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, in North America, where A. glycines populations can reach economic injury levels, the establishment of B. communis as a classical biological control agent has been unsuccessful to date.  One hypothesis for this lack of establishment is a type IV (or 'domed') functional response of B. communis, 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 all mummies were collected after 10 days. Fitness parameters of the parasitoid offspring were measured, including emergence rates, sex ratios, and hind tibia lengths. The results of this experiment were consistent with a type IV functional response, as parasitism per capita increased with host density between 10 and 500 aphids per plant and decreased between 500 and 1000 aphids per plant. The decrease in growth rates exhibited by B. communis while exposed to extremely high aphid densities suggests that there will be a limit to the parasitoid’s ability to control aphid populations during an outbreak.  It is possible that this negative relationship, in conjunction with the critical time period for management, could affect when and where we release the parasitoids in soybean aphid-infested fields. Furthermore, the more that is known about parasitoid ecology, the easier it will be to forecast successful establishment of future agents.