Caterpillars are the eating machines of the garden. A single large caterpillar can strip a cabbage leaf in a day, and a colony of them can defoliate a young plant in a week. They are also, confusingly, the babies of butterflies and moths — which means the gardener's response is a balancing act: protect the food crops, but leave the wild corners and the flowers for the butterflies. The good news is that caterpillars are one of the easiest pests to control by hand, because they are large, visible and slow, and because birds eat them by the thousand.
This guide explains how to identify the main garden caterpillars, what damage they cause, and organic-first control in order — from inspection and hand-picking through to barriers, beneficial insects and the biological spray Bt. If the damage on your plants does not match, the pest identifier helper can help you compare, and our corn earworm guide covers that specific caterpillar in detail.
How to identify caterpillars
A caterpillar is the larva of a butterfly or moth: a soft-bodied, segmented creature with three pairs of true legs near the head and several pairs of fleshy prolegs further back. Garden caterpillars range from tiny green loopers a few millimetres long to fat hornworms 10 cm long, and they come in green, brown, black, striped and spotted. The colour is often camouflage, so the best identification is by the host plant and the damage.
The most common garden caterpillars are the cabbage white caterpillars — velvety green, about 30–40 mm long, found on cabbage, broccoli, kale and other brassicas. The tomato hornworm is large, green, with a horn on its tail, and feeds on tomatoes and peppers. The cutworm is a fat, grey-brown caterpillar that lives in the soil and cuts seedlings off at the base at night. The leaf miner is a tiny caterpillar that lives inside the leaf — see our leaf miners guide for that one. If you are unsure which caterpillar you have, note the plant it is on and check your local extension service.
Lifecycle in general terms
Butterflies and moths lay eggs on the leaves of the host plant, often on the underside. The eggs hatch into caterpillars, which feed and grow, moulting several times, for two to four weeks depending on species and temperature. They then pupate — in a chrysalis on the plant, in a cocoon in the soil, or in leaf litter — and the adult emerges to mate and lay eggs again. There can be two or three generations in a season, so a few caterpillars in spring can become many by midsummer.
Because the caterpillar is the only feeding stage, the damage is concentrated in a short window — but that window repeats through the season. The adults are the ones to watch for: white butterflies over the brassica bed in spring mean caterpillars a week later, and a hornworm moth at dusk means hornworms on the tomatoes soon after.
Damage signs
Caterpillar feeding is easy to recognise: large, irregular holes chewed from the edges of leaves inward, sometimes with only the midrib left, and dark green or black droppings — frass — on the leaves below. On brassicas, the caterpillars often feed on the undersides and in the heart of the plant, and the damage is worst in the centre of the head. On tomatoes, hornworms strip leaves and eat green fruit. On seedlings, cutworms cut the stem at soil level, and the plant falls over overnight.
The damage is often worse than it looks at first, because caterpillars hide on the undersides of leaves and in the heart of the plant, and because they feed mostly at night or in the early morning. If a brassica is full of holes and you cannot see the culprit, look under the leaves and in the centre — the caterpillars are usually there, well camouflaged.
Organic-first control, in order
1. Inspect first. Check the undersides of leaves and the hearts of brassicas weekly, especially after you have seen butterflies over the bed. Look for eggs — tiny yellow or white dots on the leaf underside — and for small caterpillars and frass. Early detection matters: a caterpillar found at 5 mm is far less damaging than one found at 50 mm.
2. Hand-pick. This is the most effective control for a home garden. Pick caterpillars off by hand and drop them in a bucket of soapy water, or move them to a wild corner away from the crops. Do it in the morning, when they are feeding and visible, and check the leaf undersides and the plant hearts. A few minutes a day through the season will keep numbers low, and it costs nothing.
3. Barriers. Fine insect mesh or row cover over brassicas, put in place at transplanting and sealed at the edges, keeps the butterflies from laying eggs and is the most reliable control for cabbage whites. This is the method most experienced growers rely on, and it also protects against other pests. For cutworms, a cardboard collar around the stem of each seedling, pushed a few centimetres into the soil, stops the caterpillar reaching the stem.
4. Traps. Pheromone traps catch male moths and tell you when they are flying, which helps you time your inspections and your row cover. They will not control a caterpillar population on their own, but they are a useful early warning, especially for hornworms and cutworms.
5. Beneficial insects and birds. Birds are the most important predators of caterpillars — a single pair of blue tits can feed thousands of caterpillars to their chicks in a season. Encourage birds with water, shelter and nest boxes, and avoid broad-spectrum sprays that kill the birds' food. Parasitic wasps lay their eggs inside caterpillars, and ground beetles eat cutworms in the soil. Plant flowers nearby to support the wasps, and keep a wild corner for the birds.
6. Least-toxic options last. Bt — Bacillus thuringiensis — is a naturally occurring bacterium that kills caterpillars when they eat it, and it is harmless to people, pets, birds and most other insects. It is the best least-toxic option for caterpillars, but it must be applied thoroughly to the leaves the caterpillars are eating, and it works best on young caterpillars, so timing matters. Spray in the evening, reapply after rain, and check your local extension service for the right product and timing. Avoid broad-spectrum insecticides: they kill the birds' food and the parasitic wasps, and they make the problem worse the following year.
Prevention
Prevention is about barriers and timing. Cover brassicas with mesh from the day they are transplanted, and keep the cover on until harvest. Rotate brassicas to fresh ground each year, and clear away crop debris at the end of the season, since some caterpillars pupate in the debris. Plant brassicas where you can reach them easily for weekly inspection, and check the planting calendar for zone 3 in May for transplant timing. Grow flowers and herbs nearby to support parasitic wasps, and keep nest boxes and water for the birds. Accept that some caterpillars belong in the garden — the ones on the wildflowers and the nettles in the wild corner are the next generation of butterflies, and they are worth keeping.
Frequently asked questions
Will caterpillars kill my plants? A heavy attack can defoliate a young plant and ruin a brassica head, but a mature plant usually recovers once the caterpillars are removed. The main loss is the harvest itself, especially on brassicas, where the caterpillars feed in the heart of the plant.
Should I kill all caterpillars? No. Caterpillars are bird food, and butterflies are pollinators. Protect the food crops with mesh and hand-picking, and leave the caterpillars on wildflowers, nettles and ornamental plants alone — they are part of the garden.
What is Bt and is it safe? Bt is a naturally occurring soil bacterium that kills caterpillars when they eat it. It is considered safe for people, pets, birds and bees, and it does not harm most other insects. It works best on young caterpillars, so apply early and thoroughly, and check your local extension service for advice.
How do I stop cutworms cutting my seedlings? Use cardboard collars around each seedling, pushed a few centimetres into the soil. Check the soil at the base of fallen seedlings for the fat, grey-brown caterpillar, and pick it out by hand.
Why are there so many white butterflies over my brassicas? They are cabbage whites, laying eggs on the underside of the leaves. Cover the brassicas with fine mesh before the butterflies arrive, and check the leaf undersides weekly for eggs and young caterpillars.
Caterpillars are a pest where hand-picking and mesh beat any spray. Cover the brassicas, check the leaves weekly, pick the caterpillars off, and let the birds do the rest — and the harvest will survive. For the specific caterpillar of sweetcorn, see our corn earworm guide, and for general help use the pest identifier helper.