Damage to the outer leaves is also a clear sign of their presence. Beside the holes in the outer leaves, the damage to the cabbage head may be visible in the inner leaves when the heart is cut through. Caterpillars and their excrement are also often found on the plants. All types of Brassica crops are affected, including cabbages, cauliflower, Brussels sprouts, swede and turnips. Also some weeds may be affected.
The butterfly has a black body and gleaming white wings with a conspicuous black tip on the forewing (and two black dots in females). A few weeks after emerging from the pupal stage, females lay greenish-yellow eggs on the underside of leaves. After hatching, the caterpillars start to feed on the plant tissues. The caterpillars bore into the hearts of cabbages.
The pest can be controlled by parasitoids, Cotesia glomerata attacks the larvae of P. brassicae, while pteromalus puparam controls it during the pupal stage. Products based on the naturally occurring bacterium, Bacillus thuringiensis or Saccharopolyspora spinosa (spinosad), are very effective when sprayed thoroughly on upper and lower leaf surfaces. These insecticides do not persist in the environment. A pathogenic nematode, Steinernema carpocapsae, is also available against the caterpillars and must be used when foliage is wet, for example during cool dull weather.
Always consider an integrated approach with preventive measures and biological treatments, if available. Products based on the active ingredient pyrethrum, lambda-cyhalothrin or deltamethrin can be used against the caterpillars. Extracts of pyrethrum can be applied several times and up to one day before harvest. For lambda-cyhalothrin and deltamethrin, a maximum of 2 applications is recommended and a seven-day harvest interval must be respected.