Cutworms Causing Problems in S.D. Sunflower07/05/2016
We have received reports of cutworms feeding on newly emerged sunflower plants. As their name implies, cutworm caterpillar feeding often results in the stem or stalk of a new plant being fed around or through. This feeding injury results in the plant being “cut” as the stalk is no longer able to support the weight of the plant. Depending on the age of the plant, this method of feeding may also remove an entire portion of the stalk, which also results in the cutting injury.
There are several species of cutworms that could potentially cause issues in South Dakota sunflower. These species include the redbacked cutworm, dingy cutworm, darksided cutworm, and army cutworm. However, during outbreak years army cutworm typically peaks in April or May, which reduces the potential for these to cause injury to sunflowers.
Redbacked cutworms are approximately 1 inch long, and are dull gray to brown in color. The distinguishing characteristic for these caterpillars are the two dull red stripes present on the back of their bodies (Figure 1).
Dingy cutworms are similar in length as redbacked cutworms, but can be identified by their dull brown color. In addition, they have a thin line down their back that is lighter in color, and a series of diagonal markings on their sides (Figure 2).
Darksided cutworms are also approximately 1 inch in length, and have pale brown back and lighter underside. They can be distinguished by numerous, indistinct stripes present on the sides of their body (Figure 3).
Cutworm Scouting & Management
Cutworms are nocturnal feeders, and hide beneath the soil surface during the day. This can complicate scouting as defoliation or cutting may be observed, but not the cutworm species responsible. To scout for cutworms in sunflowers, examine newly emerged sunflower fields twice per week until approximately one month after initial emergence. Because cutworms burrow into the soil a hand trowel can be useful to excavate the caterpillars for identification. While in the field walk in a “W” or “Z” pattern, and examine five locations within the pattern for cut plants. At each location use the trowel to examine a 1 square foot area within the row. Dig approximately 2 inches down, and carefully sift through the soil when examining for caterpillars. If an average of 25-30% of the plants within the five locations are cut, or if one caterpillar per square foot is observed then the threshold for cutworms in sunflowers has been exceeded. Foliar insecticides can be used for cutworm management after plants have emerged. Please refer to the current edition of the South Dakota Pest Management Guide: Alfalfa and Oilseeds for a list of insecticides currently labeled for cutworm management in sunflower.
Source: Adam Varenhorst, South Dakota State University Extension