Low soybean aphid numbers were coupled with scarcity of a parasitic wasp in 2019: What does this mean for biological control of soybean aphid in 2020?

by Jonathan Dregni (Scientist, Entomology), Carl Stenoien (Post-doctoral Associate, Entomology), George Heimpel (Professor, Entomology) and Robert Koch (Associate Professor & Extension Entomologist)

The soybean aphid’s most dedicated natural enemy is an Asian parasitic wasp (or parasitoid) called Aphelinus certus (abbreviated as A. certus throughout this article). This tiny aphid-sized wasp was first found in eastern North America in 2005, and by 2011 it had reached Minnesota. It may be especially useful as a biological control agent against soybean aphid because it uses the aphid as a host for its offspring, so it tracks the aphid very closely. In 2019, with generally low aphid numbers throughout Minnesota (and much of the Midwest), we found no A. certus in many counties, as you can see on the maps. We assume A. certus numbers will rebound if soybean aphid numbers increase in 2020, but we will have to rely on scouting this summer to be sure.

A parasitic wasp stinging (laying its eggs in) a soybean aphid.

Possible explanations for low soybean aphid numbers in 2019 include extreme cold combined with patchy snow cover early in the winter and the wet spring conditions with intense storms. In contrast, the relatively mild winter conditions and snow cover this winter may favor soybean aphids for 2020. In addition, the snow cover may also be good for A. certus. Our studies indicate that A. certus overwinters in soybean fields, while the soybean aphid overwinters on buckthorn. Reduced tillage may improve the parasitoid's chance of surviving winter, which in turn could help control the aphid. Ongoing research is examining these relationships and will hopefully inform future management recommendations.

From our field surveys, we have also found a suite of other parasitic wasps called hyperparasitoids. These hyperparasitoids attack and feed on the A. certus that are feeding on soybean aphid. Six species of these hyperparasitoids have been found attacking A. certus in Minnesota. Improving our understanding of these hyperparasitoids is important because they may be reducing the biological control of soybean aphid offered by A. certus. This is also an understudied segment of our local biodiversity.

If you are interested in having us visit your soybean fields to monitor populations of soybean aphid and parasitic wasps as part of future surveys, please contact Jonathan Dregni at dreg0005@umn.edu. Thanks to U of MN Extension colleagues Angie Peltier, Lisa Behnken, Jared Goplen, and Bruce Potter for help with field surveys across the state.

Maximum soybean aphid and parasitoid (mummy) densities per field by county, Minnesota 2019. Maximums were reached in mid- to late August. Tan colored counties with faint outlines were not sampled.