How do I stop caterpillars eating my cabbages?

Why netting is the only reliable answer, the mesh size that actually excludes the butterflies, and what to do when they get in anyway.

Difficulty: beginner
Time: 1 hr
Read: 3 min

Short answer

Net the crop before the butterflies arrive, not after. Use insect mesh with holes no larger than about 7mm, held clear of the leaves on hoops so the butterflies cannot lay through it, and tucked in all round. Everything else is damage limitation.

Two butterflies and a moth do the damage: large white, small white and cabbage moth. All lay on brassicas from May onwards and the caterpillars can reduce a plant to ribs and stalks in under a week.

What you'll need

Insect mesh or fine netting
Hoops or a frame
Pegs or bricks
Bacillus thuringiensis spray (optional)

Step by step

Net before they arrive.From late April or early May, before the first butterflies are flying. Netting a crop that already has eggs on it simply protects the caterpillars.
Choose the right mesh.Around 7mm mesh excludes the butterflies. Standard bird netting has holes far too large and does nothing against them.
Hold the netting off the leaves.Butterflies land on netting and lay through it onto any leaf touching the mesh. Hoops or a frame that keeps a clear gap is essential.
Seal the edges.Peg, weight or bury the edges all the way round. A butterfly that gets under the net lays undisturbed and protected.
Check leaf undersides weekly.Clusters of yellow eggs from the large white, single eggs from the small white. Rubbing them off takes seconds and prevents the entire problem.
Pick caterpillars off by hand.On a small plot this is genuinely effective. Look along the leaf ribs and in the growing point where they hide.
Use a bacterial spray if numbers are high.Bacillus thuringiensis is specific to caterpillars and works best on small ones. Spray leaf undersides thoroughly in the evening and repeat after rain.
Grow a sacrificial patch of nasturtiums.Large whites lay readily on nasturtium. A patch away from the brassicas absorbs some of the pressure and makes the eggs easy to find and remove.
Clear and dig over in autumn.Pupae overwinter on stems, fences and sheds nearby. Clearing old brassica stumps reduces next year's local population.
Accept losses on outer leaves.Chewed outer leaves on a cabbage or a sprout plant matter far less than a stripped growing point. Concentrate effort on the heart of the plant.

Tips

Bird netting does not stop butterflies. You need about 7mm insect mesh.
Netting touching the leaves is netting they can lay through.
One weekly check of leaf undersides in June prevents most of the damage.

Common mistakes

Draping netting straight over the plants — Butterflies land on the net and lay through it onto leaves pressed against the mesh. The net must be held clear on hoops or a frame.
Netting in July after damage appears — The eggs and caterpillars are already inside. Check and clear the plants first, then net, or you have simply built them a predator-free enclosure.

If it doesn't work

Caterpillars under the netting

Cause: Eggs laid before netting, or gaps at the edges — Fix: Clear the plants by hand, seal every edge, and check the mesh is not touching leaves.

Plants reduced to ribs in days

Cause: Large white caterpillars feeding in a group — Fix: Hand pick, spray Bt on any remaining small caterpillars, feed and water — brassicas often recover.

Small holes but no caterpillars visible

Cause: Flea beetle or slugs, not caterpillars — Fix: Different problem and different treatment. Check at dusk for slugs and look for tiny jumping beetles on seedlings.

Damage inside a cabbage heart

Cause: Cabbage moth caterpillar, which burrows in — Fix: Harder to control. Netting from May and prompt removal of eggs is the only real answer.

Questions people ask

What mesh size do I need?

About 7mm or finer. Standard bird netting has much larger holes and will not exclude cabbage white butterflies at all.

Does companion planting keep them off?

Not reliably. Nasturtiums are useful as a sacrificial crop that concentrates egg laying somewhere you can see it, but no plant repels the butterflies from your brassicas.

Will my cabbages recover from being stripped?

Often, if the growing point survived. Remove every caterpillar, feed with a nitrogen feed and water. Plants that lost the growing point will not.