At this time they are small larvae (1/2″-3/4″ long) and feeding in the lower foliage. Scout for armyworms at grassy margins of the fields, low, weedy areas in fields or in lodged grain; populations are more likely to develop in these areas first. Armyworms prefer the edges of leaves first and are messy, wasteful eaters. They generally retreat during the day under soil and plant residue on the ground and feed more often beginning at dusk, it’s easier to scout for armyworm damage than the armyworms themselves. Look for leaves that have been notched/cut, partially eaten leaf material on the ground, and small round pellets (armyworm frass, i.e. poop) near the base of the plants.
Consider applying insecticide if: there are 4-5 armyworm larvae per sq. ft., caterpillars are ¾ – 1 ¼ in. long, leaf feeding or head clipping is found, and parasites are not evident. By the time armyworms are more than 1 ½ in long, they have stopped feeding and are getting ready to pupate. At this point the damage has already been done and control applications will probably not provide an economic return.
There are a number of insecticides registered for use against armyworms in small grains, check the label for rates. At this time of year, be certain to check the PHI as well.