True Armyworm control in Wisconsin crop fields and pastures

Moth

True armyworm moths are sand-colored and have a wingspan of 1.5” with a definitive white dot in the center of each forewing and dark markings on the hind wings. 

Egg

True armyworm eggs are greenish-white and are laid in rows or clusters on leaves. 

Larvae

True armyworm larvae are brownish-green, hairless, and have alternate dark and light stripes down their backs. Their heads are pale brown with dark markings. Fully grown larvae are about 2″ long.

Pupae

True armyworm pupae are dark brown and approximately ¾ of an inch long. They are sharply tapered at the tail end with a much more rounded head end. 

True armyworms reproduce by complete metamorphosis which includes four stages, egg, larval, pupal, and adult moth. Females lay eggs in the evening and at night in rows or combinations of rows on the leaf blades of preferred host plants. Moths often choose to lay eggs in grasses and small grains since the plants help to protect egg viability and are the desired source of food for the larval stage of the insect. Seven to ten days following egg lay, larvae emerge and begin to feed on lush green foliage. The first generation is usually small but can cause economic damage to wheat, corn planted after grass cover crops, and fields with uncontrolled grassy weeds. After feeding for 3-4 weeks, the full-grown larvae pupate and emerge two weeks later as adult moths. In WI, true armyworms normally have 2-3 generations per season, with each generation lasting 5-6 weeks. The second larval generation, which appears in July, is the largest and most damaging generation to Wisconsin crops. The success of this generation    produces later, more injurious, generations of armyworms. The fall generation is typically not injurious and is often heavily parasitized by beneficial insects, fungi, and viruses. True armyworm does not overwinter in Wisconsin as moths migrate to Wisconsin each spring.

Damage and symptoms 

The larvae tend to feed at night or on cloudy days and hide in the soil or under foliage during the day. First generation damage can be very difficult to locate and will catch many crop scouts by surprise. Pay close attention to corn planted after grass cover crops and corn fields which have high grassy weed populations. In wheat and other small grains detection is also difficult. Pay special attention to areas where crop density is high and/or where lodging has occurred. Second generation infestations may occur throughout a corn field during July if grassy weeds such as foxtail, quack grass, goosegrass, and nutsedge are present for oviposition in the field. In this case, plants in scattered areas of the field will have ragged leaves from larval feeding. The other type of infestation results when armyworms migrate from pastures, spring seeded small grains, or grassy hay fields, and invade the outside rows of corn. Damage is usually highest along the field edge or near grassy spots. 

Non-chemical natural control

Several braconid wasps and tachinid flies help keep armyworm numbers down, as do birds, toads, skunks, and some domestic fowl.

Cultural control

Since female moths prefer to lay eggs in grassy areas, keeping grassy weeds controlled will lessen the possibility of problems. Avoid planting susceptible crops in low wet areas or in rotations following sod. If this is unavoidable, be sure to plow in the fall of the previous season to decrease early spring egg-laying sites. Killing grass with an herbicide or tillage may drive armyworms to the susceptible crops.