Evidnce for a domed functional response in the soybean aphid parasitoid Binodoxys communis.

The soybean aphid, Aphis glycines, is native to Eastern Asia where populations are regulated, in part, by natural enemies such as the braconid parasitoid wasp Binodoxys communis. However, the establishment of B. communis in North America as a classical biological control agent where A. glycines populations have reached economic injury levels, has been unsuccessful to date. One potential hypothesis for this lack of establishment could be a type IV (or 'domed') functional response, i.e., increased parasitism per capita with increased host density to a threshold level, beyond which the parasitism per capita decreases. The functional response of B. communis on A. glycines was measured at six aphid densities, ranging from 10 to 1000 per soybean plant. Aphids were placed on plants two days prior to exposure to a single, mated, one-day old female B. communis. The parasitoid was left on the plant for 24 hours and 10 days later all mummies were collected. Emergence rates, sex ratios, and hind tibia lengths of the offspring were measured as fitness parameters of the parasitoid. The results of this experiment were consistent with a type IV functional response, as parasitism per capita increased with number of aphids per plant and decreased between 500 and 1000 per plant. This may indicate a limit to this parasitoid's ability to control A. glycines populations during outbreaks. Furthermore, the more that is known about parasitoid ecology, the easier it will be to forecast successful establishment of future agents.