🐛✗ Tested and false“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.
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- 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.
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- 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.
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“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.
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“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.
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