Western bean cutworms are primarily a pest of corn in Kansas, and activity is generally isolated to the western part of the state (Figure 1). This pest overwinters as prepupa in the soil in earthen cells about 3 to 9 inches deep. The moths begin emerging in southwestern Kansas in mid-June and in early July for northwestern Kansas, usually around the time that corn is close to tasseling. White eggs are laid in masses on the upper surface of leaves and gradually turn purple as they get closer to hatching (Figure 2). After about a week, the eggs hatch and caterpillars (Figure 3) enter corn ears to feed on the developing kernels. Mature caterpillars exit the corn ears and enter the soil to overwinter. There is a single generation of this pest each year.
Pheromone trap counts of western bean cutworm moths are not a metric for determining losses in corn, but can be utilized to help detect when moths start to emerge from the soil in order to properly time scouting efforts. Early detection of egg masses is important, as any chemical control, if warranted, needs to happen before larvae enter corn ears. When the field is almost fully tasseled, an average of 6-8% of plants with eggs or small caterpillars justifies control measures. Control will be reduced if applications are delayed until all silks have emerged or if larvae have already entered the ear tips.
Figure 1. Adult western bean cutworm moth. Photo by Adam Sisson, Iowa State Univ., Bugwood.org
Figure 2. Maturing western bean cutworm eggs. Photo from Marlin Rice.
Figure 3. Western bean cutworm caterpillar. Photo from K-State Entomology.
Currently, trap counts indicate that the western bean cutworm flight is light in southwest Kansas and began slightly later than in 2024. In northwest Kansas, trap counts of adult moths have been increasing over the last several weeks. The moth flight should conclude in early August. Until then, any at-risk fields should continue to be scouted.
Control options and additional information on western bean cutworm can be found in the 2025 Corn Insect Pest Management Guide: https://www.bookstore.ksre.ksu.edu/pubs/MF810.pdf.
Anthony Zukoff, Extension Entomology – Garden City
azukoff@ksu.edu
Tags: insects western bean cutworm