Cabbage White Caterpillars is a common pest that affects kale, broccoli and cabbage in Scotland. Here is what it looks like, how to spot the damage, and which controls have been tested.
What does it look like?
Large white caterpillars are yellow and black with obvious hairs; small white caterpillars are pale green and covered in short, velvet-like hairs.
The colour difference is the practical field distinction between the two commonest species.
🌱 Plants at risk
Cabbage White Caterpillars also affects these crops:
Symptoms
RHS: 'Holes are eaten in the outer leaves of all brassicasThe cabbage family: cabbage, kale, broccoli, cauliflower, Brussels sprouts, kohlrabi, swede, turnip and more, sharing the same pests, diseases and rotation rules. Full definition → and damage may also be seen on the inner leaves of cabbages when the heart is cut through.'
Life cycle
Large and small white butterflies produce two generations over the summer; cabbage moth runs two or three overlapping generations, and the diamond-back moth can manage several more in a good year. Caterpillars are typically seen June-July and again August-September for the white butterflies, and July-September for cabbage moth, with activity overall running May through October. Cabbage moth pupates and overwinters in the soil; cabbage white butterfly caterpillars instead pupate above ground, on any suitable vertical surface.
Controls: what actually works
Not every method here works equally well: some are backed by real evidence, others are simply traditional and untested.
Physical barrier: grow brassicas under fine netting or horticultural fleeceA lightweight, breathable fabric laid or draped over plants to trap a little warmth and keep off frost, wind and some flying pests. Full definition → to exclude adult butterflies
RHS: 'Growing brassicas under fine netting or horticultural fleece can exclude adult butterflies.'
Same logic as carrot fly: exclude the adult and there are no larvae. Barriers are the one control that works on both of Scotland's main brassica insect problems.
Tolerate populations
Tolerate them where you can: they feed parasitoid wasps, social wasps and birds. Accepting some damage lets natural predation do the population control for you.
Inspect regularly and hand-pick eggs and caterpillars
Inspect regularly to catch infestations early, and hand-pick eggs and caterpillars.
Checking leaf undersides for egg clusters before they hatch is the high-leverage version: one wipe removes what would be dozens of caterpillars.
Biological control: pathogenic nematodes
Pathogenic nematodes are among the recommended controls.
Proprietary repellent product (not named, see policy)
One proprietary repellent product is also recommended. We do not name it. The option is described here so you know it exists, without us endorsing or linking a specific brand.
Natural predators of cabbage white caterpillars
These 3 creatures eat cabbage white caterpillars, and each has its own page here on how to get more of them in your garden: what to plant, or what to order and when.
Myths about cabbage white caterpillars
5 common claims about cabbage white caterpillars, checked: 3 busted1 disputed0 untested1 hold up
“Any garden netting will keep cabbage white butterflies off brassicas”
Mesh size and installation both matter. RHS specifies fine nets of 4-7mm aperture for excluding white butterflies, and adds a condition gardeners often miss: the netting has to be held clear of the crop, because a female butterfly can still lay eggs through the mesh wherever a leaf is pressed against it. A loose length of coarse mesh draped straight over the plants, or any net touching foliage, leaves an opening the claim assumes doesn't exist.
What to do: Use netting rated at 4-7mm or finer, and support it on hoops so it stays off the leaves as the plants grow.
In Scotland: Fine mesh around 0.8mm also keeps out aphids and flea beetle, both regular problems on Scottish brassica beds, so it earns its place even where butterfly pressure alone might not justify it.
1 source
- RHS: Insect-proof meshFine nets with a 4-7mm mesh give good protection against white butterflies as long as the foliage does not touch the net.
“All cabbage white caterpillars are the same pest, so it doesn't matter which one you're looking at”
The two commonest species look different and cause different damage. Large white caterpillars are yellow and black with obvious hairs and feed mostly on outer leaves, leaving visible holes but sparing the heart. Small white caterpillars are pale green with fine velvety hairs, far easier to miss on a leaf, and bore into the centre of the plant, so a cabbage that looks fine from outside can already be damaged inside.
What to do: Check leaf undersides for both caterpillar types, and if you find the plain green small white caterpillar, split the heart open rather than assuming outer-leaf damage is the full extent.
1 source
- RHS: Cabbage caterpillarsLarge white caterpillars are yellow and black with obvious hairs and stay mostly on outer leaves, while small white caterpillars are pale green with short velvety hairs and can be more damaging as they bore into the hearts of cabbages.
“Planting strong-smelling herbs like sage, thyme or mint next to brassicas stops cabbage white butterflies laying eggs there”
A field trial that interplanted collards with a mixture of companion herbs found the opposite of the intended effect: egg numbers went up, not down, and the eggs were laid in denser clusters rather than being spread out or avoided. Sage performed worst of all, drawing significantly more Pieris rapae eggs onto the nearby collards than plots without it. The mechanism gardeners rely on, that a strong scent confuses or masks the crop from the butterfly, was not what the caterpillars' actual egg-laying behaviour showed.
Our read This is one study on one herb mixture with collards, not a survey of every herb combination, so it doesn't prove every aromatic plant fails. What it does show is that the effect can run backwards, which is reason enough not to rely on herb borders as your only protection.
What to do: Treat aromatic herb borders as a nice-to-have for pollinators, not pest control, and use netting as the actual barrier against egg-laying.
1 source
- The Canadian Entomologist: The influence of companion herbs on egg distribution of the imported cabbageworm, Pieris rapae, on collard plantsCompanion herbs did not repel P. rapae but instead significantly encouraged colonisation of the insect and produced an aggregated egg distribution, with sage drawing the most eggs of the herbs tested.
“Growing nasturtiums near brassicas works as a trap crop, drawing cabbage white caterpillars away from the vegetables”
The chemistry behind the claim is real but cuts both ways. Nasturtium is an acceptable host plant for Pieris rapae: adults will lay eggs on it and caterpillars that hatch there feed and develop normally, which is the basis for using it as a decoy. But caterpillars already feeding on cabbage refuse to switch onto nasturtium and will starve rather than eat it, because the leaves contain a strong antifeedant compound their gut isn't primed for. So nasturtium can pull in egg-laying adults, but it will not rescue a cabbage bed that already has caterpillars on it, and there's no field trial confirming that a nasturtium border actually reduces damage on the brassicas planted next to it rather than just adding a second host plant to the garden.
What to do: Plant nasturtiums if you like them, but don't rely on them instead of netting, and don't expect them to draw off caterpillars that are already established on your brassicas.
2 sources
- Journal of Chemical Ecology: Rejection of host plant by larvae of cabbage butterfly (Renwick et al.)Garden nasturtium is an acceptable host plant on which Pieris rapae eggs are readily laid and hatching larvae develop normally, but larvae transferred from cabbage to nasturtium refuse to feed and starve, due to a feeding deterrent identified as chlorogenic acid.
- RHS: Cabbage caterpillarsNasturtiums are listed alongside edible and ornamental brassicas as a plant cabbage caterpillars feed on.
“Bacillus thuringiensis (Bt) spray is safe to use around bees and other beneficial insects”
University extension guidance recommends the caterpillar-specific strain of Bt precisely because it spares natural enemy populations while still being very effective against cabbage caterpillars, especially when applied to young larvae. It has to be eaten to work, and it only affects caterpillar-stage insects, which is why it doesn't harm bees, birds or predatory insects that aren't chewing on treated leaves.
Our read 'Beneficial insects' isn't the same as 'every other caterpillar.' Bt sprayed on brassica leaves will also kill non-pest moth and butterfly larvae that happen to feed on the same plant, so it isn't as narrowly targeted as 'safe for beneficial insects' can make it sound.
What to do: Spray it directly on the foliage when caterpillars are small, reapply after a week or two since it breaks down in sunlight and rain, and avoid blanket-spraying plants that aren't actually infested.
1 source
- UC IPM (University of California Statewide Integrated Pest Management Program): Cabbage looper, cole crops pest management guidelinesBacillus thuringiensis is very effective against cabbage looper and imported cabbageworm when applied to young caterpillars, and using the caterpillar-specific strain avoids damaging natural enemy populations.
Frequently asked questions
What should I do first if I find cabbage white caterpillars in my garden?
Start with physical barrier. After that, tolerate populations. Work through the rest in order of how well-evidenced they are if the first two aren’t enough on their own.
Will cabbage white caterpillars kill my plants?
They rarely kill established plants. The damage is usually cosmetic on outer leaves, and natural predators often bring numbers under control without you doing anything.
Should I just leave cabbage white caterpillars alone?
In most gardens, yes. cabbage white caterpillars are food for birds, wasps and predatory insects, and a healthy garden carries some without trouble. Tolerance is also RHS's official position now, for that same reason.
Sources
1 source, recorded with what each was used for
- RHS1: biodiversity framing and tolerance advice, five species, large/small white caterpillar identification, damage description, netting/fleece, hand-picking, pathogenic nematodes, proprietary repellent