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Watch for True Armyworms in Wheat

This week we observed some true armyworm caterpillars in winter wheat fields. The caterpillars were still relatively small, which means they will continue feeding for some time. So far, the true armyworm caterpillars were still feeding on the leaves of the nearly mature wheat, but they have the potential to also clip heads off of the plants. 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). For South Dakota, the moths generally arrive during June and July. Depending on the seasonal migration and their location in South Dakota, one or two generations of true armyworms are possible.

Identification

Dark green and tan curled caterpillars with an orange stripe along their sides.
Figure 1. True armyworm caterpillars can vary in color. Courtesy: Adam Varenhorst

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.

Dark green caterpillar with an orange stripe feeding on a corn leaf.
Figure 2. True armyworm caterpillars have an orange stripe on each side of their body and dark bands present on their abdominal prolegs. Courtesy: Adam Varenhorst

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).

Scouting Strategy

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. Courtesy:Varenhorst

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.

Dark colored caterpillar feeding on wheat head.
Figure 4. True armyworm feeding on wheat beards. Courtesy: Christopher Nelson

Caterpillar feeding can reduce yields, especially if the flag leaf is removed prior to the soft dough stage.

As plants mature, fewer nutrients are available in the leaves and the caterpillars will move to the heads to either feed on the beards (Figure 4) or cut the stem below the head.

Management Options

If thresholds of true armyworms are exceeded, please refer to the 2019 South Dakota Pest Management Guide: Wheat.

Table 1 contains a list of insecticides labeled for true armyworm management in wheat. Remember to double-check pre-harvest intervals to ensure that harvest is not greatly delayed by the insecticide being used.

Table 1. Insecticides for true armyworm management in wheat.*
Insecticide
(Active Ingredient)
Rate Pre-Harvest Interval
Bolton
(chlorpyrifos+gamma-cyhalothrin)
5-18 fl oz 7 days for harvest 30 days for straw
Coragen or Prevathon
(chlorantraniliprole)
2-20 fl oz 1 day
Fastac EC
(alpha-cypermethrin)
1.3-3.9 fl oz 14 days
Mustang or Mustang Maxx
(zeta-cypermethrin)
1.28-4.3 fl oz 14 days
*This is list is not comprehensive. Insecticides that are listed were chosen based on having pre-harvest intervals of 14 or less days.