Cabbage White Caterpillars: How to Stop Them

If your kale or cabbages are suddenly full of holes and you can see green or yellow-and-black grubs on the leaves, you have cabbage white caterpillars. The single most reliable fix is fine insect netting put on from late spring, before the butterflies can reach your plants to lay their eggs. Everything else — hand-picking, encouraging predators, a bacterial spray — is a back-up for when something has already got through. This guide shows you how to recognise the damage, why brassicas get hit so hard, and exactly how to stop it, ranked from most to least effective.

How to recognise them

Two butterflies are responsible, and they cause slightly different damage. Knowing which one you have makes the checking easier.

Large white (the "cabbage white") lays its eggs in neat clusters of 20–100 on the undersides of leaves. The eggs are skittle-shaped and bright yellow, easy to spot once you know to turn the leaves over. The caterpillars that hatch are yellow-green with black blotches and fine hairs, and they feed together in groups, often out in the open on the outer leaves. A cluster can strip a kale leaf to its ribs in a day or two.

Small white lays eggs singly rather than in clusters, so they are much harder to find. Its caterpillars are velvety pale green and well camouflaged against the leaf. Worse, they tend to burrow into the heart of cabbages and the tight centre of a kale plant, where you won't see them and where they leave gritty droppings behind.

That sums up the difference people most often ask about: large white caterpillars are the obvious yellow-and-black ones in gangs on the outside; small white caterpillars are the sneaky green ones in the middle.

The tell-tale signs to look for:

• Holes in the leaves, ragged rather than clean-edged, often starting on outer leaves.
• Yellow egg clusters on leaf undersides (large white) from about late May onwards.
• Caterpillars themselves — turn the leaves over and check the heart.
• Frass — the dark green-brown droppings that collect in leaf joints and the crown. Finding frass but no caterpillar usually means a small white grub is hiding deeper in.

Why brassicas are targeted

Kale, cabbage, broccoli, cauliflower, Brussels sprouts and swede all belong to the same plant family — they are all brassicas. Cabbage white butterflies have evolved specifically to feed on this family, homing in on the mustard-oil compounds that give brassica leaves their distinctive smell. To us that smell is faint; to an egg-laying female it is a beacon that says "lay here, the caterpillars will have plenty to eat".

That's why a single uncovered kale plant in an otherwise mixed bed can still be found and covered in eggs within days of the butterflies being on the wing. It isn't bad luck — the plant is broadcasting exactly the signal the butterfly is looking for. Leafy brassicas like kale and cabbage suffer most because the caterpillars want soft leaf; tougher-leaved or fast-growing crops cope a little better, but nothing in the family is immune.

If you're growing kale for the first time, it's worth reading the full kale growing guide alongside this — getting strong, well-fed plants in the ground early is part of the defence, not a separate job.

How to stop them

Here's what actually works, in order of how much difference it makes.

1. Fine insect netting from late spring (the key method)

This is the one that does almost all the work. Cover your brassicas with fine insect mesh (sometimes sold as "enviromesh" or "butterfly netting") before the butterflies start flying, which in most of the UK means from mid to late May. The mesh physically stops the females landing to lay eggs, so there are simply no caterpillars to deal with.

A few things make netting work rather than fail:

• Use fine mesh, not bird netting. Bird netting has holes a butterfly walks straight through. You want a mesh around 1.3–1.5mm.
• Hold it clear of the leaves. Drape it on hoops, canes with upturned pots, or a low frame so it doesn't touch the foliage — a butterfly will happily lay through netting that's resting on a leaf.
• Seal the edges. Weigh the mesh down with bricks, stones or soil all the way round, or use ground pegs. Any gap and the butterflies find it.
• Lift it to check and weed, then close it up again. You can leave kale netted right through to autumn.

This is the same logic as covering carrots against carrot root fly — a physical barrier beats every spray. If you're planning a bed from scratch, factor netting in from the start; our guide on how to start a vegetable garden covers thinking about pests before you sow rather than after.

2. Weekly egg and caterpillar checks

If you don't net — or something gets in through a gap — the next best thing is simply looking. Once a week from June to August, turn over the leaves and check the heart of each plant. Rub off any yellow egg clusters with your thumb before they hatch; this is the easiest possible intervention because one wipe removes dozens of future caterpillars.

When you find caterpillars, pick them off by hand and squash them or drop them in a tub of soapy water. It feels slow, but on a few plants it genuinely keeps on top of the problem. Pay special attention to the centre of each plant, where small white caterpillars hide — follow the frass to find them.

3. Encourage natural predators and parasitic wasps

A garden with a bit of life in it does some of this work for you. Birds, ground beetles and wasps all take caterpillars, but the most useful ally is a tiny parasitic wasp (Cotesia glomerata), which lays its eggs inside large white caterpillars. You may spot the result: a sluggish caterpillar surrounded by little yellow silk cocoons. Leave those caterpillars alone — they're already finished, and you're growing the next generation of helpers.

You encourage these predators by not spraying broad insecticides (which kill the wasps too) and by growing some flowers nearby — single, open flowers like calendula and poached-egg plant give the adult wasps nectar. This won't replace netting in a bad year, but it tips the balance over time.

4. Bacterial Bt spray — a last resort

If a plant is being overwhelmed and you've left it too late to net, there is an organic-approved spray based on Bacillus thuringiensis (Bt), a naturally occurring bacterium that affects caterpillars but not people, pets, bees or the parasitic wasps. It's sold in the UK under a few brand names for caterpillar control on brassicas.

Treat it as a rescue measure, not routine. You have to spray it onto the leaves the caterpillars are eating, reapply after rain, and time it while they're small. Netting from the start is far less faff and avoids spraying anything at all — which is why Bt sits at the bottom of this list, not the top.

How to prevent it next year

The whole problem is much easier to design out than to fight, so a little planning goes a long way.

Net early, before you think you need to. The most common mistake is waiting until you see the first butterfly — by then eggs are already on the leaves. Get the mesh on as the plants go out in late spring and the question never comes up. Use the planting calendar to line up your sowing and planting-out dates so you know when the vulnerable window starts.

Grow strong plants. A vigorous, well-fed kale plant shrugs off a bit of leaf damage that would set a struggling seedling back badly. Good soil, steady watering and not letting plants get pot-bound before planting all help. The kale guide covers feeding and spacing for sturdy plants.

Be honest about companion planting. You'll read that nasturtiums act as a "sacrificial" trap crop and that strong-smelling herbs confuse the butterflies. There's a grain of truth — nasturtiums do attract large whites and can pull some pressure off — but they don't protect your kale on their own, and a row of herbs won't stop a determined butterfly. Treat companion planting as a small bonus on top of netting, never as a substitute for it. Anyone who tells you marigolds alone will keep your cabbages clean is overselling it.

Tidy up in autumn. Clear away the old brassica stumps and any chrysalises tucked under leaves and pots once cropping finishes, so fewer butterflies overwinter near your bed.

Get the netting on early and the rest of this barely matters. Cabbage white caterpillars are one of the few brassica problems you can almost entirely prevent — and once you've netted one year, you'll do it automatically every year after.