Identification and control of the small white butterfly on vegetable brassicas

The small white butterfly is one of two species commonly known as ‘cabbage whites’. There are two generations a year, and it is the second generation that is most damaging to brassica crops.

• Larvae are well-hidden in foliage and infestations may not be detected until after damage has occurred
• The second generation usually causes more damage than the first

Scientific name: Pieris rapae

Adults are white butterflies with a wingspan of 50 mm. Forewings have conspicuous black tips. The female has two small black spots on the upper surface of each forewing, while the male has one black spot in the middle of each forewing.

Eggs are yellow and bottle-shaped.

Larvae are solitary and have a green velvety appearance, with a dorsal yellow line and elongate yellow patches along their sides. Larvae are 25 mm in length when mature.

When fully grown, the larva usually pupates on the plant, to which it is attached by a silken girdle.

Nov–Apr: Pupae overwinter.

May–Jun: Adults (first generation) emerge and eggs are laid singly on the undersides of leaves of host plants.

May–Jul: Larvae feed/pupae formed (first generation).

Aug–Sep: Adults (second generation) emerge and lay eggs.

Aug–Oct: Larvae feed/pupae formed (second generation).

Non-chemical control

Several polyphagous predators attack this pest, including some birds and small beetles. Natural mortality of larvae can be high thanks to parasitic wasp species. The larvae continue to feed for some time after they have been parasitised, so crop damage is not reduced immediately.

The larvae can be controlled with products based on Bacillus thuringiensis (Bt). The possibility of control with other biopesticides is under evaluation in the UK.

Monitoring

Capture adult butterflies in yellow water traps or on yellow sticky traps.

Thresholds

None established in the UK.

Insecticide resistance

None known.