Cutworms

If you plant out tomato or cabbage seedlings and find one lying on the ground the next morning, cut cleanly through at the soil line, a cutworm did it. Cutworms are the caterpillars of several night-flying moths in the family Noctuidae, including the black cutworm (Agrotis ipsilon) and the variegated cutworm. They do most of their damage in spring and early summer, when transplants and seedlings have thin, tender stems.

How to recognize them

• Larvae are smooth, plump caterpillars, gray, brown or nearly black, up to 1 1/2 to 2 inches long when grown. When disturbed they curl into a tight C shape.
• Adults are dull brown or gray moths that fly at night and are rarely noticed. The moths do no damage themselves.

Cutworms hide in the top inch or two of soil during the day and feed at night. When you find a plant that has been cut, scratch gently through the soil within a few inches of the stem; the caterpillar is usually there.

Life cycle

Depending on the species, cutworms spend the winter in the soil as partly grown larvae or pupae, or the moths migrate north each spring (the black cutworm cannot survive northern winters). Females lay eggs on low weeds, grass and plant debris. The larvae feed for several weeks, pupate in the soil, and there can be one to several generations a year. Beds that were weedy, grassy or in sod the previous year usually have the most cutworms.

Damage

Most species chew through stems at or just below the soil surface, often several plants in one night, and eat little of the plant they cut down. Climbing cutworms crawl up and feed on leaves, buds and fruit instead.

Commonly attacked plants include tomato, pepper and eggplant transplants, cabbage, broccoli and other brassicas, beans, sweet corn, lettuce, and many flower seedlings.

Prevention and control

Collars. The simplest and most reliable protection. Put a collar about 3 inches tall around each transplant's stem when you plant it, pushed an inch or so into the soil: a cardboard tube, a strip of light cardboard stapled into a ring, or a paper cup with the bottom cut out. Leave it until the stem has thickened, after which cutworms cannot cut through.

Weed and prepare beds early. Clear weeds and turn under or remove plant debris at least a week or two before planting, so the young larvae have nothing to feed on and starve before your transplants go in. Tilling or digging in fall also exposes overwintering larvae to birds and cold.

Hunt at night. Go out with a flashlight an hour or two after dark and pick off any cutworms you find on or around plants, or dig them out of the soil next to a freshly cut plant in the morning.

Beneficial nematodes. Steinernema carpocapsae nematodes, watered into moist soil, attack cutworms where they hide. Ground beetles, birds and parasitic wasps also eat cutworms, so avoid broad-spectrum insecticides that kill these predators.

Bt and spinosad. Bt (Bacillus thuringiensis, kurstaki strain) kills caterpillars only after they eat it, and works best on small larvae. Because cutworms feed at night at the soil line, apply it in the evening to the lower stems and the soil surface around plants. Spinosad products, including baits labeled for cutworms, also work. Both are much less effective against large, fully grown cutworms, so collars remain the first line of defense.

See the garden pests page for other pests, and tomato hornworm for another caterpillar that Bt controls.