Impact
Armyworms, Pseudaletia unipuncta, can occasionally cause severe damage to crops throughout the San Luis Valley. This insect prefers grass crops, including small grains, but will feed on some broadleaf crops such as potatoes after grain maturity. The greatest threat in small grains is head clipping in barley.

Description and Symptoms

Mature larvae are about I 1/2 inches in length, smooth-bodied, and dark grey to greenish-black in color. They are characterized by five stripes, three on the back and two on the sides, running the length of the body. The stripes on the sides are pale orange with a white outline. The head capsule is remarkable for its honeycomb of black markings.

Dense vegetation is preferred for egg laying.

Newly hatched larvae move with a looping (inchworm) action.

Larvae feed at night and on cloudy days, and hide under crop debris during sunny periods.

Several generations can occur per year with the first damage occurring very early in the growing season.

Larvae overwinter in upper layers of the soil or under crop debris.

Population increases are dramatic from a single hatch, which is a serious
threat to neighboring fields. The new larvae may feed and move together like an army.

Crop damage can range from early season seedling destruction to late season clipped heads. Larvae feed on green plant material throughout the season. Head clipping is more common in barley.

Large flocks of blackbirds in a grain field have been noted in the San Luis Valley as an indication a high population of army cutworms.

Integrated Management

Scout often for armyworm in field margins, low areas with rank growth, or areas of lodged plants. Look for feeding damage, droppings around base of plant, or dropped plant material. Check for larvae in and under debris, around damaged plants and in heads of barley or wheat.

Treat armyworm infestations if all of the following conditions are met:

1) larval counts exceed action threshold; 2) worms are 3/4 to 1 1/4 inches in length; 3) most larvae are not parasitized (look for white eggs behind the head or small brown cocoons attached to the body); and, 4) leaf feeding or head clipping is evident. Insecticides labeled for these crops can be found in the current Colorado Pesticide Guide for Field Crops.

This material is based on work supported by the San Luis Valley Water Quality Demonstration Project Best Management Practices Advisory Committee, Colorado State University Cooperative Extension, Farm Service Agency, Natural Resources Conservation Service, and the U.S. Department of Agriculture under authorization as 1991 U.S. Department of Agriculture Water Quality Demonstration Project.