Populations of true armyworm (Mythimna unipuncta) exceeding economic thresholds have been reported in Minnesota. Be sure to scout corn that was planted into a grass cover crop or where dense grassy weeds were controlled post-emerge. It may be worth the effort to check near grassed waterways or grassy field margins, as well.

It can be confusing, but these true armyworms are not the same as the tent caterpillars that feed on broadleaf trees and shrubs. Adult moths are attracted to areas of dense grass vegetation to deposit eggs. These grasses include grassy weeds, live rye or wheat cover crops, or grasses in field borders. True armyworm larvae prefer to feed on grasses, which is why the adults lay their eggs in grass. When the larvae’s food source is depleted, they will migrate in groups to find a new food source. These migrating swarms or “armies” eat and destroy crops as they move. They can easily cross a road and feed well into a field on the other side, and can do so quickly.

Because grassy weeds are attractive to the moths, pay close attention to field borders and areas within the field that have or had high grass weed pressure. If not killed before moths arrive, grass cover crops, particularly winter rye, may also be attractive egg laying sites.

While scouting for armyworm, check for chewing damage on crop leaves and the presence of feces, called frass, as well as looking for the larvae themselves. Armyworm larvae tend to feed at night and hide in whorls (in corn) or under leaf litter throughout the day. Be sure to check deep in the corn whorl if you’re scouting on a sunny day. Armyworms are often more active on cloudy days, and it’s generally best to scout early in the morning or at the end of the day around sunset.

Whenever live larvae are present, there’s potential for future damage. Treat whorl stage corn when 25% of plants have two larvae per plant or if 75% of plants have one larva or more.

If you detect armyworms migrating from areas bordering fields, you may want to spot treat these infested areas which can be effective if you identify the problem early enough. Watch for armyworms migrating out of maturing small grains into corn.

All Bt traits can have difficulty controlling large populations of large armyworm larvae.

Typically, an armyworm issue means larger, less susceptible larvae moving from weeds and field borders into corn. Additionally, insects must eat the Bt to be affected. As a result, damage armyworms can do considerable damage to corn even with an effective Bt protein.

There’s a Handy Bt Trait Table that shows which Bt proteins control various insect species.

While several Bt traits control fall armyworm (FAW) only, the Viptera trait is promoted for control of true armyworm. You can find the trait table at z.umn.edu/HandyBtTable.

If control is necessary, contact your local Extension Educator, Claire LaCanne or visit extension.umn.edu/corn-pest-management/armyworm-corn for more information.

Claire LaCanne is the agricultural extension educator for Rice and Steele counties. Reach her at lacanne@umn.edu, or at 507-332-6165 (Rice County) or 507-444-7691 (Steele County).

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