Why Are Caterpillars Destroying My Kale?

Kale is one of the favourite foods of cabbage white butterfly caterpillars — and a single plant colonised by a cluster of large white caterpillars can be stripped bare in three or four days. This is one of the most dramatic and visible pest problems in the vegetable garden, and it can happen surprisingly fast in summer and early autumn when butterfly populations peak. Acting as soon as you see the first signs is crucial.

The two main caterpillar pests

The large white butterfly (Pieris brassicae) lays batches of 20–100 bright yellow eggs on the underside of kale leaves. The hatching caterpillars are yellow and black, highly visible, and feed in groups. They are voracious — a large cluster can defoliate a kale plant completely within a week. The small white butterfly (Pieris rapae) lays single pale eggs and its caterpillars are a pale, well-camouflaged green. Small white caterpillars are harder to spot but equally damaging individually; they bore into the heart of the plant as they mature. Both species have two to three generations per year from spring to autumn.

The most reliable control: exclusion netting

Fine insect mesh (enviromesh or butterfly net) draped over kale and secured at the edges so butterflies cannot get underneath is the single most effective control. Applied before the butterflies begin laying — from late spring — it prevents egg laying entirely. If you use netting consistently, you can grow kale with almost no caterpillar damage. The main challenge is that kale grows tall and the mesh needs periodic adjustment. Some growers use hoops to tent the mesh above the plants rather than laying it flat.

Hand-picking as daily management

Without netting, check the underside of every leaf twice a week and remove egg batches (scrape them off with a fingernail) and caterpillars (pick off into soapy water or crush). The large white's conspicuous yellow eggs are easy to find — a few minutes checking regularly makes a significant difference. This approach works well for small numbers of plants but becomes impractical if the garden is large or populations are very high.

Biological control

Bacillus thuringiensis (Bt) is a naturally occurring soil bacterium whose spores are toxic to caterpillars but safe for people, pets and beneficial insects. Available as a spray, it is effective against small caterpillars shortly after hatching and is approved for organic use. It must be applied to the leaves the caterpillars are eating and reapplied after rain. It is most effective as an early intervention rather than a last resort when caterpillars are already large and the damage is severe.