Bean leaf beetle is considered to be one of the most important pests of soybean. Damage from adult bean leaf beetles feeding on soybean pods causes the greatest damage to yield potential and quality. Regular scouting to assess your soybean fields for damage is important for managing this pest.
What to Watch For
Bean leaf beetle (BLB) can be a destructive pest of soybean. They overwinter as adults and can cause damage throughout the growing season on soybean foliage and developing pods.
There are generally two to three generations of BLB every season, depending on your geographical location (two generations are typical in the north central region). The adult beetle is small (about 1/4 inch long). Colors and spot patterns of BLB can range considerably. The most reliable diagnostic feature is the prominent black triangle at the front of the wing covers (Figure 1). This marking is always present and distinguishes BLB from other beetles in soybean.
Impact on Your Crop
Heavy BLB populations can cause serious leaf and pod damage to your soybeans. It is essential that you scout for BLB during the pod-fill growth stages. For information on reproductive stages of soybean, see Soybean Flowering Through Maturity. BLB chews small round holes in soybean leaves and feeds on the green tissue of pods, leaving a thin membrane over the seed (Figure 2). This thin membrane can crack when the seed matures, leaving an entry hole for plant pathogens that can cause discolored, moldy, shriveled and/or discolored soybeans. Pod clipping (feeding at the base of the pod) can cause complete pod loss. Several viruses, including bean pod mottle virus (BPMV), can be transmitted by BLB. Infection by BPMV can result in plants that remain green after harvest maturity (green stem syndrome) and seeds that are mottled.
Tips to Manage
Your soybean fields should be scouted regularly (every 5 to 7 days) during the pod-fill growth stages. Counts of adult beetles using sweep nets as well as assessing pod damage are useful tools for determining the need for an insecticide application. Deciding when to treat to prevent further pod damage from bean leaf beetle (BLB) is based on pod injury levels. Select 10 plants at random from around the field and examine all the pods. Count the total number of pods as well as pods that have scar injury and determine the percentage with pod injury on those 10 plants. Treatment may be warranted if 10 to 15% exhibit pod injury and BLB adults are still present and active. A sweep net is an efficient way to determine if BLB adults are still present. In several locations throughout the field, take sets of 10 sweeps per location to determine BLB presence or absence. Sampling in the mid-morning after the dew has dried or early evening is best.
There are a variety of insecticides labeled for bean leaf beetle control in soybean. Keep in mind that when you apply a late summer treatment to prevent additional pod damage, you need to be aware of pre-harvest intervals for many of the insecticides, which range from 14 days to 60 days. The amount of time left before your planned harvest of a field may determine the insecticide chosen.
For information on early season bean leaf beetle damage and defoliation thresholds for vegetative stages, see Early-Season Bean Leaf Beetle in Soybean.
1 Bean leaf beetle. Purdue University. Field Crops IPM. http://extension.entm.purdue.edu.
2 Cook, K. 2003. Defoliators abundant in many soybean fields. University of Illinois. http://bulletin.ipm.illinois.edu.
3 Ratcliffe, S.T., Gray, M.E., and Steffey, K.L. 2004. Bean leaf beetle. University of Illinois Extension. Integrated Pest Management. http://www.ipm.illinois.edu.
4 Hammond, R.B., Michel, A., and Eisley, J.B. 2009. Bean leaf beetle on soybean. Ohio University Extension. FC-ENT-0023-09. http://ohioline.osu.edu
5Holshouser, D. 2009. Green stem syndrome in soybeans. Virginia Cooperative Extension. 2912-1430. http://www.ext.vt.edu.
Web sources verified 04/04/2018.