By Terry Basol
When it comes to soybean, one of the most common types of injury is defoliation from insects. This damage can be easily detected in the canopy by observing holes in the leaves or along leaf margins as insects with chewing mouthparts consume leaf tissue. The insects most responsible for defoliation are bean leaf beetles, Japanese beetles, and grasshoppers. There are numerous caterpillars that also cause soybean defoliation, including green cloverworm, soybean looper and alfalfa caterpillar.
Soybean plants can tolerate a fair amount of feeding without suffering significant yield loss due to their capacity to compensate from defoliation by insects. Research has shown a key factor that drives soybean yield loss from defoliation is the reduction of light interception of the canopy. As long as the remaining leaves can intercept at least 90% of the light, soybean can still maintain 100% of their yield potential. Therefore, the size of the remaining soybean canopy is crucial when making pest management decisions. The larger the size of the canopy, the greater tolerance for leaf loss.
Timing of insect feeding as it relates to the soybean growth stage is also important to consider when making a treatment decision. Soybean can tolerate defoliation in the vegetative stages more than during reproductive stages. Environmental and growing conditions should also be considered, as favorable conditions will favor regrowth and compensation (due to ample moisture, optimum temperatures, etc.). Replicated data over multiple growing regions consistently show economic thresholds for soybean defoliation are 30% in the vegetative stages and 20% in the reproductive stages. This threshold applies to the entire field. Sometimes these pests cause significant injury only along field margins. In these situations, perimeter treatments may be more cost effective, if practical.