The Role of Enemy-Mediated Competition in Determining Fitness of a Generalist Herbivore

Abstract

The influence of indirect competition on insect communities is not well understood. We studied how the fitness of a gregarious lepidopteran species, fall webworm, Hyphantria cunea (Drury), is affected by another gregarious caterpillar species, western tent caterpillar, Malacosoma californicum (Packard). Fall webworm and tent caterpillars both feed on chokecherry, Prunus virginiana L., and create silken structures in which they reside until just prior to pupation. Preliminary observations suggested that predators and parasitoids reside in tent caterpillar tents after tents have been abandoned by the caterpillars, thus increasing the presence of enemies near tents. We hypothesized that when reared near tent caterpillar tents, fall webworms are less fit because of increased predation from nearby enemies (enemy-mediated competition). We tested our hypothesis at two field sites by placing fall webworm larvae in the field with or without a tent caterpillar tent nearby. We compared differences in survival rates, parasitism rates, and fitness between the larvae reared with and without tents nearby. At one field site, webworm larvae near tent caterpillar tents disappeared ∼30% faster than did larvae without tents nearby. Although survival or fitness did not differ significantly between larvae reared with or without tents nearby, distance separating fall webworm larvae from a tent caterpillar tent might be important. Our results suggested that enemy-mediated competition might affect the fitness of fall webworm larvae and should be investigated further.