True armyworms have seen a resurgence in the western Corn Belt with the advent of more cover crop acres (Figure 1). True armyworms overwinter in southern states as pupae in the soil. In late April and early May, the moths migrate north. The larvae prefer to feed on grass crops. True armyworm can range in color from dark green to almost black. On the side, there are white, orange and dark-brown stripes along the entire body. Once the larvae reach maturity, they are about 1.5 inches long. The head capsule is yellow brown with a network of veins that result in a mottling appearance. The larvae can feed and migrate in a mass, hence the name true armyworm.
The best scouting procedure is to look for the band of damage in the field (Figure 2). The area of prior feeding and the yet-to-be damaged area of the field will not contain larvae. The greatest number of larvae will be in a band between the damaged and undamaged areas of the field.
They feed at night or on very cloudy days and hide in residue during the day. Grass cover crops, especially cereal rye, provide the perfect nursery for armyworms in the spring.
The armyworm threat can be reduced by terminating the cover crop in late February or early March, thus eliminating the green material that is attractive to migrating female moths. If cereal rye is left to mature in late spring and the subsequent cash crop is planted in green rye, the armyworm threat increases. As the cover crop is terminated, the armyworms will leave the rye and begin feeding on the emerging cash crop. Corn and grain sorghum planted into green rye are at risk. The damage occurs in the matt of the cover crop residue and goes undetected until the stand is lost.