Keep an Eye on Your Wheat: True Armyworm Caterpillars May be Present

Dark green and tan curled caterpillars with an orange stripe along their sides.
True armyworm caterpillars. (Credit: Adam Varenhorst, SDSU Extension)

Written collaboratively by Adam Varenhorst, Philip Rozeboom, Patrick Wagner, and Brad McManus.

While scouting wheat this week, we observed true armyworm caterpillars that ranged from 1/4” to 1/2” in length. Every year, true armyworm caterpillars pose a threat to South Dakota wheat fields, and they most frequently show up during the end of June and early July. However, strong southerly winds during the spring can cause populations to show up earlier. Although true armyworms can feed on numerous crops, they are primarily an issue for corn and wheat. True armyworm caterpillars feed on the leaves and can cause severe defoliation when large populations are present. In South Dakota, by the time the caterpillars start showing up, most corn is beyond the susceptible stage so the main concern is their presence in wheat.

True armyworms are migratory pests that start each season in the Southern U.S. During the northward flight, true armyworm moths are more attracted to fields that contain living ground cover (e.g., grass, weeds, early season crops). Depending on the seasonal migration and their location in South Dakota, one or two generations of true armyworms are possible.

Identification

Caterpillars of the true armyworm can vary greatly in color from light brown to dark green or sometimes almost black (Figure 1). Fortunately, there are some other characteristics that can be used reliably to identify them. True armyworm caterpillars have an orange stripe on each side of their body that runs from their head to the end of their abdomen. In addition, true armyworm caterpillars will have dark bands on each of their abdominal prolegs (Figure 2). The last characteristic is the network of black lines that are present on their orange head capsule (Figure 3).

Color

Dark green and tan curled caterpillars with an orange stripe along their sides.
Figure 1. True armyworm caterpillars can vary in color from light brown, to dark green, to almost black. (Credit: Adam Varenhorst, SDSU Extension)

Striping

Dark green caterpillar with an orange stripe feeding on a corn leaf.
Figure 2. Caterpillars have an orange stripe on each side of their body and dark bands on their abdominal prolegs. (Credit: Adam Varenhorst, SDSU Extension)

Head

Dark green caterpillar that has an orange head with a network of black lines.
Figure 3. True armyworm caterpillars have a network of black lines present on their orange head capsule. (Credit: Adam Varenhorst, SDSU Extension)

Scouting and Management

True armyworm caterpillars are generally not present at large enough populations to be noticed. However, they can be easily scouted for with either sweep net sampling or individual plant observations. When scouting, the entire field should be examined. If examining individual plants, the threshold is 2 caterpillars per square yard. Sometimes the caterpillars will be found at the base of the plant or on the soil near the plant. If scouting with a sweep net, the threshold is 40 caterpillars per 30 pendulum sweeps. Remember when using a sweep net to walk in a W or Z pattern through the field. Caterpillar feeding can reduce yields, especially if the flag leaf is removed prior to the soft dough stage. Large populations of these caterpillars can lead to rapid defoliation of the wheat, and in some cases result in the stand loss that affects large areas of the field. These caterpillars have also been observed in the past clipping seed heads prior to harvest.

If thresholds of true armyworms are exceeded, please refer to the current South Dakota Pest Management Guide: Wheat.

Related Topics

Wheat Insects

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