How Do I Stop Cabbage White Butterflies on Sprouts?

The cabbage white butterfly — both the large white (Pieris brassicae) and the small white (Pieris rapae) — is one of the most persistent threats to Brussels sprouts in temperate gardens. Adults emerge in spring and continue laying eggs through summer into early autumn, meaning unprotected plants face wave after wave of hungry caterpillars throughout the growing season.

Understanding the Threat

The large white butterfly lays eggs in batches of 20 to 100 on the undersides of leaves. The small white lays single eggs, making them harder to spot. Both species can complete two or three generations per year in warm summers, so populations build up quickly if left unchecked. Caterpillars from the large white feed in groups and strip leaves fast; small white caterpillars are solitary and often bore into the centre of forming sprouts where they are hidden until the damage is severe.

Insect-Proof Netting: The Best Defence

Fine mesh netting with a maximum aperture of 0.8 mm is the most reliable way to stop cabbage whites. The netting must be supported on hoops so it sits clear of the leaves — butterflies will lay eggs through mesh that rests directly on plant surfaces. Fix the edges firmly to the ground with pegs or bury them in soil to stop butterflies finding gaps. Put netting in place at transplant time and keep it on until you harvest. Check every week for gaps or sagging caused by plant growth.

Removing Eggs by Hand

If you prefer not to use netting, inspect the undersides of leaves every two to three days throughout spring and summer. Large white eggs are bright yellow and easy to spot in their clusters. Small white eggs are white and torpedo-shaped, laid singly. Wipe off or crush any egg clusters you find. This is time-consuming but satisfying and removes the problem at source before any damage occurs. Even with netting in place, check for eggs in case butterflies have got in.

Companion Planting as a Deterrent

Interplanting Brussels sprouts with strong-scented herbs such as sage, rosemary, and thyme is widely practised as a deterrent. The aromatic compounds are thought to confuse butterflies seeking brassicas. Nasturtiums planted as a trap crop can draw egg-laying butterflies away from the sprouts. These methods reduce rather than eliminate the problem and work best in combination with regular inspection.

Biological Controls and Sprays

Bacillus thuringiensis var. kurstaki (Btk) is an organic spray that kills caterpillars but is safe for other wildlife. Spray it on the leaves in the evening and reapply after rain. It works best on small, newly hatched caterpillars. Neem oil disrupts the feeding and development of young caterpillars and can be used as a preventative spray. Avoid spraying broad-spectrum insecticides during flowering of companion plants as these will also kill beneficial pollinating insects.

Encouraging Natural Predators

Parasitic wasps such as Cotesia glomerata lay their eggs inside large white caterpillars, dramatically reducing populations. These wasps are present in most gardens and are favoured by a diversity of flowering plants providing nectar. Birds, particularly blue tits and great tits, take both caterpillars and chrysalises. A garden that supports this predator community provides a constant background check on cabbage white populations throughout the season.