On small grains, the threshold is 5 worms per square foot. I have no idea who created these thresholds but seem to be the only guidelines at the moment. Infestations usually starts in grasses injured by hail and lodged to the ground. Armyworm moths migrate into the state from the south. Fields with a lot of foxtails also appear to attract armyworm moths.
Numerous chemicals are available for use against armyworm on corn.
Insecticides for use on small grains have a lot of restrictions and so make sure you read and follow the fine prints on the label.