When it comes to soybean, one of the most common types of injury that happens to this crop is defoliation from insects. This damage can be easily detected in the canopy by observing holes in the leaves or along leaf margins, as the insects with chewing mouthparts consume the leaf tissue.

The insects most responsible for defoliation are bean leaf beetle, Japanese beetle and grasshoppers. There are numerous caterpillars that also cause soybean defoliation, including green cloverworm, soybean looper and alfalfa caterpillar. In the case of Japanese beetle, the injury caused to the leaf tissue of soybean results in skeletonized leaves, where only the veins of the leaf remain.

Although it can look distressing when scouting, soybean plants can tolerate a fair amount of feeding and have a great capacity to compensate from defoliation by insects. Research has shown a key factor that drives soybean yield loss from insects that cause defoliation is the reduction of light interception of the canopy. Soybeans have demonstrated the capability of withstanding a great amount of loss to the leaf area. As long as the remaining leaves can intercept at least 90% of the incident light, research shows that soybeans 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 or leaf loss it can withstand from feeding.

Timing of feeding

Time of occurrence (insect feeding) as it relates to the soybean growth stage is also important to consider when making a treatment decision. Soybeans can tolerate defoliation in the vegetative stages more than the critical reproductive stages of growth. Environmental and growing conditions should also be considered, as conducive conditions will favor optimum plant capacity for regrowth and compensation (due to ample moisture, optimum temperatures, etc.). Replicated data over multiple growing regions consistently show economic thresholds for soybean 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 along field margins, and perimeter treatments may be more cost-effective if practical.

It is difficult to estimate defoliation in the field and defoliation can easily be overestimated. This is due to the varying species and what area of the canopy they feed on. Bean leaf beetles primarily feed on the upper canopy, whereas other species feed on the lower canopy, making it difficult to assess an overall defoliation estimation, as different injury levels will occur in different portions of the canopy. Because of this, all areas (upper, mid and lower) of the canopy need to be evaluated for a valid defoliation estimate. To help reduce bias, use the leaflet sampling method shown in Figure 1 (below). It can also be found at the ICM Blog Post, bit.ly/icmsoydefoliation.

Defoliation estimation

Practicing the estimation of defoliation is important to help train your eyes and minimize the chances of overestimation when conducting evaluations in the field. Consequently, there are some great tools available to help. One of them is training that is offered online from the Crop Protection Network website called Soybean Insect Defoliation Training at bit.ly/soydefoltraining.

FIGURING DEFOLIATION: Scouting recommendation for estimating defoliation in soybean. (Graphic by Erin Hodgson)

This allows you to hone your assessment skills to accurately estimate the amount of leaf area covered by defoliation caused by insect pests. One of the unique features of this tool is that it allows you to change the leaflet percent defoliation prior to taking a practice quiz. Another relatively new tool available is a phone/tablet app that is free and open sourced called LeafByte. This app uses your camera to measure the total leaf area, consumed leaf area, and percent consumed or defoliated. You can learn more about how the app works at bit.ly/leafbyte.

In summary, it’s important to scout and monitor soybeans as they emerge and develop from the vegetative growth stages through the reproductive stages, as the insects that cause defoliation can cause plant injury throughout the growing season. Identification is always important to know what is causing the observed injury. However, for pests that cause defoliation, they can be lumped together; don’t try too hard to distinguish feeding by species. Remember to consider the soybean growth stage, environmental conditions and size of the soybean canopy. A larger canopy can tolerate more defoliation than a smaller canopy, as the larger canopy has more leaf area.

Basol is a field agronomist for Iowa State University Extension.