Now that western bean cutworm moths have been reported in Iowa this summer, farmers should review the moth trap data for their county to determine the need to scout their fields for this corn insect pest. Iowa State University Extension entomologist Erin Hodgson, along with Integrated Pest Management specialist Adam Sisson and Laura Jesse of the ISU Plant and Insect Diagnostic Clinic, provide the following information.
Western bean cutworm, or WBC, moths have been reported in several Iowa counties. The first reported moth of 2012 was captured in Benton County, located in the east-central part of the state, on June 18. With the higher-than-average accumulated heat units, moths may have emerged prior to this date, before traps were in place.
Western bean cutworm eggs. Frank Peairs, Colorado State University, Bugwood.org
Trap data by participating counties can be viewed by the public at the North Central (NC) ipmPIPE webpage. At the NC ipmPIPE webpage, click on a highlighted county to see the number of recorded moths in that county. If captures occur on consecutive days in a trap and moth numbers are increasing, this is the signal to begin scouting at a location. Mills County, in southwest Iowa, experienced this June 20. The presence of adult moths in traps indicates only that scouting efforts should begin in an area.
Western bean cutworm larvae that have just emerged. Frank Peairs, Colorado State University, Bugwood.org
Map of Iowa accompanying this article shows predicted dates of emergence
Adult emergence can also 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% adult emergence, which is predicted at 1,319 DD. Note that 50% adult emergence (peak) is predicted at 1,422 DD, and scouting should continue for seven to 10 days afterwards. The map of Iowa (accompanying this article) shows the predicted dates of approximately 25% and 50% adult emergence based on the DD model.
May 1 to 21 accumulated degree days (base 50° F) and normals are used to predict when approximately 25% (top number) and 50% (bottom number) adult western bean cutworm adult emergence occur.
How to scout cornfields for presence of western bean cutworm
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 (see photos 2 and 3) on the top three to four leaves. Management options and descriptions of WBC are outlined in a previous ICM News article, Use Treatment Thresholds for Western Bean Cutworm.
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. Insecticide application must be timed correctly, before larvae enter the ear to feed. The suggested application timing is 90% to 95% tassel emergence, or 70% to 90% hatch if tassels have extended.