HOST-PLANT-MEDIATED INTERACTIONS BETWEEN A GENERALIST FOLIVORE AND ITS TACHINID PARASITOID

1. We investigated the consequences of feeding on different species of host-plants on the performance of a generalist lepidopteran larva (Platyprepia virginalis (Bvd.) Arctiidae) and its interactions with a tachinid fly parasitoid (Thelairia bryanti Curran). 2. Growth rate of P. virginalis was not different for larvae reared individually on either poison hemlock, Conium maculatum L., bush lupine, Lupinus arboreus Sims, or fiddle neck, Amsinckia menziesii (Lehm.) under laboratory conditions. 3. Further, larvae caged in the field on either hemlock or lupine did not differ in developmental rates or pupal mass. In 1 of 2 years survivorship was higher for moth larvae caged on hemlock. 4. The parasitism of P. virginalis by T. bryanti was, however, strongly influenced by host plant. In the early spring of 1990 and 1991 we collected small woolly bear larvae from either lupine (1990) or hemlock (1990 and 1991); some were dissected to determine parasitism rates by T. bryanti and the remainder were returned to the field and caged on either hemlock or lupine foliage. 5. Dissections showed that a larger proportion of moth larvae collected from hemlock were parasitized by T. bryanti than moth larvae collected from lupine. Conversely, caterpillars caged on lupine were more likely to have flies emerge from them than caterpillars caged on hemlock, suggesting that host diet influenced success of developing fly larvae. 6. Moth larvae caged on hemlock were more likely to survive emergence of fly larvae and complete development (55%) than moth larvae caged on lupine (40%). 7. Parasitoid pupal mass was not related to host-plant diet in cage experiments. However, flies emerging from moth larvae that subsequently survived were heavier than flies that emerged from larvae that died. 8. Overall, moth larvae did equally well or slightly better when caged on hemlock while flies did better if their host caterpillar fed on lupine. Given that woolly bear larvae on lupine have a reduced chance of being parasitized (indicated by dissection results), but a greater chance of surviving parasitism when feeding on hemlock, we speculate that selection should favour multiple host use by woolly bears.