Western bean cutworm or WBC is a corn insect pest that has expanded its range across Iowa and the eastern Corn Belt, and is moving 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.
Iowa State University Extension Integrated Pest Management specialist Adam Sisson, along with Laura Jesse of ISU's Plant and Insect Diagnostic Clinic, and ISU Extension entomologist Erin Hodgson offer the following information and scouting guidelines.
Emergence of WBC adults can be predicted using a degree-day model
Western bean cutworm adult emergence can be predicted using a degree day or DD model developed in Nebraska, explains ISU's Erin Hodgson. This DD model is based on the accumulation of DD (base 50°F) from May 1. Scouting should begin at 25% 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% adult emergence based on the DD model.
When scouting for WBC, the ISU Extension specialists say you should examine 20 successive plants in 5 different areas of a field. On these plants, check for the presence of eggs or young larvae (See Photo 1 and Photo 2) on the top three to four leaves. Management options and descriptions of Western Bean Cutworm are outlined in a previous Iowa State University Extension ICM News article.
For field corn, if 5% to 8% of plants have eggs or larvae, an insecticide treatment may be warranted. For sweet corn, the threshold is reduced to 4% for the processing market and 1% 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, says Hodgson. If density thresholds are met, the suggested application timing is 90% to 95% tassel emergence, or 70% to 90% hatch if tassels have extended.