Cutworms
Also known as Black cutworm, Agrotis ipsilon, Turnip moth caterpillar
In short
Cutworms are soil-dwelling caterpillars of several night-flying moths. They hide in the soil by day and at night chew through the stems of seedlings and young transplants at or just below soil level, so plants are found toppled or wilting.
How it looks
Plump, smooth-skinned caterpillars, usually dull grey-brown but sometimes pale green or creamy white, often with faint markings that match the soil. Fully grown they are about 2.5–5 cm (1–2 inches) long. They have three pairs of true legs and five pairs of prolegs and curl tightly into a C shape when disturbed. Found in the top few centimetres of soil close to damaged plants, and on plants mainly at night. The black cutworm (Agrotis ipsilon) looks greasy grey.
Signs
Seedlings or transplants cut off near the soil line overnight; stems gnawed or ringed at the base; sudden wilting of a plant whose stem base or roots have been eaten. Some climbing species chew holes in leaves or bore into lettuce and cabbage heads or young fruit.
Life cycle
Moths lay eggs on plants or soil; the caterpillars feed at night and hide in soil by day, then pupate in the soil. Depending on species and climate, larvae may pupate before winter or overwinter and finish feeding the next spring, and they can be found at any time of year.
What favours it
Weedy, neglected or recently cleared ground, crop residues, and freshly dug beds; new transplants and seedlings in open ground or large outdoor containers.
Prevention
- Clear weeds and plant debris before planting.
- Put a collar of card, plastic or a cut bottle around the stem of each transplant, pushed a little into the soil.
- Cover seedlings with fleece or fine insect mesh.
Treatment
Start with the first step and move down only if the problem persists.
- Search the soil around a cut plant and hand-pick the caterpillars; checking by torchlight at night is very effective.
- Use sticky collars on perennial crops to stop climbing cutworms.
- Apply insect-parasitic nematodes (Steinernema) to moist soil when it is warm enough (RHS gives 12–20 °C).
- On larger plants, Bacillus thuringiensis subsp. kurstaki or spinosad sprays; follow the product label.
Pros and cons
Cons
- Can wipe out newly planted seedlings in one night.
Look-alikes
- Vine weevil larvae
- Legless, creamy-white, C-shaped grubs with a brown head that feed on roots inside pots rather than cutting stems.
- Slugs and snails
- Leave slime trails and graze leaves and stems rather than cleanly cutting stems at soil level.
- Damping-off
- Seedlings collapse from a water-soaked, pinched stem base with no chewing.
Affected plants
Seedlings and transplants of vegetables and ornamentals; also lettuce, potato tubers and root vegetables.
Related
Sources
Written in our own words from these sources. Check a product label before using any treatment.