Cabbage white caterpillars: how to recognise and treat it

Pest

Symptoms

Holes eaten in the outer leaves of brassicas; caterpillars and their droppings on the plants.
Large white — yellow-and-black caterpillars with obvious hairs, feeding in groups mostly on the outer leaves; clusters of yellow, skittle-shaped eggs.
Small white — pale green, velvety caterpillars with a thin yellow line along the back; single yellow eggs on leaf undersides.
Small white caterpillars bore into cabbage hearts, so damage also shows inside when the head is cut.

Pick off egg clusters and caterpillars as soon as you see them.
Spray Bacillus thuringiensis var. kurstaki (Bt), which works best on young caterpillars; follow the label.
Water on a nematode biological control sold for caterpillars, in cool, damp weather so the leaves stay wet.
Neem (azadirachtin) products, where approved in your country; follow the label.

Spinosad, pyrethrins or lambda-cyhalothrin on young caterpillars; follow the label dose and pre-harvest interval.

Reference information, not a guarantee. Read and follow the label of any product: dose, safety and the days to wait before harvest.

Grow brassicas under fine netting or fleece that does not touch the leaves, so butterflies cannot lay eggs through it.
Check plants regularly from spring; caterpillars appear mainly in June–July and August–September.
Spare the parasitic wasp Cotesia glomerata — its yellow cocoons next to a dead caterpillar are a sign of natural control.
Mix brassicas with unrelated flowering plants, which draws in parasitic and predatory wasps.

Figures checked against these sources.