Yesterday, beautiful little seedlings were just poking their heads up in the garden. This morning, tragedy! Some seedlings are missing altogether, while others have been beheaded, their fragile tops cut off, lying neatly beside them. If you’re wondering what horrible thing has befallen your plants, the answer is simple: cutworms. You may be able to find one taking a daytime nap by probing the earth with your fingers. Of course, you can kill any cutworms you find by tossing them on a hard surface and stepping on them, but that won’t help the seedlings that have lost their heads. The only immediate solution is to reseed or transplant new seedlings to replace those that were demolished.
Know their cycle to get control
Once you’ve experienced cutworms’ damage, you’ll have no trouble understanding why the larvae of certain night-flying moths are called “cut” worms. The term is applied to about 200 species with slightly different eating habits. Most are gray-brown caterpillars, with the characteristic habit of curling up in the shape of the letter C when disturbed.
Cutworms sleep by day just under the soil surface or occasionally in moist debris on the surface. Some will even make tunnels and feed just below or above the soil surface. These cutworms are the ones most likely to chop down seedlings. Other types remain in the soil and feed on underground stems and roots, causing plants to wilt. Many of these caterpillars are climbers that eat leaves or buds of larger plants or trees. Regardless of the parts of plants they are likely to eat, all of these cutworms come out at night to do their devastating damage.
Grass and weeds harbor eggs. The adult moths of many cutworm species share a similar life cycle. In spring, they are attracted to grasses and weeds to lay their eggs. They prefer weeds with multiple stems and many basal leaves that produce low, dense growth. So the worst cutworm infestations in the vegetable garden generally occur where grassy areas have recently been broken up to create a new planting bed. When the young caterpillars hatch, they begin feeding on the nearest vegetation, slowly expanding their range as they grow. Consequently, if you’re planning to enlarge your garden in early spring by cultivating a nearby grassy or weedy area, do it at least two to three weeks before planting any vegetable seeds. If cultivated too soon before vegetable planting, cutworm larvae that have already hatched may migrate to the vegetables in search of food.
In fall, you can begin a program to reduce cutworm damage the following spring. Thickly mulch the garden area. Compost, weeds, hay, leaves, or even newspaper can make a good smothering mulch. Monitor the area in late winter and spring when the ground warms sufficiently for plant growth. Keep it as free of weeds and grass as possible. Remove any vegetation that might tempt cutworm moths to lay their eggs nearby, or continue adding mulch to deter weed growth.
Seek alternatives to toxic insecticides
If mulching in the previous season doesn’t rid you of cutworms, you can treat the garden with Btk (Bacillus thuringiensis var. kurstaki). This is a naturally occurring bacterial insecticide specific to caterpillars, and it is harmless to humans and to the natural enemies of the pests. Products with this active ingredient are available under various trade names; just be sure to follow package directions when it comes to application.
Because it’s hard to determine when cutworm-moth eggs are starting to hatch, spray Btk at weekly intervals while your seedlings are emerging and still small. Repeat applications are necessary because Btk dissipates quickly in the environment and may wash off plants with rain or irrigation. The most important thing to understand about Btk is that it has to be eaten by the cutworms to be effective. It will not kill the cutworms through contact alone, so you’ll have to tolerate a little plant damage while the tiny worms ingest the poison. Also, Btk is ineffective against cutworms that feed below the soil surface. Btk is a living product, so protect it from moisture and high temperatures during storage.
Heterorhabditis bacteriophora and Steinernema carpocapsae, two species of insect-attacking nematodes that are otherwise harmless in the garden, are known to eat cutworms. Both are available as commercial products from nurseries and mail-order suppliers. Keep in mind, however, that nematodes work best in loamy or sandy soils. They are also susceptible to drying out, so apply them to a moist soil surface in the late afternoon and water them in thoroughly. They will remain viable in the soil longer if there are some alternate sources of food for them. Ensure this by incorporating compost into the soil beforehand. Composted organic material usually attracts fungus gnats, and their larvae serve as food for the nematodes in between any cutworm meals.
If cutworms didn’t have the annoying habit of chopping down seedlings, then we might pity them for they have many natural enemies. Most are too small to notice, like parasitic braconid wasps or tachinid flies. Others are more visible, like meadowlarks, blackbirds, toads, moles, and shrews. The best approach to preserving these natural enemies is to avoid the use of poisons, especially those like Sevin that contain the active ingredient carbaryl, which is toxic to many natural enemies. Include flowering plants in the garden to provide nectar and pollen for the beneficials, food to tide them over until their insect prey hatches. It’s worthwhile to make the extra effort that alternative pest-management methods sometimes require to keep cutworms in check and your harvests healthy.
Collars can thwart cutworms
Not all vegetables lend themselves to being raised in containers or flats and then transplanted. But those that do, such as lettuce, cabbage, broccoli, and beans, can be protected during the stage when they are most vulnerable to cutworms.
STEP 1: Keep seedlings in their containers in a sunny, protected indoor or outdoor location until they are 4 to 5 inches tall and have several true leaves.
STEP 2: Make a set of protective barriers/collars by cutting in half-frozen juice containers, cardboard toilet-paper tubes, or plastic cups.
STEP 3: Transplant the sturdy seedlings into the ground, and surround each with a protective barrier. Each barrier should be sunk into the ground an inch deep.