Monitor Alfalfa for Cutworm Activity

 

During the spring, there are a lot of insects to scout for in alfalfa. One pest group that should be focused on right now are cutworms. However, scouting for cutworms can be challenging as they hide under the soil surface during the day and are active at night. Furthermore, injury from cutworms can sometimes be overlooked until large patches of an alfalfa field have been “cut”. There are a couple of cutworm species that are active earlier in the season than others. These include the army cutworm and the pale western cutworm. For anyone scouting alfalfa at the moment, these cutworm caterpillars are more than likely observed below-ground and still mostly inactive.

 

 

Army cutworms overwinter in South Dakota as caterpillars in the soil. Pale western cutworms may also overwinter as caterpillars, but more often as eggs. Pale western cutworm caterpillars hatch and begin feeding once soil temperatures reach 70 °F. Caterpillars of both species may be observed in alfalfa fields, but pale western cutworms are more common in winter wheat. Because army cutworms overwinter as caterpillars, they begin to feed when the soil is approximately 40 °F and are typically the first cutworms that are observed injuring plants. Soil temperature data is available online at the Mesonet at SDState (select “soil temp” from the layer menu).

 

 

Army cutworm caterpillars are typically gray and have lighter patterns or splotches on their bodies (Figure 1). They also have a pale stripe on the center of the back that runs the length of their body.

 

Figure 1. Army cutworm caterpillar. 

 

In comparison, pale western cutworms are grayish-white with no discernible patterns (Figure 2).

 

Figure 2. Pale western cutworm caterpillar. 

 

 

The two cutworms also differ in their feeding habits. Army cutworms will defoliate plants and, when food is limited, they may devour the entire plant.

 

In comparison, pale western cutworms feed below the soil surface and will notch or cut plant stems. Pale western cutworms always feed above the moisture line in soil, and during wet conditions they will actually feed on the above-ground tissue of the plant.

 

 

 

To scout for army cutworms in alfalfa, monitor new growth for signs of defoliation. Any delay in green-up may be a sign of cutworm activity. If army cutworms are observed in a field, their population density should be determined by scouting several square foot areas within the field. Dig the soil in each square foot area and determine the number of army cutworm caterpillars that are present. For newly established alfalfa stands, the threshold for army cutworms is 2 caterpillars per square foot. For established alfalfa, the threshold is 4 or more army cutworm caterpillars per square foot. There aren’t thresholds for pale western cutworms in alfalfa, but approximately 1-2 caterpillars per square foot may cause issues.

 

 

These cutworms can be managed using foliar insecticides that are timed in the late afternoon or evening (when possible). Both army cutworms and pale western cutworms are below-ground or under plant residue during the day. Refer to the 2019 South Dakota Pest Management Guide: Alfalfa and Oilseeds for a list of products that are labeled for cutworm management in alfalfa.

 

If alfalfa is going to be destroyed and replanted to another crop, it is important to know if the cutworms will still be present. When planting a new crop in a previously infested alfalfa field, consider using an in-furrow insecticide to reduce the risk of cutworm feeding.