Studies of the ecological factors that determine top-down impacts of natural enemies on insect herbivores have direct bearing on efforts to biologically control insect pests. Caterpillar behavior, morphology, social behavior and population density have all been shown to be important predictors of attack by parasitic wasps and flies that use caterpillars as hosts for their developing offspring. Caterpillars in the slug caterpillar family (Limacodidae) are well defended, both physically and chemically. Previous research on 14 members from this family suggested that parasitoids preferentially parasitize Limacodid larvae with strong physical defenses against predators. We hypothesized that this preference for ovipositing in defended hosts may increase the likelihood of survival for the parasitoid’s offspring due to a decreased incidence of predation of the parasitized host. If this is the case, it suggests that host selection by female parasitoids may be governed in part by their ability to exploit “enemy-free space” provided by their hosts.

We tested part of the “enemy-free space” hypothesis for Limacodid parasitoids by examining predator responses to four species of slug caterpillars. We tested the hypothesis that larvae with aposematic and/or highlighted physical defenses would suffer less predation from generalist predators than cryptic and/or undefended larvae. We conducted paired predation trials of non-spiny versus spiny caterpillars and of more-spiny versus less-spiny caterpillars with field-collected predators (assassin bugs and lace-wing nymphs) under laboratory conditions. Predator responses were quantified (N=24 trials) and analyzed using a Chi-square analysis.

Results/Conclusions

Results indicated that predators preferentially chose prey that are not physically well-defended. Observational data from trials using paper wasp colonies and ant colonies added further support for the effectiveness of larval defenses against invertebrate generalist predators. Together, our experimental and observational data support the hypothesis that parasitoids may exploit enemy-free space by preferentially attacking well-defended herbivorous hosts because these herbivores are avoided by generalist predators.