Should I Spray Soybeans for Insects?
By Kelly Nichols, Ag Agent, UME Montgomery
Right now, you’re likely to find a variety of insect pests in soybean fields, including Japanese beetle, green cloverworm, bean leaf beetle, grasshopper, brown marmorated stink bug, Mexican bean beetle, and silver spotted skipper. (Click on links for pictures.) All of these insects can certainly take a bite (or several!) out of soybean leaves and pods. But should we spray?
Generally, the threshold for soybean insects is 30% defoliation during vegetative stages and 15% during bloom through pod fill. This is often more than what may be aesthetically acceptable; see Figure 1 for a guide to varying amounts of leaf defoliation. If the pest has not reached the threshold, it is not economical to spray. For specific insect pest thresholds, see the IPM Threshold Guide for Agronomic Field Crops.
When scouting, sample different locations across the field in order to have a better sense of the overall pest population. Also consider the weather and part of the field where the pest is found. Pests such as the two-spotted spider mite and grasshoppers flourish in drought conditions; they also start at the edge of the field and work their way in. If they reach the threshold only in the field edges, insecticide applications can be limited to the edges instead of the whole field.
We also cannot forget about the beneficial insects such as spiders, lady beetles, damsel bugs, and green lacewings. (Click on links for pictures of the larval and adult growth stages.) These insects can have a voracious appetite; depending on the species and growth stage, a lady beetle can eat 50 to a few hundred aphids per day, for example. If a pest is not at the threshold and there are beneficials present, don’t spray, as the insecticide will reduce the beneficial insect population, too.
This article appears in July 2026, Volume 17, Issue 4 of the Agronomy News.