Cabbage white caterpillars are the green and yellow-black caterpillars that strip cabbages, kale, broccoli and other brassicas to bare stalks in high summer, hatched from eggs laid by the two cabbage white butterflies. The only reliable way to stop them is to keep the butterflies off the plants in the first place, with fine insect netting over the crop from May onwards. Once caterpillars are present, picking them off by hand and squashing the yellow egg clusters on the leaf undersides is the honest answer, because there is no spray a home gardener can rely on to do it for you. Miss a week in July and August and they will reduce a healthy plant to a skeleton with alarming speed.
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The first year I grew brassicas properly I did everything right except the one thing that mattered. Beautiful cabbages, well fed, well watered, and I stood back rather proud of them in late July. Ten days later I lifted a leaf and the underside was crawling. Not a few caterpillars, hundreds, and the plants were more hole than leaf. I had watched the pretty white butterflies flitting about the plot all month and thought nothing of it. That is the whole lesson of this article in one story: the butterfly you ignore in June is the ruined crop you find in August.
Which Butterfly, Which Caterpillar
Two species do nearly all the damage, and it helps to know them apart because they behave differently. The large white is the bigger butterfly, with bold black wing tips, and it lays clusters of yellow, skittle-shaped eggs on the undersides of leaves. Its caterpillars are yellow and grey-green with black blotches, hairy, and they feed in gangs out in the open, often stripping the outer leaves first. The small white is slightly smaller with less black on the wings, and it lays single eggs rather than clusters. Its caterpillars are velvety green, well camouflaged against the leaf, and they burrow into the heart of a cabbage where you cannot see them, which makes them the sneakier of the two. There is also the cabbage moth, whose brown caterpillars do the same job at night. In practice you treat all of them the same way, but the large white is the one you usually spot first because it is so much more obvious.
How to Tell Caterpillars Are the Problem
The damage is distinctive once you know it. You will see ragged holes eaten through the leaves, often starting at the edges and working in, and on a bad infestation only the tough leaf veins left behind, giving that skeletal look. Look for the caterpillars themselves on the undersides of leaves and tucked into the growing heart, and for scatterings of dark green or brown frass, which is a polite word for caterpillar droppings, on the lower leaves and around the plant. Above all, check the leaf undersides for the eggs before they even hatch: the large white lays neat clusters of upright yellow eggs, and finding and rubbing those off is ten seconds of work that saves you a hundred caterpillars later. If the holes are accompanied by silvery slime trails instead of frass, you are dealing with slugs and snails rather than caterpillars, which is a different problem with a different answer.
The Only Thing That Really Works: Netting Them Out
Prevention beats cure so completely with this pest that it is worth saying plainly: the single most effective thing you can do is stop the butterflies ever reaching the plants. A physical barrier of fine insect mesh, put over the brassicas from May and kept on through summer, means no eggs, no caterpillars, and no work beyond checking it now and then. Ordinary butterfly netting with a mesh around 7mm is enough to keep both cabbage whites out, and it is the one piece of kit I would not grow brassicas without after that first disastrous year. The two things that matter in using it: hold it clear of the foliage on hoops or canes, because a butterfly will happily lay through mesh that is resting on a leaf, and tuck the edges right down to the soil or weigh them with stones, because they will find any gap. A tunnel of netting over a bed of cabbages is not the prettiest thing on the plot, but it is the difference between whole leaves and lace.
What to Do If They Are Already Here
If the caterpillars have already arrived, and in a British summer they will find unprotected plants within weeks, the honest answer is hands and vigilance rather than a magic bottle. Go over the plants at least twice a week, more in warm weather, and check the undersides of leaves and deep into the hearts. Rub off any egg clusters, and pick off and squash or drop into soapy water every caterpillar you find. It is not glamorous but it genuinely works if you are consistent, because you are breaking the cycle before the next generation lays. On a heavy infestation, a strong jet of water knocks many off, and standing the outer leaves back to expose the caterpillars to birds helps too. The biological control Bacillus thuringiensis, a naturally occurring bacterium sold for caterpillar control, is the one genuinely effective treatment beyond hand-picking, though availability to UK amateurs varies, so check what is currently sold before relying on it. What does not work well is most general bug sprays, which struggle to reach caterpillars tucked into a cabbage heart and which also kill the ladybirds and hoverflies you want to keep.
Working With Nature Instead of Against It
You do not have to wage total war. Cabbage white caterpillars have plenty of natural enemies, and a garden that encourages them suffers less. Birds, especially in spring, eat huge numbers, so leaving some access rather than netting absolutely everything has a place. A tiny parasitic wasp, Cotesia glomerata, lays its eggs inside large white caterpillars, and those sickly caterpillars surrounded by little yellow cocoons are the wasp doing your job for you, so leave them be. Growing some flowers among the veg to bring in hoverflies and other predators helps the whole plot, which is one of the arguments in my piece on plants for pollinators. Some gardeners plant nasturtiums as a sacrificial crop that the whites lay on in preference, though in my experience that just breeds more butterflies unless you then remove the caterpillars, so treat it as a decoy you still have to police, not a fix.
Keeping Brassicas Healthy Enough to Cope
A strong plant shrugs off light damage that would set back a weak one, so good growing is part of pest control. Firm planting, steady moisture and sensible feeding all help, and the brassica cluster on this site covers each in turn: growing brassicas for the overview, feeding without soft growth, since soft sappy growth from too much nitrogen is exactly what pests love, and watering brassicas properly. Net the plants, keep them growing steadily, check them twice a week through summer, and you will eat the cabbages instead of watching the caterpillars eat them. It really does come down to those few habits, and the netting is the one that does most of the work.
Frequently Asked Questions
How do I stop cabbage white caterpillars eating my plants?
Cover the brassicas with fine insect netting from May, held clear of the leaves on hoops and tucked down at the edges, so the butterflies cannot land and lay. This is by far the most effective method. If plants are already affected, check the leaf undersides twice a week and pick off the caterpillars and rub off the yellow egg clusters by hand.
What do cabbage white caterpillars look like?
There are two kinds. Large white caterpillars are yellow and grey-green with black blotches, hairy, and feed in groups out in the open. Small white caterpillars are velvety green, well camouflaged, and burrow into the heart of the plant. Both leave ragged holes in the leaves and dark droppings, and can strip a plant to its veins.
Will caterpillar-damaged cabbages still be alright to eat?
Usually yes. Remove the badly eaten outer leaves and check the heart carefully for hidden caterpillars, washing the leaves well. A plant reduced to bare veins may not recover enough to heart up, but lightly holed leaves are perfectly fine to eat once cleaned. The damage is cosmetic more than anything until it becomes severe.
What is the difference between the large white and small white butterfly?
The large white is bigger with bold black wing tips and lays clusters of yellow eggs, its caterpillars feeding openly in gangs. The small white is slightly smaller with less black on the wings and lays single eggs, its green caterpillars hiding in the plant’s heart. Both are commonly called cabbage whites and both attack brassicas.
Does spraying work against cabbage white caterpillars?
Not reliably. General bug sprays struggle to reach caterpillars tucked into a cabbage heart and also kill beneficial insects like ladybirds and hoverflies. The biological control Bacillus thuringiensis is the one genuinely effective treatment beyond hand-picking, though its availability to UK home gardeners varies. Netting and hand-picking are the dependable methods.
When are cabbage white caterpillars worst?
From July through to September, after the butterflies have been on the wing through early summer. Warm weather speeds up their life cycle, so there can be more than one generation in a season. This is why netting needs to go on in May, before the first eggs are laid, rather than after you first see damage.
A Sensible Place to Start
If you grow brassicas, buy the netting before you need it, not after. Put it over the plants in May on hoops that hold it clear of the leaves, tuck the edges down to the soil, and you have solved the problem before it starts. If the caterpillars are already among your cabbages this summer, do not despair and do not bother with sprays: get into a rhythm of checking the leaf undersides twice a week, rubbing off the yellow eggs and picking off the caterpillars, and you will save the crop. It is ten minutes that feels like a chore and pays you back in whole, unlaced cabbages. I learned that the hard way, standing over a bed of skeletons one August, and I have never grown a brassica uncovered since.