Thursday, August 7, 2008: 10:30 AM
102 A, Midwest Airlines Center
Background/Question/Methods The preponderance of host specificity among insect herbivores may be due to ecological advantages of dietary specialists over generalists. One possible ecological advantage is the degree to which host plants provide herbivores with defenses or refuges from enemies (enemy-free space). Previous work suggests that specialist herbivores can enjoy an enemy-free space advantage over generalists by more effectively using host plant-derived chemical defenses against generalist predators. In this study, we compare frequencies of parasitism among 90 species of early season specialist and generalist caterpillars in a temperate forest (Middlesex Co., CT, USA). These caterpillars feed on young leaves, which contain relatively high concentrations of nutrients and relatively low levels of secondary metabolites. Consequently, our measurement of enemy-free space most likely entails caterpillars' use of host plants as a refuge from attack by parasitoids. We systematically collected middle- to late-instar caterpillars of macromoths from eight common forest tree species at three field sites over a four year period (2004-2007), reared them in the laboratory on natural host plants to determine frequencies of mortality due to parasitoids, and classified them as specialists or generalists according to their observed and published diet breadth. We defined specialists as herbivores using host plants within a single taxonomic family.
Results/Conclusions We found that specialists enjoyed an overall reduction in mortality from parasitoids compared to generalists. This general difference was specifically due to mortality from hymenopteran parasitoids, as no such difference was found for parasitism by tachinid parasitoids. Among various possible explanations, one stands out as particularly interesting: specialist caterpillars may be superior to generalists at interfering with odor signals released by host plants to attract parasitic wasps.
