Western Bean Cutworm: A threat to Iowa's fields this summer

Posted By Larry Sailer, Musings of a Pig Farmer


ABOVE: Western Bean Cutworm Larva (Iowa State University)

The Western Bean Cutworm (WBC) has moved into Iowa — and the rest of the Midwest — and it’s making itself at home. The insect first appeared in Iowa in 2000, and has been confirmed in all 99 counties, according to Ag extension experts. Not only are the insects in Iowa, they’re marching east with confirmed cases in Michigan and Ohio.

It is possible to control the pests, but it will take careful scouting and well timed insecticide application. There is no doubt that Western Bean Cutworm is here, and we are going to have to deal with it long-term.

While scouting your fields this season, follow these tips for dealing with Western Bean Cutworm:

  • Plants are most vulnerable to cutworm damage just prior to pollination.
  • These cutworms feed on silks, tassels, and ears depending on the time of emergence.
  • Cutworm feedings cause direct effects on yield, but it also allows other pests to invade wounded plants. Yield losses can reach up to 30-40 percent.
  • WBC’s over-winter in the larval stage and emerge in the pupil stage in May and June.
  • In mid-July, the pest will be fully developed as an adult. During this time, farmers should scout for moths both in the air and lying in the upper third of the plant.
  • Adults will invade during the late whorl stage to begin laying their eggs.
  • Scout for clusters of eggs on the upper-third of the plant. Eggs may also be found on the underside of leaves.
  • Eggs hatch five days later and WBC larvae begin to feed for the following 3 to 5 weeks.
  • In September, the pests return to the soil and begin to prepare for the wintering stage.
  • Farmers should follow these general rules of thumb when scouting and determining WBC thresholds in their fields this season. Treat if…
    • Eight percent of plants show presence of egg masses or larvae, and 90-95% of tassels has emerged.
    • If tassels are already emerged at the time of diagnosis, 70-90% of the eggs should be hatched before application of insecticide.
  • Timing of insecticide application is critical for control of the WBC. Once the insect tunnels into the silk channel, treatment by insecticide is almost-impossible.


ABOVE: Western Bean Cutworm Moth (Iowa State University)


ABOVE: Western Bean Cutworm eggs (Iowa State University)

Agronomics