Today, I heard about some armyworm feeding on corn in northeast and northwest Iowa. In the NE Iowa field, rye was planted last fall and killed late this spring. Armyworms tend to be aggregated, or found in big groups, and can cause significant injury seemingly overnight. The caterpillars are messy feeders - they shred corn plants and leave frass pellets in the whorl. I helped write up an ICM News article about armyworms last summer that might be useful. Thanks to Ben Wikner for the notification and great photos!