A field-based assessment of the parasitoid Aphelinus certus as a biological control agent of soybean aphid in North America

Damaging outbreaks of soybean aphid continue to occur in North America despite the valuable biological control services provided by resident natural enemies. The adventive parasitoid Aphelinus certus Yasnosh (Hymenoptera: Aphelinidae) has recently established in North America and has been hypothesized to be a key component of the resident natural enemy community, but there have been few studies evaluating the efficacy of this parasitoid in suppressing soybean aphid populations. We used an exclusion cage study to quantify the effect of A. certus on soybean aphid population growth at four sites spanning western and east-central Minnesota from 2017 to 2019. There were minimal differences in soybean aphid population growth between experimental treatments that excluded natural enemies and control treatments, suggesting that parasitism of soybean aphid by A. certus did not have a strong impact on soybean aphid population growth during this study. Because, for example, A. certus larvae can reduce host reproduction prior to mortality (resulting in underestimates of effects in short-term studies), our results reflect the challenges of using exclusion cages to assess the effects of individual natural enemy species, especially those with complex life cycles.