We have started to received reports of true armyworm caterpillars showing up in South Dakota wheat fields. So far, the reports have originated from winter wheat fields in Central South Dakota. However, in the next few weeks populations may show up in additional areas of the state in both winter wheat and spring wheat.
Although true armyworms will feed on leaf tissue, the real concern is generally their head clipping activities later in the season. True armyworms are migratory pests that start each season in the Southern U.S. During their northward flight, true armyworm moths are more attracted to fields that contain living ground cover (e.g., grass, weeds, early season crops) and as a result these are the areas where eggs are laid. For South Dakota, the moths generally arrive during June and July. Depending on the seasonal migration timing and also the location in South Dakota one or two generations of true armyworms are possible.
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, the true armyworm caterpillars will have dark bands on each of their abdominal prolegs (Figure 2). The last characteristic that can be used is the network of black lines that are present on their orange head capsule (Figure 3).
Figure 1. True armyworm caterpillars can vary in color. Courtesy: Adam Varenhorst.
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.
Figure 3. True armyworm caterpillars have a network of black lines present on their orange head capsule. Courtesy: Adam Varenhorst.
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 also 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 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 or cut the head.
If thresholds of true armyworms are exceeded please refer to the 2018 South Dakota Pest Management Guide: Wheat.