Cutworms

The pale western cutworm is a large (1.5 to 2 inches when fully grown) grayish caterpillar with no conspicuous markings except for a dark inverted V on the head. They live in the soil at the base of plants and are thus not seen until damage is apparent.

The black cutworm larva is gray to dark brown above and has a greasy appearance. Faint light stripes run lengthwise down the body. It also lives in soil, and like the pale western cutworm is usually not seen until damage is found.

The granulate cutworm is about an inch long when mature, dark gray in color, and the surface of its body is covered with black granules. It lives in the soil and cuts plants off below ground.

The variegated cutworm is a dark gray caterpillar with a light stripe on the side and small yellow to orange spots on top of the abdominal segments. Fully-grown larvae may be 1.5 to 2 inches long. Variegated cutworm is a climbing species, and while mostly nocturnal, may frequently be found feeding during the day.

The army cutworm is pale greenish gray to brown with the back pale-striped and finely splotched with white and brown.

Several other species of cutworms may be found in sugarbeets and their habits and control are similar to the species listed above.

The subterranean species (pale western, black, and granulate cutworms), feed largely underground, cutting plants off below the soil line. Frequently, many plants in a row will be cut off during the night; often this is the first indication of a problem. The black cutworm is especially active and has the habit of cutting off many plants while feeding. The granulate cutworm is primarily a pest of sugarbeet grown near alfalfa in the Imperial Valley. On occasion it migrates out of the alfalfa fields into sugarbeet, where it can consume young plants or clip them off below the ground as they feed. Granulate cutworm moths have been found to lay eggs on sugarbeet, and hatching larvae kill plants by eating them to the ground or by clipping them off at ground level, thus reducing plant stands.

The variegated and army cutworms are aboveground feeders and cut the plants off at or above the soil line. They also climb onto older plants and feed mostly on young foliage in the center of the crown. They generally cause only minor damage at this point.

Cutworms are attacked by a number of predators, parasites, and diseases. Many of these natural control agents are not effective on pale western and black cutworms because of their subterranean nature. It is not known if any of these natural enemies can control cutworm populations, but their presence should be noted.

Cutworms often build up in rotation crops preceding sugarbeet, such as alfalfa and cereals. If surveys indicate the presence of substantial numbers of cutworms in these crops, sugarbeet should not be planted. Spring plowing and discing are also useful in reducing cutworm numbers. Keep fields weed-free, especially eliminating grassy weeds that serve as alternate hosts for cutworms. Cutworms may also build up in high numbers if grassy weeds are prevalent in the crop preceding sugarbeets.

Monitor for cutworms during stand establishment by looking for plants lying on their sides that have been chewed at the soil surface or that are completely missing. If plants are missing completely, gently dig in the area where a seedling would be expected to try to find the intact root system as evidence of cutworm damage.

No economic thresholds have been established for cutworms, and the decision to treat depends on the severity of injury. Carbamate (methomyl, carbaryl) insecticides do not control the granulate cutworm; check with your farm advisor concerning the availability of materials to control this pest.