Allocation of searching time within the soybean canopy by parasitoids attacking the green cloverworm

Abstract

We examined the searching behavior of a guild of primary parasitoids which attack the green cloverworm,Plathypena scabra (Fabricius), as well as that of an associated hyperparasitoid. We hypothesized that self-superparasitism is an important constraint on the foraging behavior of primary parasitoids, and therefore these parasitoids should avoid portions of the soybean canopy where parasitized caterpillars accumulate. Conversely, we hypothesized that the hyperparasitoid preferentially searches parts of the canopy where parasitized caterpillars accumulate. In a greenhouse experiment, we found that exposure to parasitoids [eitherCotesia marginiventris (Cresson) orDiolcogaster facetosa Ashmead] resulted in the accumulation of caterpillars lower in the canopy. In a field experiment, we measured the amount of time parasitoids spent searching in each of three strata (upper, middle, bottom) of the soybean canopy. Leaf area in each stratum was used to calculate expected values for search effort. The time spent searching each of the strata was proportional to leaf area for all primary parasitoids, exceptD. facetosa, which spent significantly more time searching the top stratum of plants than predicted by leaf area in that stratum. The hyperparasitoidMesochorus discitergus (Say) tended to search the bottom stratum of the canopy. Thus only one of the three primary parasitoids appears to search in a manner that would reduce its rate of encounter with previously parasitized green cloverworms. The hyperparasitoid searching pattern may increase its probability of encountering parasitized caterpillars, thereby increasing its foraging success.