Caterpillars Are Eating Holes Through All My Cabbage Leaves

Finding large, ragged holes through cabbage leaves — sometimes with the leaf skeletonised and just the veins remaining — is caterpillar damage. The large cabbage white butterfly (Pieris brassicae) and small cabbage white butterfly (Pieris rapae) are the primary culprits in UK gardens. A third species, the cabbage moth (Mamestra brassicae), is less visible as its caterpillars feed inside the head at night. Between them, these three species can devastate an unprotected brassica patch from late spring through to October.

Identifying large and small cabbage white caterpillars

Large cabbage white caterpillars (Pieris brassicae) are bright yellow with black spots and black hairs, 40–45 mm when fully grown. They feed in groups and can rapidly skeletonise a leaf, leaving only the veins. The eggs are yellow, oval, and laid in large clusters of 30–100 on the leaf underside. Small cabbage white caterpillars (Pieris rapae) are pale green, unmarked, much harder to see, and feed individually rather than in groups. They can bore into the head and contaminate it. Check both the leaf underside (for eggs and young caterpillars) and the head interior (for later-stage small whites).

Hand picking

For small plantings, hand picking is highly effective. Check plants at least twice weekly from May to September. Look on the leaf underside for egg clusters (remove and crush) and for caterpillars at all sizes. Early removal of eggs prevents the damage entirely. A second-year gardener who checks weekly will rarely see serious caterpillar damage; one who only checks monthly may find entire plants stripped.

Netting — the definitive solution

Fine mesh netting (1.25 mm or finer) installed over a frame and secured at the edges before butterflies begin laying prevents the problem entirely. The main UK cabbage white season is late April through September, with peak laying in June and August. Install netting before May and keep it in place until October. Check for any gaps where butterflies could enter — a single large white female can lay 300 eggs per season. Netting is particularly important in urban gardens with populations of both Pieris species.

Bacillus thuringiensis (Bt)

Where caterpillars are present and netting has not been used, Bacillus thuringiensis var. kurstaki (sold as DiPel or Xentari) is an effective organic biological insecticide that kills caterpillars without harming other insects or mammals. Apply as a foliar spray when caterpillars are small for best results. Available from specialist horticultural suppliers.