Why Are Caterpillars Eating My Brussels Sprouts?

Finding caterpillars chomping through your Brussels sprout leaves is a frustrating but very common experience. Two species are responsible for nearly all the damage in most gardens: the large white butterfly (Pieris brassicae) and the cabbage moth (Mamestra brassicae). Understanding the difference between them helps you target the right control at the right time.

Large White Butterfly Caterpillars

The large white butterfly lays bright yellow eggs in tight clusters, usually on the underside of outer leaves. These hatch into pale yellow and black caterpillars that initially feed gregariously, covering the leaf surface in a wriggling mass before dispersing across the plant as they grow larger. They produce a strong smell and leave dark frass (droppings) on leaves. A single egg batch can produce enough caterpillars to defoliate a plant within two weeks.

Cabbage Moth Caterpillars

The cabbage moth is less visible because it lays eggs singly and the resulting caterpillars are greenish-brown, blending well with plant tissue. They tend to bore into the heart of the plant or directly into forming sprouts rather than feeding on outer leaves. You may notice entry holes in sprouts and a foul smell before you see the caterpillar itself. This species has two or three generations per year and remains active later into autumn than the cabbage white.

Checking for Eggs Before Damage Starts

The most efficient approach is to inspect the undersides of leaves every few days during the butterfly flight season, which runs roughly from late spring through early autumn. Remove egg clusters by hand and squash them or drop them into a bucket of soapy water. This single habit reduces caterpillar numbers dramatically without any chemical input. Bright yellow cabbage white eggs are easy to spot; cabbage moth eggs are smaller, whitish-green, and harder to find.

Picking Caterpillars by Hand

Hand-picking remains one of the most effective controls for small gardens. Go over plants carefully twice a week, checking the undersides of leaves, around the growing tip, and inside any forming sprouts. Drop caterpillars into a container of soapy water. Wearing gloves makes the job more comfortable. Even if you miss some, regular disturbance reduces the population significantly over the course of a season.

Netting as a Physical Barrier

Fine insect-proof mesh with an aperture of no more than 0.8 mm, supported on hoops well clear of the leaves, is the most reliable long-term solution. It prevents butterflies and moths from reaching the plants to lay eggs at all. Put the net in place at transplanting time and secure the edges firmly. Check regularly that it has not sagged onto leaves, which would allow butterflies to lay eggs through the mesh.

Biological and Spray Controls

Bacillus thuringiensis (Bt) is a naturally occurring soil bacterium sold as a concentrate. It is harmless to most other creatures but is lethal to caterpillars when they eat sprayed foliage. Apply it in the evening when caterpillars are most active and reapply after rain. Pyrethrin-based organic sprays are a faster-acting alternative but will also affect beneficial insects, so use them only as a last resort when populations are high.