True Armyworms

True armyworms are showing up in SW Wisconsin and NE Iowa. Armyworms are an occasional pest in the area, but seem to me more numerous than normal is year. Bob Rose and Chris McLimans are both reporting fields with high enough pressures that growers need to take action to prevent stand and yield losses.

This pest will rarely cause trouble in conventional-tilled fields, but growers should be checking reduced and no-till fields. Another likely area of infestation would be fields that had wheat stubble or rye planted as a cover crop that was taken off in the spring. Any kind of ground cover makes a great place for the moths to lay their eggs as they fly up from the south. When the eggs hatch, usually about 7 to 10 days, they begin to feed and will continue for 5 to 6 weeks. At the end of the feeding period they will pupate and reemerge as moths to lay eggs and start the cycle again, we usually see up to 3 generations per year. The second generation will usually appear in July, and is the most damaging.

This insect will usually feed on the leaf margins and will stop at the midrib. Young corn plants have a remarkable ability to recover from the feeding, an ISU study showed that corn in the V7 to V9 stage with 50% defoliation recovered after treatment and only showed a 2-6% yield loss.
Treatment should be considered in V7-V8 stage if larvae are less than 3/4 inch in length, the population is larger than eight larvae per plant, and there is 25% or more defoliation. If armyworms are less than 3/4 inch, they still have about a week to feed before they pupate. At 1 ½ inches, the larvae are about done feeding and treatment is not going to give any economic benefit. There are many common insecticides labeled for armyworm control, you will need to check with your local chemical supplier to see what pricing and availability is for your area.