Armyworm

Pseudaletia unipuncta (Haworth)

Appearance and Life History

Larva Larva
Photo by J. Obermeyer

Several species of armyworms can be found in the Midwest every year. However, the development of economically damaging populations depends on a number of factors such as; cropping practices, insect migration patterns, parasites and predators, weather conditions, etc. For example, several weeks of cool wet weather in the spring favor armyworm development and reduces the normal activity of parasites and predators, thus influencing the growth of armyworm populations.

Adult moth with characteristic white dot on forewing Adult moth with characteristic white dot on forewing
Photo by J. Obermeyer

The adult armyworm is a pale brown moth with a white dot in the center of each forewing. The young larva is green in color and moves about in a looping motion. A full-grown larva is dull-green to brown in color with alternating light and dark stripes running the length of its body. Upon reaching larval maturity, it is about 1-1/2 inches (38 mm) long.

The larvae feed primarily on grain crops and grasses, attacking other plants only when preferred foods are not available. Infestations usually develop in grass pastures, fence rows, roadsides and in small grain fields where crops have lodged or are matted against the ground. Once the larvae have consumed the readily available food, or small grains mature, they move into other crops, most notably corn. This usually happens during May and early June. An exception to this pattern may develop in no-till corn fields where cover crops are used, or in corn fields with many grassy-type weeds. Armyworm moths are attracted to the grasses in these fields for oviposition. When the larvae hatch in these fields, they can immediately cause damage throughout the field. This is in contrast to their appearance along the edges of tilled corn fields.

Armyworm larvae usually feed at night and during cloudy days, hiding beneath crop debris or in the whorl of plants at other times. Because of their nocturnal habits, their presence may not be realized until the crop begins to show feeding damage.

Armyworm Life Cycle

Damage

Young corn plant showing Armyworm damage Young corn plant showing Armyworm damage
Photo by Purdue University

Armyworm feeding gives corn a ragged appearance, with defoliation occurring from the leaf edge toward the midrib. Damage may be so extensive that most of the plant, except leaf midribs and the stalk, is consumed. Such a highly damaged plant may recover, however, if the growing point has not been destroyed.

Damage in early whorl corn Damage in early whorl corn
Photo by Purdue University

Sampling Method

Scouting is necessary only in fields where larvae or their damage is noted. When scouting early in the season, be especially watchful for signs of armyworm feeding damage where corn was no-tilled into a grass cover-crop or in areas of corn fields that border small grains or grassy areas.

If larvae or their damage are found, sampling should be conducted in at least 5 areas of the field. In each of the 5 areas, randomly select a plant as the first of a sample set of 20 consecutive plants. Carefully examine each plant to determine the percentage of damaged plants in each sample set. Also note the number of larvae observed and estimate their size (length). On a sunny day, larvae may be found deep in the corn whorl or on the ground under plant residue. Finally, estimate the percentage of plants damaged by armyworm and note the location(s) of the infestation. The percentage of plants showing feeding damage can be estimated on a per plant basis or as a portion of the total field acreage (i.e., if 5 acres of a 50 acre field have armyworm feeding damage, report it as 10% of the field with damage). Since some infestations may be outside the sampling area, i.e. border rows, rows between sampling areas, etc., any area with damage should be noted and evaluated.

Management Guidelines

Corn Insect Control Recommendations: E-series 219-W (PDF)

If more than 50% of the plants show armyworm feeding and numerous live larvae are less than 1-1/4 inches (31 mm) long, a control may be necessary. Larvae greater than 1-1/4 inches (31 mm) will soon be pupating and the application of a control is not usually justified since the damage has already been done. If armyworms are detected migrating from areas bordering fields, or from waterways or other grassy areas within fields, spot treatments in these infested areas are possible if the problem is identified early enough.

Good grass control before planting will lower the chance of an armyworm outbreak. This reduces moth egg laying activity and larval migration into fields. Watch for armyworms migrating out of maturing small grains into corn. Planting into standing grass cover crops and then applying burn-down herbicides may increase the chance of armyworm attack.

If control is necessary, contact your state Cooperative Extension Service or click here for control materials and rates.

IPM Tip Armyworm