Bryan Jensen, UW Extension, UW-Madison Integrated Pest Management Program
No worries just yet. More of a feeling than anything else. Based on a few calls and emails, be on the lookout for armyworms. The summer generation can be cryptic. Corn is tall and unless you are actively scouting populations can “magically appear”. Wheat is not out of the woods until combined and oats could be an excellent habitat for the summer generation. Throw pastures on the list as well.
Based on your location in Wisconsin, you may be at the tail end of the spring generation or starting the summer generation now. Infested fields are hard to predict so spot checking all field is a good idea especially if you have grassy weeds that have escaped control.
Armyworm larvae have tan heads with a net-like pattern of lines, yellow belly, faint orange stripes on sides and darker striping on back. The intensity of striping can vary from those which are faint to those which are so dark colored that lines are not easily visible. Base your identification by looking at several individuals.
Economic thresholds and guidelines for corn is to treat when either 75% of the plants have one armyworm/plant or 25% of the plants have two or more larvae AND the larvae are 1 inch or less in length. Treating in small grains is suggested if there are 3 or more armyworm/sq ft. But be careful of head clipping.