Print PageVarious ecological factors may influence host use by insect parasitoids, including both top-down and bottom-up factors. One important parameter that differs among habitats is the amount of sunlight that reaches plants. Light level is known to influence a suite of plant nutritional and resistance traits which in turn affect herbivore performance, but the extent to which these bottom-up effects cascade up to affect higher trophic levels is unclear. Here, we asked: what are the direct and indirect effects of light environment on host use by parasitoids? Specifically, we aimed to understand how light environment (light gap v. shaded forest understory) and leaf type (sun v. shade leaves) affected the performance and incidence of parasitism of two species of moth caterpillars, Acharia stimulea and Euclea delphinii (Limacodidae). We manipulated the leaf type of potted white oak (Quercus alba) saplings by growing them in either full shade or full sun during leaf expansion. These saplings were then transported to the forest, factorially placed in light gap and shaded habitats, and stocked with sentinel caterpillars to be exposed to parasitism. Additional caterpillars were reared in sleeve cages (protected from parasitism) to isolate light environment and leaf treatment effects on caterpillar performance.
Results/Conclusions
In the ‘exposed’ experiments, we found that light environment, but not leaf type, affected the likelihood of parasitism. Euclea delphinii caterpillars were 11.5 times more likely to be parasitized in sunny light gap habitats than in shaded understory habitats, a significant difference. This pattern was consistent for both tachinid fly and wasp parasitoids across two separate experiments. Overall, parasitism was high for E. delphinii (31-86%). In contrast, the incidence of parasitism overall was very low for A. stimulea (<1-18%), resulting in non-significant effects of both habitat and leaf type on the likelihood of parasitism for this species. The ‘bagged’ experiments revealed complex bottom-up effects on caterpillar performance. Both caterpillar species developed more quickly when feeding on sun leaves than shade leaves (irrespective of habitat), whereas greater pupal mass was attained in shade habitat than in sun habitat (irrespective of leaf type). Our results suggest that the direct effects of light environment on rates of parasitism supersede any indirect effects resulting from altered leaf quality. These conclusions are strengthened by the consistency of this pattern across parasitoid guilds and time periods, however, it is clear that parasitism is extremely temporally variable.
