Cut corn plants detected in South Dakota, damage caused by black cutworms

Adam Varenhorst
South Dakota State University Extension
Black cutworm caterpillar.

Cut corn plants have been detected in South Dakota. The damage is due to black cutworm feeding.

Black cutworm is a migratory moth that moves into South Dakota during early spring from the southern Unites States. Although black cutworm caterpillars injure corn by feeding on leaf tissue, the serious damage occurs when caterpillar feeding results in the cutting of young corn plants, leading to significant stand loss and the possible need to replant sections of a field.

Fields with residue or weeds are more attractive to the moths, which may result in increased risk. No-tilled fields and low-lying areas are typically the most attractive for the black cutworm moths.

Although most of the corn in South Dakota is past the point where cutworms can cause cutting, late-planted fields or fields that had delayed emergence can still be at risk and should be scouted.

Identification

Black cutworm caterpillars range in color from light-gray to brown to black and have a pale band that runs the length of the body along the top of the caterpillar.

Caterpillars appear to have rough or “pebbly” texture because their skin is covered with pairs of uneven-sized spots or tubercles located on the sides and near the top of each abdominal segment. These spots or tubercles can be used to separate black cutworm from dingy cutworm (dingy cutworms have equal sized spots), which less-frequently damages corn.

Head capsules of black cutworm caterpillars have two black stripes. Black cutworm caterpillars also have three pairs of true legs and four pairs of abdominal prolegs. When fully mature, caterpillars are approximately 1.5 inches in length.

Adult black cutworms are small, brown moths that are difficult to separate from the myriad of other small brown moths observed in South Dakota. Distinguishing characteristics of black cutworm moths include the last third of the forewing being lighter in color compared to the rest of the wing, forewings patterned with dagger-like black markings and hindwings that are typically white or gray.

Lifecycle

Black cutworms do not overwinter in the Corn Belt, so moths migrate into South Dakota during early spring. Females lay eggs on low-growing, broadleaved plants, but they will also lay eggs on dead plant debris if more preferable host plants are not present. Moths are also attracted to low-lying areas of fields due to increased moisture. Caterpillars feed at night and hide under soil clods or in cracks during the day. Black cutworm caterpillars feed on a wide variety of crops, including corn, wheat, several different vegetables and turf grasses.

Pupation occurs in the soil. Like most moths, adults are nocturnal, feed on nectar and rest in grasses during the day. There are multiple generations of black cutworm per year; however, only the first generation typically causes significant injury to corn, because the black cutworm caterpillars are able to feed around the stem completely.

Injury to corn

Black cutworm caterpillars feed on corn tissues close to the soil surface; caterpillars prefer to not climb plants. Stand reduction occurs when caterpillars become large enough to cut corn plants at the base.

Corn plants can tolerate black cutworm feeding after V5 leaf stage. Injury to corn is associated with tillage practices that allow for plant residues or weeds in the spring, fields previously planted to sod or legumes and low or poorly drained areas within a field.

Scouting

Scouting for black cutworm is difficult, because caterpillars are nocturnal and hide during the day. Therefore, scouting revolves around finding corn plants with signs of caterpillar feeding, either on the leaves, around the base of the stalk or the presence of cut plants.

Scouting should begin during corn emergence (VE) and continue weekly until V5 leaf stage. An insecticide treatment is recommended if greater than five percent of corn plants are cut and any black cutworm caterpillars that are found are less than one inch in length.

Management

There are several preventative management practices available for management of black cutworm. Many seed treatments are labeled for black cutworm, including those with the active ingredients clothianidin and thiamethoxam. There are also Bacillus thuringiensis (Bt) corn hybrids labeled for management of black cutworm, specifically hybrids that produce either Cry1F or Vip3A Bt toxins.

Management of weeds within a field also can reduce the risk of injury to corn from black cutworm. Fields with reduced tillage or no tillage management can attract egg-laying females, since they lay eggs on low-lying weeds and plant debris. The presence of weeds within a field can also reduce feeding on corn plants, as black cutworm larvae can more successfully develop on some weed species compared to corn. However, pre-emergent termination of weeds or incorporation of weeds and debris into the soil via tillage should occur no less than two weeks before planting corn to ensure black cutworm caterpillars starve before corn emerges.

Therapeutic management of black cutworm used in conjunction with scouting for cut corn and caterpillars is also effective. Currently registered insecticides for management of black cutworm in corn are listed in the current edition of the South Dakota Pest Management Guide: Corn.