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Scout for True Armyworms in Oats and Wheat

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.

During most years, we start worrying about true armyworm activity in wheat fields in mid-July. However, the strong southerly winds that we experienced during the last two weeks pushed several insect pests north ahead of their normal schedule. One of those pests included the true armyworm moths. We have received reports of true armyworm caterpillars defoliating oats near Beresford, SD and it is likely that this pest may also be present in wheat fields. True armyworms can cause substantial defoliation when present in large populations, so scouting should begin now to ensure that this doesn’t occur.

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. This means that the true armyworm caterpillars could still be an issue regarding head clipping as another generation might be observed later this season.

Identification

Left: Figure 2-A. Dark green caterpillar with an orange stripe running the length of its body feeding on a corn leaf. Right: Figure 2-B. Dark green caterpillar that has an orange head with a network of black lines.
Figure 2. (A) True armyworm caterpillars have an orange stripe on each side of their body and dark bands on their abdominal prolegs. (B) True armyworm caterpillars have a network of black lines on their orange head capsule. 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. In addition, true armyworm caterpillars will have dark bands on each of their abdominal prolegs (Figure 2-A). The last characteristic is the network of black lines that are present on their orange head capsule (Figure 2-B).

Scouting and Management

Dark colored caterpillar feeding on wheat head.
Figure 3.True armyworm feeding on wheat beards. Courtesy: Christopher Nelson.

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. When using a sweep net, remember 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. 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 3) or cut stems below the heads.

If thresholds of true armyworms are exceeded, please refer to the latest. 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.