(7/20) WI ⚠🐛 – Scout for Western Bean Cutworm

Potential Larval Damage

Small mass of cream-colored eggs on corn leaf.

Mature egg mass with purple and pink eggs on corn leaf, indicating the eggs are close to hatching.

Small tan larvae with black head capsules in mass on corn leaf.

Early Western bean cutworm eggs (top), mature Western bean cutworm eggs (middle), and hatching larvae (bottom). Photo credit: J. Obermeyer, Purdue University.

It’s time to scout for Western bean cutworm – a dry bean and corn pest. This moth’s emergence passed the 25% mark in Southern WI on 7/20/2026 and headed to hit 50% this week. In Central WI counties will hit 25% emergence this week as well.

Critical Economic Information

The economic threshold for insecticide treatment is reached by scouting for egg masses and larvae. For field corn, treat when 5% of plants scouted have egg masses or small larvae. For processing sweet corn, treat when 4% of plants scouted have egg masses or small larvae.

Chemical Control

Refer to A3646, Pest Management in Wisconsin Crops, by following these links for true armyworm insecticide recommendations in corn (pg. 76). See spray thresholds above.

Scouting

Scout a minimum of 20 consecutive plants in five locations in a corn field for egg masses and larvae. Look for egg masses on the upper surfaces of new leaves and leaves that haven’t unfolded above and below the ear zone. Look for young larvae on leave axils, tassels and silks.

Corn leaves with one small tan egg mass on the upper leaf blade, pointed to by a large red arrow. — Photo of WBCW egg mass on the upper leaf of corn prior to tasseling (indicated by red arrow). Photo credit: J. Obermeyer, Purdue University

Learn more with UW-Madison Extension’s Western bean cutworm resource and DATCP’s Field Note’s section on Western bean cutworm.

If you found this information on the web, you can sign up for insect pest text alerts here.