The factors determining host use by insect parasitoids remain an important area of research in ecology and in biological control applications. Evidence for a positive relationship between parasitoid size and fitness in a number of parasitoid groups suggests that host choice should be under strong selection. Host size (a proxy for developmental stage) is likely to be a primary cue for ovipositing parasitoids due to its potential influence on offspring development time, the risk of multiparasitism, and host immunocompetence. Here, we use both fly and wasp parasitoids reared from slug caterpillars to answer the following questions 1) How does a community of parasitoids sharing a common set of hosts partition these resources? 2) Does host size influence adult parasitoid size and/or sex allocation? Do flies and wasps respond in the same manner? 3) For a common host caterpillar species, how does host plant quality affect adult parasitoid size? (i.e., does host plant quality ‘cascade up’?). Taking a community approach, we measured the hind tibia lengths of wasp and fly parasitoids reared from 10 common host species of generalist caterpillars (Limacodidae) reared on seven different host plant species.

Results/Conclusions

We found that parasitoids partition their limacodid host resources based on size and solitary wasps use host size as a sex determinant in oviposition. Wasps tend to be reared from caterpillars collected in early instars, while flies tend to be reared from caterpillars reared in late instars. Solitary wasps choose significantly larger host caterpillars for their daughters than they do for their sons. Host plant quality indirectly affects parasitoid size, and presumably parasitoid fitness, through its direct effects on the size and performance of the caterpillar host. The host plants that produced the largest pupae (black cherry and white oak) in the absence of enemies also yielded the largest flies, when controlling for host species. These differences could contribute to host plant-specific differences in parasitoid foraging. Host plant effects may be most likely to ‘cascade up’ in parasitoids with delayed development (e.g., tachinid flies) where host plant effects on host caterpillars are fully realized. Interspecific competition for host resources in this system is likely and could lead to partitioning of development stages among parasitoid taxa.