Consequences of herbivore diet breadth for bird predation and parasitism

Background/Question/Methods

The enemy-free space hypothesis (EFSH) predicts that dietary specialist herbivores are superior to generalists in using host plants for defense or refuge from enemies. The EFSH has received support from tests with generalist invertebrate predators of insect herbivores. Here, we test the EFSH with respect to bird predation and parasitism in an assemblage of dietary specialist and generalist caterpillars living on 8 common tree species in a temperate forest (Middlesex Co., CT, USA). In addition, we analyze effects of bird predation on parasitism of caterpillars to evaluate an extension of the EFSH: host use by parasitoids that minimizes bird predation of their offspring. To quantify bird predation on caterpillars, we ran a bird-exclusion field experiment over the same 3-6 week period over 4 years (2008-11). To measure mortality from parasitoids, we collected caterpillars from the bird predation experiment and a previous 4-year field study (2004-2007), reared them in the laboratory on their natural host plants, and recorded emergence of parasitoids, classified as flies (Tachinidae) or wasps (Hymenoptera: Parasitica). We classified caterpillars as specialists if they used host plants within a single taxonomic family based on our data and published records.

Results/Conclusions

In support of the EFSH, dietary specialist caterpillar species (9 species) experienced reduced bird predation and parasitism relative to generalist species (31 species). The enemy-free space advantage of specialists was greater with respect to bird predation than with respect to parasitism. Although total parasitism and wasp parasitism were not related to bird predation across generalist or specialist caterpillar species, mortality from flies was negatively related to bird predation across generalist caterpillar species. The latter result suggests that the EFSH applies to host selection by parasitoids when their offspring are subjected to intense, indirect predation. Taken together, these results suggest that enemy-free space can be an important agent of natural selection for host use by herbivores and parasitoids.