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The decision to treat for foliar-feeding insects in soybean is based upon a threshold for plant defoliation, but it is important to know what insect pests are out there, since the management options can vary by pest. In addition to insects that feed on plant foliage, you may find other yield robbers such as piercing/sucking insects, i.e., soybean aphid and various stink bug species. The first step in managing any insect pest is identifying what is out there. This is most easily determined by using a sweep net to collect insects from random locations in the field. Fifteen to twenty sweeps from a few random areas is sufficient. One of the most important things that sweep samples will determine is whether or not the insects that fed upon the leaves are still present. If there are no insects, there is obviously no need to treat.

Common defoliators of soybean in Illinois: Although there are numerous species of insects that can be found in a soybean field, a few that are commonly seen, year in and year out, are the bean leaf beetle, Japanese beetle, green cloverworm, soybean looper and grasshopper.

 Above: Bean leaf beetle. Courtesy of Adam Sisson, Iowa State University

Above: Japanese beetle. Courtesy of Daren Mueller, Iowa State University.

Above: Green cloverworm. Courtesy of Daren Mueller, Iowa State University.

Above: Soybean looper. Courtesy of Clemson University - USDA Cooperative Extension

Above: Grasshopper. Courtesy of Adam Sisson, Iowa State University

Scouting

Scouting to determine what pests are present is critical, but the amount of defoliation is ultimately most important in making a management decision. 

To determine the amount of defoliation caused by insect pests: 

1. Select at least ten plants from representative areas spread throughout the field.
2. Randomly choose a leaf from the top, middle and bottom portion of each plant sampled.
3. Use a reference to help you determine the amount of defoliation that is actually present. Often times people overestimate the amount of defoliation, so having a visual reference such as this University of Minnesota Visual Guide for Estimation of Soybean Defoliation can be extremely helpful. Different insects will also exhibit different feeding behaviors; some will skeletonize leaves while others will leave larger holes throughout. The guide gives examples of both.
4. Finally, record the amount of defoliation for each leaf, and determine the mean percent defoliation per field by dividing the total percent defoliation from your samples by the number of samples taken. Use this number to determine if treatment is necessary.

Thresholds and treatment

Treatment thresholds are based on levels of defoliation; this can be from a single insect species or a combination of many. Surprisingly, soybeans can withstand a significant amount of feeding, and in some studies small amounts of feeding have actually shown an increase in yield. Thresholds vary based on the growth stage of the soybean plant. For plants that are still in vegetative growth stages (all the way up to R1), a 30% – 40% defoliation threshold has been established. For soybeans in reproductive stages (R1 to R6), the threshold drops to 20%. 

References:

University of Minnesota, Visual Guide for Estimation of Soybean Defoliation

Photos courtesy of contributors to Bugwood.org