By Adam Sisson, Integrated Pest Management; Laura Jesse, Plant and Insect Diagnostic Clinic; and Erin Hodgson, Department of Entomology
Western bean cutworm (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 18th. With the higher than average accumulated heat units, moths may have emerged prior to this date, before traps were in place.
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 the southwest part of the state, experienced this around June 20th. The presence of adult moths in traps indicates only that scouting efforts should begin in an area.
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 percent adult emergence, which is predicted at 1,319 DD. Fifty percent adult emergence (peak) is predicted at 1,422 DD, and scouting should continue for 7 to 10 days afterwards. 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 —21 accumulated DDs (base 50 F) and normals are used to predict when approximately 25 percent (top) 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 (Figures 2, 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 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. Insecticide application must be timed correctly, before larvae enter the ear to feed. The suggested application timing is 90 to 95 percent tassel emergence, or 70 to 90 percent hatch if tassels have extended.
Figure 2. Western bean cutworm eggs. Frank Peairs, Colorado State University, Bugwood.org
Figure 3. Western bean cutworm larvae that have just emerged. Frank Peairs, Colorado State University, Bugwood.org
Adam Sisson, Integrated Pest Management, can be reached at 515-294-5899 or e-mail firstname.lastname@example.org. Laura Jesse, Plant and Insect Diagnostic Clinic, can be reached at 515-294-0581 or e-mail email@example.com. Erin Hodgson, Department of Entomology, can be reached at 515-294-2847 or e-mail firstname.lastname@example.org.