Today, while scouting for bean leaf beetle south of Ames, my lab found a few early vegetative plants with soybean aphid. Actually, it was Ashley Dean who found them. It was the ants moving on plants that caught her eye. Taking a closer look revealed a few small aphid colonies feeding on the undersides of the trifoliates. That is pretty typical from my experience – finding the first soybean aphids of the season is by seeing ants or lady beetles. It is not unusual to see soybean aphid in mid-June.
Soybean aphid is the only species in Iowa that will colonize soybean. After developing on their overwintering host, buckthorn, winged adults will migrate to soybean and potentially product 15+ generations. Initial infestations in soybean are patchy and located near field edges, but winged aphids can quickly disperse within and between fields. Long and short distance immigration is more likely after bloom. Aphids prefer to feed on the undersides of leaves and will colonize on the newest leaves. If a large colony develops and leaves are crowded, soybean aphid will feed on stems.
With the potential of many overlapping generations in a field, scout weekly from plant emergence until seed set to assess population dynamics. The economic threshold for soybean aphid is well established for the north-central region. Consider a foliar application when the average density exceeds 250 per plant (Ragsdale et al. 2007; Koch et al. 2016). Populations should be increasing and most of the plants have to be infested (>80 percent) in order to justify an application. This threshold is appropriate until plants reach mid-seed set (R5.5).