Watch for Early-Season Bean Leaf Beetles in Soybeans

KEY POINTS

  • Potential for yield losses exist in early-planted soybeans and in fields that are isolated from other soybean fields.
  • Fields at risk should be monitored closely after planting.
  • Control may be accomplished with use of seed treatments as well as foliar insecticides.

Impact on Your Crop

  • The potential for economic damage may occur when bean leaf beetle (BLB) feeding damages the growing point or cotyledons of soybean before the unifoliate leaves emerge.
  • Yield losses may also occur when BLB feeding results in excessive defoliation (30-50% or more) of pre-bloom soybean.
  • BLB can transmit bean pod mottle virus (BPMV), with early infection posing the greatest risk of potential yield loss.

Scouting

  • Scout for adult BLB as soon as soybeans emerge (Figure 1).

 

Figure 1. Bean leaf beetles can vary in color (light yellow or tan is most common) and are about 1/4 inch long. They can have 2 or 4 black spots on their backs, and a black border on the outside of each wing cover; however, these markings may be absent. Note the characteristic black triangle behind the head on both beetles above.

 

  • Seedling stage scouting requires a determination of the number of beetles per foot of row or per plant. Drop cloths or sweep nets can be used, and sampling should occur at five different locations throughout a field. Also note the extent of defoliation when counting beetles.
  • Management thresholds vary by region and recommendations can be found in your local University Extension publications or by contacting your Extension entomology specialist.

Management Options

  •  Insecticidal seed treatments help protect seedlings from early-season BLB feeding.
  • If an insecticide seed treatment was not used, a foliar insecticide treatment may be warranted when BLB feeding appears to be injuring or clipping the cotyledons and growing points of soybean, or when economic thresholds have been reached.
  • For prevention of BPMV transmission, one approach is to use a seed treatment or foliar insecticide to protect seedlings from overwintering adults followed by a foliar insecticide targeted against first generation adults.

Sources:

Hunt, T. and Jarvi, K. 2012. Scout early emerging soybeans for bean leaf beetles. University of Nebraska-Lincoln CropWatch. http://cropwatch.unl.edu; Davidson, D. 2016. Watch for bean leaf beetles. Illinois Soybean Advisor. http://ilsoyadvisor.com; Varenhorst, A. 2017. Bean leaf beetles: early season soybean defoliators in 2017. South Dakota State University Extension IGrow publication. http://igrow.org Web sources verified 02/01/18.