Host range, apparent competition, and biological control of the soybean aphid

Classical biological control agents should be selected to maximize pest suppression while minimizing non-target impact to native species. Therefore, candidate biological control agents with high attack rate on the target species undergo host range tests in quarantine to predict safety prior to release. However, organismal traits affecting safety (e.g. narrow host range) may impact efficacy at reducing pest populations. An accidentally introduced parasitoid (Aphelinus certus) which attacks the invasive soybean aphid (Aphis glycines) has spread throughout much of the soybean growing region in North America, and may be controlling the soybean aphid in some regions. We conducted a host range test on 18 native and pest aphid species covering a broad phylogenetic range within Aphididae. A two-host-one-parasitoid model was analyzed to examine conditions under which indirect interactions between hosts could increase or decrease suppression of both target and non-target aphids. Field parasitism rates were recorded on soybean aphids in cultivated fields in Minnesota and were found to be consistent with an apparent competition effect. Results will be discussed in the context of risk-benefit modelling in biological control.