It has been commonly observed that caterpillar species which appear well-defended against predators are more heavily parasitized. This has led to hypothesis that caterpillars that escape predation offer enemy-free space for parasitoids. However, this hypothesis has rarely been tested using direct measurements of both predation and parasitism. We tested this hypothesis by simultaneously measuring the effects of caterpillar shelter traits on predation and parasitism for 24 species. Predation was measured by monitoring caterpillars in the field, while parasitism was measured by rearing and dissecting caterpillars for parasitoids. The effects of three shelter traits on predation and parasitism were then analyzed while controlling for caterpillar phylogeny.
Results/Conclusions
Shelter type, shelter openness, and the presence of frass in the shelters all affected the level of caterpillar predation. Predation was elevated in three shelter types (leaf folds, leaf rolls, and leaf ties) compared to cut-and-fold shelters and webs. At the same time, different combinations of shelter openness and frass resulted in different levels of predation. Specifically, closed, frass-free shelters had the lowest predation while closed, frass-filled shelters had the highest. Parasitism, however, was not affected by shelter traits, but was strongly and negatively correlated with predation. These results support the hypothesis that parasitism is higher in host species that experience less predation. Additionally, they demonstrate that shelter traits influence predation rates. It appears that caterpillars whose shelters are highly effective against predators inadvertently make themselves better hosts for parasitoids. These two groups of natural enemies likely exert contrasting pressures on caterpillar shelters and other defensive traits depending on their relative abundance in communities.