Figure 1. Orange stripe present on each side of true armyworm caterpillar and black band on each of the abdominal prolegs. Credit: A. Varenhorst
While scouting wheat last week I observed several fields with small populations of true armyworms. This may seem unimportant as most wheat fields are nearing maturity. However, true armyworms don’t just defoliate leaves. True armyworms will initially feed on leaves, but as the wheat dries down, they will begin feeding on other areas of the plant and eventually cause head clipping.
True armyworm populations often will not reach economically damaging levels in wheat; however, head clipping will cause economic losses to occur rapidly. The drawback to managing true armyworms near harvest is that the insecticides available will delay harvest from 7-21 days after application. The specific pre-harvest interval for each insecticide is located on the label.
True armyworms can vary greatly in color from light brown to dark green or almost black (Figure 2). They can be identified based on a few distinguishing characteristics. True armyworm caterpillars have orange stripes present on the sides of their bodies (Figure 1), and a network of black lines present on their heads (Figure 3). The caterpillars have three true legs, and four pairs of prolegs that are present near the center of their bodies with a distinct black band present on each one.
Figure 2. True armyworm color comparison. Credit: A. Varenhorst
Figure 3. Network of black lines present on true armyworm head capsule. Credit: A. Varenhorst
Scouting & Management
What to look for?
True armyworm caterpillar populations are often overlooked or are simply not present in wheat. True armyworm caterpillars feed at night. During the day, they are typically found near the base of the plant or in litter on the surface of the soil.
When scouting for true armyworms in wheat, search for defoliation or head clipping in the field. If defoliation is observed, look for caterpillars on plants and in the soil around the injured plants. For wheat that is close to harvest, the threshold for true armyworm is 2 caterpillars that are less than ¾ inch in length per square foot.
The stage of the wheat and time until harvest should be considered prior to any insecticide application. Yield loss in wheat is more likely to occur if the flag leaf is removed prior to the soft dough stage. However, as plants mature there are fewer nutrients available and as a result caterpillars will move to the head and feed either on the beards or simply clip the heads. If populations of true armyworm caterpillars are observed in a maturing wheat field it is important to scout neighboring corn and later wheat fields as true armyworm caterpillars are known to move in large populations from one field to the next as resources are depleted.
For insecticide management options please refer to the current edition of the South Dakota Pest Management Guide: Wheat. Remember to always follow label instructions and wear proper personal protective equipment when applying insecticides.