Cutworms are the pest that gardeners discover in the morning, when a healthy seedling that was standing yesterday is lying flat on the soil, cut clean through at the base. The culprit is a fat, grey-brown caterpillar that hides in the soil by day and feeds at night, and it can destroy a whole row of transplants in a few nights. The good news is that cutworms are easy to prevent with a simple collar, and easy to control once you know what to look for.
This guide covers how to identify cutworms, what damage they cause, how they live, and how to control them organically.
Identifying cutworms
Cutworms are the caterpillars of several species of night-flying moth. They are smooth, fat, grey, brown or black caterpillars, usually 2–5 cm long when fully grown, and they curl into a tight C-shape when disturbed — the most reliable way to recognise one. They hide in the soil or under debris during the day, within a few centimetres of the plant they are feeding on, and come out at night to feed.
The signs are usually visible before the caterpillar itself:
- Seedlings cut clean through at soil level, often lying flat on the ground, still fresh and green
- Young plants wilting suddenly for no obvious reason
- Small holes in the leaves of low-growing plants
- The caterpillar itself, found by scratching in the top few centimetres of soil around a damaged plant — look for a fat, smooth, grey-brown caterpillar curled in a C-shape
Cutworm damage is sometimes confused with slug damage, but slugs eat leaves and leave slime trails, while cutworms cut the stem at the base and leave the plant lying flat. Damping-off disease also kills seedlings at soil level, but the stem goes thin and brown and the plant collapses, rather than being cleanly cut. If you are unsure, our pest identifier helper can help you work through the symptoms, and your local extension service can confirm an identification.
The cutworm lifecycle
Cutworms are the larvae of moths that lay their eggs on plant stems, leaves or the soil surface in late summer and autumn. The eggs hatch into caterpillars that feed briefly, then overwinter in the soil or under debris. In spring, they resume feeding — and this is when they do the most damage, because spring is when gardeners are planting out tender seedlings.
A cutworm feeds for several weeks, growing through several stages, then pupates in the soil and emerges as a moth in summer. The moth lays eggs, and the cycle repeats. There may be one or two generations a year, depending on the species and the climate. In mild climates, the cycle can be almost continuous.
Because the caterpillars overwinter in the soil and under debris, the risk is highest in beds that were weedy or grassy the previous autumn, and in beds where transplants go into soil that has not been cleared and cultivated. A clean, well-prepared bed is much less attractive to egg-laying moths.
Damage signs
Cutworms are most dangerous to young plants. A single cutworm can cut through several seedlings in one night, and a small population can destroy a whole row. They feed at the base of the stem, cutting through it cleanly, and the plant above the cut wilts and dies. They also eat leaves, especially low leaves, and can damage the roots of some crops.
The crops most at risk are the ones gardeners transplant in spring: tomatoes, peppers, cabbage, lettuce, beans and other tender seedlings. Established plants with woody stems are rarely troubled — a cutworm cannot cut through a thick, woody stem — so the danger is concentrated in the first few weeks after planting out.
Organic-first control, in order
Work through these steps in order, starting with the simplest and most effective: prevention with a collar.
1. Inspect the soil around damaged plants. When you find a cut seedling, scratch in the top few centimetres of soil around it. The cutworm is usually hiding within a few centimetres of its last meal. Find it, pick it up and drop it in a bucket of soapy water or feed it to the birds. One cutworm found is several seedlings saved.
2. Use collars — the best control there is. A collar is a tube of cardboard, plastic or metal placed around the stem of a transplant, pushed a couple of centimetres into the soil and standing 5–8 cm above it. It physically blocks the cutworm from reaching the stem. Cut the bottom off a paper cup, a toilet roll tube or a plastic bottle, and you have a collar. Put collars on every transplant in spring, and cutworm damage drops to almost nothing. This is the single most effective cutworm control, and it is free.
3. Clear the bed before planting. Cutworm moths lay eggs on weeds and plant debris, and the caterpillars overwinter in weedy, undisturbed soil. Clear the bed, remove debris, and cultivate the top few centimetres of soil a week or two before planting out. This exposes overwintering caterpillars and pupae to birds and weather, and removes the weeds the moths prefer.
4. Encourage predators. Birds are the best cutworm predators — a garden with birds will have fewer cutworms. Ground beetles, which hunt at night, also eat cutworms. Leave some habitat for them: a log pile, a patch of long grass, a few stones. Avoid spraying anything that kills them.
5. Use barriers and traps. A ring of sharp grit or crushed eggshells around a seedling can deter cutworms, though it is less reliable than a collar. A board laid on the soil near vulnerable plants makes a good trap: cutworms shelter under it during the day, and you can collect them in the morning.
6. Least-toxic treatments, last. If collars and inspection have not brought damage down, beneficial nematodes — microscopic worms that parasitise soil pests — can be watered into the soil. They are available from garden suppliers, work best in warm, moist soil, and are harmless to plants, people and pets. Bacillus thuringiensis (Bt), a naturally occurring bacterium, is also effective against caterpillars when sprayed on the foliage they eat, but it must be applied when the caterpillars are young and actively feeding, and it washes off in rain. Always read and follow the label, use only products approved in your area, and wash produce before eating. Never use a broad-spectrum insecticide for cutworms: it kills the predators that control them.
If you are unsure which product is appropriate, check your local extension service for recommendations in your area.
Prevention
Prevention is mostly about collars and clean beds. Put a collar on every transplant, every spring, without exception — it takes a minute per plant and saves the whole row. Clear beds before planting, remove debris where moths lay eggs and caterpillars overwinter, and avoid planting tender seedlings into a bed that was weedy grass the previous autumn without cultivating it first.
Rotate crops where you can, and do not plant tender seedlings in the same spot year after year if cutworms have been a problem there. Harden plants off before planting out so they establish quickly — a plant that grows fast is vulnerable for a shorter time.
When is it a serious problem vs cosmetic?
A single cut seedling is a nuisance — replace it and put a collar on the replacement. It becomes a serious problem when several seedlings are cut in one night, when a whole row is destroyed, or when you are losing transplants faster than you can replace them. In that case, act immediately: search the soil around every damaged plant, put collars on everything that is still standing, and consider nematodes if the problem persists.
Cutworms on an established plant with a woody stem are rarely a problem. Cutworms on a row of newly planted tomatoes are a serious problem. The difference is the age of the plant and the rate of loss — and morning inspection is how you tell which one you have.
Frequently asked questions
What does cutworm damage look like? A seedling cut clean through at soil level, lying flat on the ground, still fresh and green. The cut is clean, as if made with scissors, and there are usually no slime trails.
How do I find cutworms? Scratch in the top few centimetres of soil around a damaged plant, especially in the morning. The caterpillar is usually within a few centimetres of its last meal, curled in a C-shape.
Do cutworm collars really work? Yes — they are the most reliable cutworm control available. A simple cardboard or plastic collar, pushed a couple of centimetres into the soil and standing 5–8 cm above it, blocks the caterpillar from reaching the stem.
Can cutworms damage mature plants? Rarely. They cannot cut through thick, woody stems, so the danger is concentrated in the first few weeks after planting out, when stems are soft and thin.
Are cutworms the same as wireworms? No. Wireworms are the larvae of click beetles — thin, hard, yellow-brown, wire-like grubs that live in the soil and eat roots and tubers. Cutworms are fat, soft caterpillars that cut stems at soil level. The control methods are different.
The bottom line
Cutworms are a spring problem with a simple answer: put a collar on every transplant. Inspect the soil around any damaged plant, clear beds before planting, encourage birds and beetles, and reach for nematodes or Bt only if collars and inspection have not worked. For more pest guidance, see the guides hub and the garden tools.