Western Bean Cutworm in Sweet Corn and Snap Beans

Increasing trap counts over the past several weeks indicate that the Western Bean Cutworm (WBCW) adults are nearing peak flight. With the early tassling of the field corn crop this year, vegetable crops may be under increased pressure of WBCW feeding. The appropriate approach to controlling WBCW will depend on the crop and the geographic area.

Sweet Corn: Crops at the pre-tassel to full tassel stages are at the highest risk of WBCW infestation. Inspect a minimum of 100 plants across the field.  Focus on the top three-to-four upper leaves of the plant. Look for egg masses and young larvae. Egg masses are initially white and turn purple when they are within a few days of hatching. Newly hatched larvae spend a few days feeding on the tassel before moving down to the ear. Use a control threshold of 5% eggs or larvae and time the spray applications to target the newly hatched larvae.  Most insecticide sprays used to control European Corn Borer will also control WBCW.

Snap Beans: To date we have not seen significant feeding of WBCW in commercial snap beans in Ontario. However, when research plots are inoculated with egg masses, feeding damage can be significant, indicating that this is a potential pest of commercial fields.

In dry beans, the WBCW eggs are often inconspicuous and the larvae feed only at night. As a result, scouting for pest activity is very difficult.  Keep track of the pheromone trap counts in your area to determine for peak moth flight.  Peak moth flight indicates when peak egg laying takes place. Pod feeding typically occurs 10-20 days after peak moth flight.  If possible, scout adjacent field corn or sweet corn fields to help gauge the potential risk in a snap bean crop.  As with sweet corn, sprays used to control European Corn Borer will also control the Western Bean Cutworm.

Geography: Western Bean Cutworm Populations vary greatly from area to area.  Hot spots often include areas with a high proportion of sandy soils.  Cheryl Trueman, a researcher with the University of Guelph – Ridgetown Campus, has a network of trapping locations in sweet corn and snap bean fields across Southwestern Ontario (figure 1). Based on the 2012 trap counts, current hot spots include Strathroy, Thamesville and Tupperville.  For a wider range of trapping results from field corn and dry bean locations across the province, visit www.cornpest.ca