Figure 1. Small ant tending to a soybean aphid colony. Photo taken 7/20/18. Courtesy: Adam Varenhorst
Soybean aphid populations have continued to grow in the last couple of weeks. Although the majority of a field may have an average as low as 10-20 aphids per plant, there are definitely hot spots throughout the scouted fields. These hot spots can be quickly identified by the presence of soybean aphids on the newest growth of the soybean plant. In addition, many of these hot spots have ants that are tending to the soybean aphids (Figure 1). The presence of ants as well as natural enemies such as lady beetles (Figure 2) can often be used to help find heavily infested soybean plants.
Figure 2. Seven spotted lady beetle adult. Courtesy: Adam Varenhorst
As the season progresses these hot spots will likely expand. In areas with heavily infested soybean plants, the aphid excrement known as honey dew will cause the plants to first appear shiny and eventually turn dark green to black. The change in color is due to the growth of black sooty mold. At this time, no management is recommended as field averages are still well below the 250 aphids per plant threshold. We recommend to continue scouting and keep an eye on these populations.