Written collaboratively by Adam Varenhorst, Philip Rozeboom, Patrick Wagner, and Brad McManus.
Soybean aphid populations are persisting in many soybean fields throughout eastern South Dakota. In many cases, these soybean aphid populations are either at the 250 aphids per plant threshold, or very close. From our observations, most of the soybeans are at the R5 growth stage, but in some fields, soybean have reached the R6 growth stage. Since yield improvements have not been observed when soybean aphids are managed at the R6 growth stage, we advise holding off on insecticide management of these soybean aphid populations. Most of the aphids in these fields are not the robust soybean aphids that we observe in July and early August. In most of the fields, the colonies are mainly present in the lower canopy and are comprised of pale-yellow aphids that are smaller than average in size. This can be an indicator of a soybean aphid colony that isn’t as healthy as one where the aphids are present on the upper nodes of the plant and are bright green in color. In addition, the natural enemy populations in these fields are established and provide some population reduction.
As a reminder, the R5 growth stage is reached when a seed that is 1/8th of an inch long (3 mm) is present in a pod at one of the four uppermost nodes on the main stem (Figure 1). The R6 growth stage is reached when a pod contains green seeds that fill the pod’s capacity and is present on one of the four uppermost nodes on the main stem (Figure 2).
