Cutworms

Cutworm larvae vary in color, but commonly

• are dull gray to brown caterpillars with various markings,
• blend in with the soil, and
• appear smooth-skinned at a distance.

Cutworm larvae have four pairs of prolegs (leglike appendages, or fleshy stubs) in the middle of the abdomen, on segments 3 to 6. Full-grown caterpillars are 1 to 2 inches long. Cutworms commonly curl up into a C-shape when disturbed.

Distinguishing characteristics of different cutworm larvae:

• Black cutworm is gray to dark brown on top with pale lengthwise stripes. It has an overall greasy appearance and numerous tiny, black, rounded, pebblelike bumps.
• Glassy cutworm is shiny, pale green to gray, or whitish with a brown head.
• Granulate cutworm is brownish to dark gray with black blotches or specks.
• Variegated cutworm is dark gray with a light stripe on the side and small yellow to orange spots on top of the abdomen.

Adult moths have dark gray or brown front wings with irregular spots or bands and pale hind wings. Females lay hundreds of white eggs, either singly or in clusters (depending on the species). Eggs are laid on leaves or stems close to the ground. After hatching, young larvae chew on the leaf surfaces. Larvae are nocturnal: older larvae drop to the ground and tunnel into the soil during the day and emerge at night to feed.

Cutworms normally feed close to the soil surface, clipping off seedlings or chewing leaves that touch the ground. Uncommonly, cutworms bore into cabbage heads. Most feeding occurs at night; during the day cutworms are usually found just below the soil surface or under dirt clods.

Seedlings and young plants are cut off at or just below ground level; often several plants in a row will be wilted or clipped. Losses can be especially severe in fields that were recently thinned or that were precisely direct-seeded to a stand to eliminate the need for thinning. Damage often recurs in the same fields and same parts of fields year after year. Damage is worst when large numbers of cutworms are present before planting.

Cutworms migrate into crops from surrounding weeds or infested crops. Monitor for cutworms in surrounding weeds. Use cultural controls and apply insecticide bait before seedling emergence or transplanting.

Cutworms have numerous natural enemies, including predatory beetles, parasitic wasps, and parasitic tachinid flies. However, natural enemy numbers are usually not high enough to control both cutworms and the other caterpillar pests that occur in the crop simultaneously. Biological control from these natural enemies is typically not sufficient to prevent economic damage.

At least 10 days before planting:

• Remove weeds from field margins, and
• Plow fields to destroy egg-laying sites, food sources, larvae, and pupae.

Start with cultural control in an organically certified crop. Sprays of Bacillus thuringiensis and the Seduce formulation of spinosad are also available for cutworm control in organically grown cole crops.

Before seedling emergence or transplanting, monitor for cutworms on weeds around field edges. Scout specifically along cracks and the crown area of plants, as cutworms typically hide in these places during the day. If many cutworms are present, apply insecticide bait. This controls most species (except the glassy cutworm, which occurs in the southern San Joaquin Valley). Baits are more effective when few plants are present, so apply bait after weed removal and before crop emergence or transplanting.

After seedling emergence or right after transplanting, check the crop for four or more wilted plants in a row with clipped or partially cut stems. Dig around those stems and sift the soil. If you find cutworms, apply insecticide.

When plants are small or young, monitor weekly or more often depending on the severity of the infestation. Apply insecticide as soon as several severed plants in the same row are found. A directed spray at the base of plants is most effective, but a broadcast spray can also be effective.

Spray for cutworms during the late afternoon or evening, so that cutworms will be exposed to insecticide when they emerge in the evening. Certain insecticides degrade when exposed to sunlight; insecticides applied earlier in the day may not be as effective at controlling cutworms as those applied in the late afternoon or evening.