Advantage of specialist over generalist temperate forest caterpillars at avoiding bird predation

Background/Question/Methods The preponderance of host specificity among insect herbivores may be due to ecological advantages of dietary specialists over generalists. For example, specialists might be superior in using host plants for defense or refuge 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 invertebrate predators. In this study, we compare the magnitude of bird predation on an assemblage of early season specialist and generalist caterpillars in a temperate forest (Middlesex Co., CT, USA). Morphologically cryptic and dietary generalist caterpillars numerically dominate this herbivore assemblage, suggesting that any advantage in enemy-free space of specialist over generalist herbivores would most likely be due reduced density-dependent predation or improved morphological crypsis. To measure the impact of bird predation on caterpillar density, we placed bird exclosures on tree branches or entire saplings of 8 common forest tree species, with paired branches or saplings as unmanipulated controls. We systematically collected and identified caterpillars from each experimental unit over a 6-week period in 2008. We classified 61 identifiable caterpillar species as specialists (19 species) or generalists (42 species) according to their observed and published diet breadth.

Results/Conclusions Dietary specialist caterpillars suffered less bird predation than did generalists. Specifically, the magnitude of the difference in caterpillar density between bagged and unbagged experimental units (density difference) was greater for generalist than for specialist caterpillars across all 8 tree species. Moreover, specialists gained an enemy-free space advantage over generalists even when caterpillar density was taken into account. In conclusion, the enemy-free space advantage of specialist caterpillars is at least partly trait-mediated. These results support the general hypothesis that tri-trophic interactions shape herbivore adaptation to host plants, including dietary specialization.