Why Are Caterpillars Destroying My Brassicas?

Broccoli and cauliflower are among the most caterpillar-prone crops in the garden. Large cabbage white caterpillars and the smaller, well-camouflaged cabbage moth larvae can reduce a healthy plant to bare ribs within a few days. If you are seeing ragged holes in leaves, small dark pellets of frass on the leaf surface, or plants that are suddenly skeletal, caterpillars are almost certainly the cause.

Which caterpillars are responsible

The large cabbage white butterfly (Pieris brassicae) lays bright yellow eggs in clusters on the underside of leaves. The resulting caterpillars are pale yellow-green with black spots and eat in groups, stripping leaf after leaf. The small cabbage white (Pieris rapae) lays single eggs and produces solitary, green caterpillars that are very hard to spot. The cabbage moth (Mamestra brassicae) produces brown-green caterpillars that bore into the hearts of plants rather than eating the outer leaves, making them particularly destructive and hard to find until the damage is done.

Finding and removing caterpillars by hand

Check the underside of every leaf, starting with the youngest growth and the plant heart. Look for egg clusters (bright yellow and ribbed) and remove them immediately. Large caterpillars are easy to spot and pick off; small green ones need a careful systematic search. Do this every two to three days during butterfly season — which runs from mid-spring through to early autumn with several generations per year. Drop caterpillars into a bucket of soapy water to kill them.

Netting as a physical barrier

Fine insect mesh (less than 1.3mm mesh size) laid over the plants from transplanting is the most effective control. It physically prevents butterflies and moths from laying eggs. The net must be well-sealed at the edges — even a small gap is enough for a butterfly to get through. Remove the netting briefly when you need to weed, then replace immediately. Once plants are well-established and heading, the risk from caterpillars is lower since damage to outer leaves matters less.

Biological control and sprays

Bacillus thuringiensis (Bt), sold as a biological spray, kills caterpillars within a few days of application without harming other wildlife. It is particularly useful when netting is not practical. Apply it in the evening and reapply after rain. For large infestations, a pyrethrin-based spray provides faster knockdown, though it also kills beneficial insects and should be used as a last resort.