Functional response of 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, including 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 economically injurious levels, has been unsuccessful. One potential hypothesis for this (based on laboratory observations of low growth rates by this parasitoid at very high aphid densities) could be a type IV (‘domed’) functional response. Type IV functional responses demonstrate increased population growth rates with increased host density to a threshold level, beyond which parasitoid growth rate decreases. The functional response of B. communis on A. glycines was measured at five aphid densities, ranging from 10 to 500 aphids per soybean plant. Aphids were placed on plants 2 d prior to the start of the experiment, and on the third day were exposed to a single, mated, 1-d-old female B. communis. The parasitoid was left on the plant for 24 h and 10 d later all mummies on the plant were collected. If a type IV functional response is suggested by this experiment, such that the population growth rate for B. communis is lowest when aphid densities are high, then it may indicate a limit in this parasitoid’s ability to control A. glycines populations during an outbreak. From a broader perspective, the more that is known about parasitoid ecology, the easier it will become to forecast successful establishment of future agents.