Functional response and the effects of insecticidal seed treatment on the soybean aphid parasitoid, Binodoxys communis

Abstract

The soybean aphid, <italic>Aphis glycines</italic>, is native to Eastern Asia where populations are regulated, in part, by natural enemies such as the braconid parasitoid wasp <italic>Binodoxys communis</italic>. However, the establishment of <italic>B. communis</italic> in North America as a classical biological control agent where <italic>A. glycines</italic> populations have reached economic injury levels, has been unsuccessful to date. This study examined the impact of neonicotinoid seed treatment on<italic> B. communis</italic>, as well as determined the parasitoid's functional response. Seed treatments and biological control are often both used in a soybean aphid integrated pest management (IPM) regime. A field study was conducted to evaluate any impact seed treatment may have on the parasitoid. There were eight susceptible soybean plots, four of which were untreated and four were treated with a neonicotinoid seed treatment. Adult male and female wasps were released onto individual caged plants and mummies were collected 10 days later. Proportion of aphids that mummified, emergence rates, sex ratios, and hind tibia lengths of the offspring were measured as fitness parameters of the parasitoid and compared between treatments. The results suggest that even though there was an effect on<italic> B. communis</italic> fitness, the results may not be biologically significant due to inconsistent results across all parameters in the study. Another potential hypothesis for the parasitoid's difficulty as a classical biological control agents, 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 <italic>B. communis</italic> on <italic>A. glycines</italic> 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 <italic>B. communis</italic>. 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. Parasitism per capita by <italic>Binodoxys communis</italic> 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 <italic>A. glycines</italic> populations during outbreaks. Furthermore, the more that is known about parasitoid ecology, the easier it will be to forecast successful establishment of future agents.