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Time to Start Scouting For Cutworms

May 4, 2018

In previous years, cutworms have been found showing up in winter wheat fields in April or sometimes even earlier in March. This year has been much cooler than usual with most of the Midwest experiencing one of the coldest Aprils on record. Cold temperatures and late snows cause delays in insect activity, including that of cutworms. However, now that warmer temperatures are becoming more consistent, cutworms may start showing up again. There are two species that can pose a threat to winter wheat: army cutworm and pale western cutworm.

Army Cutworm
Overwintering army cutworm caterpillars emerge as temperatures warm up in the spring. They have a dark brown head and their body changes color with age. Initially, the caterpillars will be small (less than ½ inch) and have a light grayish brown body with few markings. As the caterpillars mature, they will turn a dull gray to brown color with some mottling. Eventually, they will grow up to about 2 inches in length and develop several pale stripes that run the length of their bodies.

Army cutworms are a nocturnal insect, only coming out of the ground to feed at night. For scouting, use a small shovel to dig into the top few inches of soil and count the number of caterpillars. The treatment threshold for army cutworms is about 2-4 caterpillars per square foot. An alternative scouting method is to base thresholds on cutworm feeding injury to plants. Army cutworm feeding is characterized by plants appearing cut or clipped near the soil surface. This clipping mostly happens on the tender blades of winter wheat, and rarely on the stem, crown, or meristematic tissues, which allows for regrowth to occur.

Pale Western Cutworm
Unlike army cutworms, pale western cutworms overwinter as eggs and begin feeding after they hatch in the spring. Pale western cutworms show up later than army cutworms, making them more of an occasional pest of winter wheat. The caterpillars have light gray to greenish white colored bodies with heads ranging from light to dark brown. They also have characteristic dark spots on each body segment and two distinct vertical lines on the head. Pale western cutworms can grow up to 1 ¼ inches in length.

Pale western cutworms feed near the soil surface causing “clipping” injury. They also feed through the stems, resulting in plant mortality. To scout for the caterpillars, dig into the top few inches of soil and count how many are in a square foot within a row. The threshold for pale western cutworms is 1-2 caterpillars per square foot. As with army cutworms, you may also scout by looking for the presence of clipped plants within the field.

For insecticides currently labeled for managing army cutworm and pale western cutworm, please refer to the current version of the South Dakota Pest Management Guide: Wheat.

Source: Patrick Wagner, iGrow