2013 Western Bean Cutworm Scouting Update

By Adam Sisson, Integrated Pest Management; Laura Jesse, Plant and Insect Diagnostic Clinic; and Erin Hodgson, Department of Entomology

Western bean cutworm (WBC) is a corn pest that has expanded its range across Iowa and toward East Coast states. Before corn tassels, newly emerged WBC larvae move to the whorl and feed on the flag leaf. Once tasseling begins, they move to the green silks. Older larvae feed primarily on the ear tip, but some move outside the ear, chew through the husk and feed on kernels on the side or shank of the ear. Unlike corn earworm, multiple WBC larvae may be found in the same ear. Consuming the developing kernels can cause yield losses. In addition, the damage caused by feeding can allow pathogens to enter the ear and  reduce grain quality. 

Western bean cutworm adult emergence can be predicted using a degree day (DD) model developed in Nebraska. This DD model is based on the accumulation of DD (base 50°F) from May 1. Scouting should begin at 25 percent adult emergence, which is predicted at 1,319 DD. Fifty percent adult emergence, or the peak adult activity, is predicted at 1,422 DD, and scouting should continue for 7 to 10 days after the peak. The map (Figure 1) shows the predicted dates of approximately 25 and 50 percent adult emergence based on the DD model.

 
Figure 1. May 1 – July 9 accumulated DDs (base 50 F) and normals are used to predict when approximately 25 percent (top date) and 50 percent adult western bean cutworm adult emergence occur.

When scouting for WBC, examine 20 successive plants in five different areas of a field. On these plants, check for the presence of eggs or young larvae (Photos 1, 2) on the top three to four leaves. Management options and descriptions of WBC are outlined in a previous ICM News article.

Photo 1. Western bean cutworm eggs. Frank Peairs, Colorado State University, Bugwood.org.

Photo 2. Western bean cutworm larvae that have just emerged. Frank Peairs, Colorado State University, Bugwood.org.

For field corn, if 5 to 8 percent of plants have eggs or larvae, an insecticide treatment may be warranted. For sweet corn, the threshold is reduced to 4 percent for the processing market and 1 percent for the fresh market. Alternatively, a newly developed “speed scouting” tool, which incorporates corn price into the threshold and may require examining less plants, was developed by Nebraska and can be downloaded here.

Insecticide application must be timed correctly to reach larvae before they enter the ear. If density thresholds are met, the suggested application timing is 90 to 95 percent tassel emergence, or 70 to 90 percent hatch if tassels have extended.

Adam Sisson is an extension specialist for the Integrated Pest Management. He can be contacted by email at ajsisson@iastate.edu or by calling 515-294-5899. Laura Jesse is an entomologist with the Iowa State University Extension Plant and Insect Diagnostic Clinic; contact at ljesse@iastate.edu or by phone 515-294-0581. Erin Hodgson is an assistant professor of entomology with extension and research responsibilities; contact at ewh@iastate.edu or phone 515-294-2847.