Today, our lab crew went to the Johnson Farm south of Ames, Iowa to evaluate a bean leaf beetle study. To our surprise, we found many V4-V6 plants with established soybean aphid colonies (Photo 1). It is not uncommon to find sporadic soybean aphids in June, but it is surprising to find a colony this early. Typically, we find established aphid populations on soybeans after bloom. Many of the plants had winged aphids among the colony. Typically, winged aphids this early would be dropping off nymphs and moving on to new plants, but the size of these colonies may suggest that the small plants are already overcrowded and winged aphids are being produced to disperse to new areas.
This field had a lot of activity happening. Aside from the soybean aphids, there was quite a bit of predator activity already (Photo 2). Many of the plants had parasitized aphids (known as mummies), which happens when a tiny parasitoid wasp lays an egg inside of a soybean aphid. When the egg hatches, the larvae feed on the soybean aphid from the inside-out and complete their life cycle inside of the mummy, and the adult exits. We could also find several lady beetle adults and lacewing larvae. Many of these colonies were being tended by ants – this is a relationship where ants feed on honeydew excreted by the soybean aphid and protect the aphids.
We will keep an eye out for aphids in other areas of the state. Bruce Potter in southern Minnesota found soybean aphids a few days ago, so we might be able to find them in northern Iowa. If you’re out scouting soybeans, you may be able to find soybean aphids by looking for lady beetles, which can find soybean aphids at any level of infestation. Ants crawling on soybeans may lead you to a larger colony of soybean aphid.