The larvae of night-flying moths, cutworms damage plants by feeding on both roots and foliage.
Illustration By Keith Ward
This article is part of our Organic Pest Control Series, which includes articles on attracting beneficial insects, controlling specific garden pests, and using organic pesticides.
Cutworms (Agrotis ipsilon and Peridroma saucia)
The discovery of cutworms in the garden always comes first thing in the morning, when you find seedlings that have been cut off at the soil line during the night. Direct-seeded beans and corn are frequent victims, especially when planted into soil recently cleared of weeds and grasses. Young transplants are also frequent victims.
Organic cutworm controls include protecting seedlings with rigid collars, encouraging predation by birds and thorough bed preparation. The two most common species, black cutworms (Agrotis ipsilon) and variegated cutworms (Peridroma saucia) are both found in temperate climates worldwide. In spring, moths are carried northward and become summer pests in areas where they do not survive winter.
What Are Cutworms?
Rarely seen by day, cutworms are grayish to brown smooth-skinned caterpillars that feed above-ground at night. The larvae of night-flying moths, cutworms damage plants by feeding on both roots and foliage. Black cutworms have small black dots on their bodies, while variegated cutworms are mottled brown with a faint white stripe down their backs. When encountered in the soil, cutworms immediately curl up and become still.
What Cutworm Damage Looks Like
Cutworms girdle plants by wrapping themselves around tender stems as they chew. Cutworm damage usually takes place right at the soil line, with only a nub left at the surface. Frequently the top portion of the plant is left to shrivel unconsumed. In comparison, rabbits chew off a sharp edge, often at an angle, and leave no leftovers. In summer, cutworms can crawl up plants and chew holes in the foliage that looks like damage done by slugs.
Cutworm Life Cycle
Variegated cutworms overwinter as larvae, which move to the surface to feed in spring. These individuals are the cutworms most likely to decapitate early cabbage seedlings or lettuce sown while the weather is still cool. Black cutworms emerge as adults, and promptly begin laying eggs at the bases of weeds and other host plants such as lamb’s-quarters and wild mustards. The eggs hatch in less than a week, and the larvae feed on roots and foliage for about a month before they are ready to pupate into adults. By late spring, both types of cutworms are posed to be serious pests in young stands of beans or corn.
Natural Enemies of Cutworms
The soft bodies of cutworms make them easy prey to ground beetles, and they are eagerly eaten by general nighttime predators including bats, toads and snakes. Well-known beneficial insects including tachinid flies, braconid wasps and yellow jackets claim some cutworm victims, though these predators are more active above-ground in the daytime, when few cutworms are about.
Insect-eating birds that feed by scratching at the soil’s surface, such as robins, brown thrashers, and domestic chickens can be major cutworm predators. In addition, cutworms frequently fall prey to parasitic nematodes or fungi that live in the soil.
Organic Cutworm Control
The most popular and effective way to protect plants from cutworms is to use cutworm collars — 2- to 3-inch rounds that are pressed 1 inch into the soil around vulnerable plants. Collars can be made of cardboard, metal cans, or plastic drink cups cut into rounds. Slit one side to make them easy to pop around plants.
Many gardeners also protect seedlings from cutworm damage by placing a nail, toothpick, or bamboo food skewer into the soil alongside the plant stem. Although labor-intensive, this method is practical in small plantings. Other gardeners make small moats around plants and fill them with cornmeal. Cutworms eat the cornmeal but cannot digest it, and many die.
If you have already seen cutworm damage, handpick larvae at night by flashlight. One individual cutworm can do significant damage in a bed of beans. The night after you see cutworm damage, check the bed at hourly intervals after dark and you will probably nab the culprit.
In some situations, you could try using Bt (Bacillus thuringiensis), a widely available biological pesticide that can be useful in controlling climbing cutworms that eat a lot of leaves. Bt is less effective controlling hit-and-run cutworms because they do not ingest a lethal dose. Bt can be mixed with oatmeal or cornmeal and placed in little mounds on the soil’s surface in hopes that foraging cutworms will eat it. Similarly, beneficial nematodes can be of great value when introduced under proper conditions.
More Advice on Organic Cutworm Control
New garden areas previously occupied by grass or weeds should be cultivated in fall and again in spring to expose and kill overwintering cutworm larvae and pupae. Chickens let into the plot after these cultivations will help improve the effectiveness of this technique.
Do not use plastic mulch in high-risk beds, because cutworms living beneath plastic mulch are impossible to control. Instead, enlist the help of insect-eating birds to get good organic control of cutworms in the garden. Draw their attention to newly tilled beds by providing convenient perches nearby.
Adult moths can be attracted to, and killed by, electronic bug zappers, but these devices also kill beneficial insects. If you want to passively trap the adult moths in your garden, use solar landscape lights set above broad pails of soapy water.
More information on cutworms is available from this ATTRA Q&A and Oregon State University.