Cabbage worms are the green caterpillars that eat holes in cabbage, kale, broccoli and other brassicas, and they are one of the most common pests in the vegetable garden. They are easy to miss — they are the exact green of a cabbage leaf — and they can do a surprising amount of damage in a short time, turning a perfect head of cabbage into a lace of holes and droppings. The good news is that they are easy to control by hand, and easy to prevent with a simple cover.

This guide covers how to identify cabbage worms, what damage they cause, how they live, and how to control them organically.

Identifying cabbage worms

There are two main kinds of cabbage worm, and they are often found together. The imported cabbage worm is the caterpillar of the small white butterfly — the white butterfly you see fluttering over the vegetable garden in summer. The caterpillar is smooth, velvety green, about 2–3 cm long, with a faint yellow stripe down its back. The cabbage looper is the caterpillar of a brown moth; it is also green, but it moves by looping its body, arching its back as it walks, and it has thin white stripes along its sides.

Look for:

  • Smooth, green caterpillars on the leaves, especially on the undersides and in the centre of the plant — they are the exact colour of the leaf, so look closely
  • Irregular holes in the leaves, often large, with the leaf veins left behind
  • Dark green droppings (frass) on the leaves and in the centre of the plant
  • White butterflies fluttering over the brassica bed — a sign that eggs are being laid
  • Young caterpillars, only a few millimetres long, on the undersides of leaves

The white butterfly is the most useful early warning. If you see small white butterflies over your cabbage or kale, eggs are being laid, and caterpillars will appear within a week.

Cabbage worms are sometimes confused with other caterpillars. Cutworms are grey-brown, live in the soil and cut stems at the base. Flea beetles make tiny shot-holes, not large ragged holes. Diamondback moth caterpillars are smaller, paler and wiggle violently when disturbed. If the caterpillar is smooth, green and eating large holes in a brassica leaf, it is almost certainly a cabbage worm. 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 cabbage worm lifecycle

The lifecycle starts with the small white butterfly, which lays tiny yellow eggs singly on the undersides of brassica leaves. The eggs hatch in a few days, and the young caterpillars begin feeding immediately. They grow through several stages over two to three weeks, eating more as they grow, then pupate — forming a chrysalis attached to a leaf, a stem or a nearby structure — and emerge as a butterfly a week or two later.

There are usually several generations a year, from spring to autumn, and the population builds through the summer. A single butterfly can lay hundreds of eggs, so a few butterflies in June can mean a serious infestation by August. In mild climates, the cycle continues almost all year; in cold winters, the pupae overwinter in sheltered places and emerge in spring.

Damage signs

Cabbage worms eat the leaves of brassicas — cabbage, kale, broccoli, cauliflower, Brussels sprouts and related crops. They start with small holes and progress to large, ragged holes that can strip a leaf to its veins. They feed in the centre of the plant as well as on the outer leaves, and their droppings contaminate the head of a cabbage or the florets of a broccoli, making the crop unappetising even where the damage is light.

Young plants are the most vulnerable: a few caterpillars can defoliate a seedling and kill it. Established plants can tolerate moderate damage to their outer leaves, but the caterpillars also eat the growing point, which can stop a cabbage from heading or a broccoli from forming a head.

Organic-first control, in order

Work through these steps in order, starting with the simplest.

1. Inspect regularly. Check the undersides of brassica leaves at least once a week from spring onwards, and more often in summer. Look for eggs — tiny yellow dots — and for young caterpillars, which are easiest to deal with when they are small. Squash any eggs you find.

2. Hand-pick. For a small garden, hand-picking is the most effective control. Go through the plants, turn over the leaves, and pick off every caterpillar you find. Drop them in a bucket of soapy water or feed them to the birds. Do this weekly, or twice weekly in summer when the population is building. It is quick, free and surprisingly effective.

3. Use row covers. The best prevention is a fine insect mesh or row cover over the brassica bed from planting to harvest. It physically blocks the butterflies from laying eggs, and it works completely — no butterflies, no eggs, no caterpillars. Use a fine mesh (not fleece, which overheats in summer) supported on hoops so it does not rest on the leaves, and seal the edges with soil or bricks so butterflies cannot crawl underneath. This is the single most reliable control for cabbage worms.

4. Encourage beneficial insects. Parasitic wasps lay their eggs inside cabbage worm caterpillars, and the wasp larvae eat the caterpillar from the inside. If you find a caterpillar covered in small white cocoons, leave it alone — those are the wasp cocoons, and the wasps that emerge will protect your garden. Birds also eat caterpillars, so a garden with birds and habitat will have fewer cabbage worms. Plant flowers that attract predators, and avoid spraying anything that kills them.

5. Use companion planting. Strongly scented herbs such as thyme, rosemary, sage and mint, planted around the brassica bed, may help confuse the butterflies and reduce egg-laying. The effect is modest, but it is worth trying alongside other methods.

6. Least-toxic sprays, last. If hand-picking and covers have not brought numbers down, Bacillus thuringiensis (Bt) is the standard organic treatment for caterpillars. It is a naturally occurring bacterium that is harmless to people, pets and beneficial insects but lethal to caterpillars when they eat it. Spray it on the leaves, especially the undersides, and repeat after rain or every week while caterpillars are present. It works best on young caterpillars, so apply it early. 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 cabbage worms: it kills the parasitic wasps and other 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 row covers and inspection. Cover the brassica bed with fine mesh from the day you plant out, and keep it covered until harvest — this alone prevents most cabbage worm damage. Inspect weekly, squash eggs, and hand-pick any caterpillars that get through.

Rotate brassicas where you can, and do not plant them in the same spot year after year, because pupae overwinter in the soil and nearby debris. Clear away old brassica plants at the end of the season rather than leaving them to rot in the bed, and remove weeds in the brassica family — such as wild mustard and shepherd's purse — which can harbour the caterpillars.

Plant brassicas in a sunny, airy spot with good airflow, water steadily and feed moderately. Mulching keeps watering steady and the soil healthy, which helps plants recover from light damage. Our fertiliser calculator can help you avoid over-feeding, which produces soft growth that caterpillars favour.

When is it a serious problem vs cosmetic?

A few small holes in the outer leaves of an established cabbage are cosmetic — the plant will still head, and you can peel off the damaged leaves at harvest. It becomes a serious problem when caterpillars are eating the growing point, when young plants are being defoliated, when the head or florets are contaminated with droppings, or when numbers are climbing fast in summer. In that case, step up hand-picking, cover the bed, and use Bt if needed.

Cabbage worms on a mature kale plant that you harvest leaf by leaf are a nuisance. Cabbage worms eating the centre out of a young cabbage are a serious problem. The difference is the age of the plant, the part being eaten and the rate of increase — and weekly inspection is how you tell which one you have.

Frequently asked questions

What are the white butterflies in my vegetable garden? They are small white butterflies — cabbage whites — and they are laying eggs on your brassicas. Each butterfly can lay hundreds of eggs, so a few butterflies in June mean caterpillars in July. Cover the bed with fine mesh to stop them.

Will cabbage worms kill my plants? They can kill young seedlings by defoliating them, and they can prevent a cabbage from heading or a broccoli from forming a head. Established plants usually survive moderate damage, though the crop may be unappetising.

Are cabbage worms safe to eat? They are not poisonous, but you do not want to eat them. Wash all produce thoroughly before eating, and check the leaves and florets carefully — a soak in salted water will bring any hidden caterpillars out.

Do cabbage worms attack other plants? They mainly attack brassicas, but they will also eat nasturtiums and other members of the brassica family. They do not usually attack tomatoes, beans or other unrelated crops.

Can I use washing-up liquid on cabbage worms? It is not effective against caterpillars and can damage leaves. Hand-picking, row covers and Bt are the reliable methods.

The bottom line

Cabbage worms are easy to see, easy to pick off by hand and easy to prevent with a fine mesh cover. Inspect weekly, squash eggs, hand-pick caterpillars, cover the bed, encourage parasitic wasps, and use Bt only if the gentler steps have not worked. For more pest guidance, see the guides hub and the garden tools.