild parsnip contained higher concentrations of four of five furanocoumarins, including xanthotoxin, quantified from the collected seeds. Clutch sizes of C. sosares were reduced when reared from webworms collected from wild parsnip compared to cowparsnip, whereas there was no difference in webworm pupal size between the two plant species. The decrease in C. sosares clutch size was likely due to furanocoumarins encountered in hemolymph as indicated in our artificial diet study with C. sosares.;Host caterpillars represent finite resources for both Copidosoma species, and individual larvae receive access to less of the host as primary clutch size increases. Because mixed-sex broods contain siblings while single-sex broods contain genetically identical individuals, intensity of competition may vary between mixed-sex and single-sex broods. We compared body size between brood types and related mixed-sex clutch size to survival and sex ratio. Regardless of brood type or species, individual body size decreases with increasing primary clutch size, although body size is more negatively affected by clutch size in mixed-sex broods. Furanocoumarins reduce the quality of the host caterpillar, possibly increasing the intensity of competition between clonemates and siblings during development.