Spatial Distribution of Progeny in a Parasitoid of a Gall Midge and Its Relevance to Applied Biological Control

The gall midge Rhopalomyia californica Felt and its parasitoid guild comprise a good model system for assessing issues in applied biological control of insect pests. The midge is native to Northern California where its larvae develop in terminal galls on Baccharis pilufans DC. Galls are multichambered (one midge per chamber) and thus represent discrete host patches in space and time. Midge larvae are exploited by a guild of seven parasitoid species, including Platygaster californica (Ashmead), which is often the dominant member of the guild. The natural history of Rhopalomyia and Platygaster is described. Analysis of percentage parasitization per gall in response to spatial variation in gall size (chambers per gall) revealed that parasitization by Platygaster was either directly density dependent or density independent. Many of the density-independent responses at one study site were associated with habitat disruption brought about by malathion-bait sprays. The overall impact of Platygaster was probably reduced by interspecific competition and an apparent propensity by ovipositing females not to fully exploit a host patch. The latter could be due to several factors, including a host refuge from parasitization, egg limitation, or "spreading the risk" by ovipositing females (rather than maximizing oviposition per host patch). These results illustrate how preintroductory assessment of the ecological potential of a biological control agent can result in an underestimate of an agent′s intrinsic potential when the candidate agent is a member of a competitive (or highly interactive) guild, and suggest that, in competitive enemy guilds, the probability that a given guild member can suppress the host population when acting alone is lower than might otherwise be expected. The value of model systems in addressing practical questions in biological control is also discussed.