Biology and management of the large white butterfly on brassicas

The large white butterfly is one of two species commonly known as ‘cabbage whites’. Feeding larvae often leave behind skeletonized leaves. This species lays its eggs in large batches, so damage does not tend to be widespread, but some plants will be severely affected.

• The larvae feed within plant foliage and infestations may not be detected until damage has occurred
• Eggs are usually laid in large numbers on only a few plants, so these can be severely damaged while other plants escape
• This species is not as problematic as the small white butterfly

Scientific name: Pieris brassicae

Adult butterflies have a wingspan of 60–70 mm. The tips of the forewings are black and the female has large black spots on the upper surface of each forewing.

Eggs are yellow and flask-shaped (1.5 mm high and 0.6 mm at their base).

Young larvae are initially pale green, turning mottled blue-green.

Larvae are 25–40 mm long when mature, with three yellow longitudinal stripes along the body. Larvae are covered with black markings and groups of short, stiff white hairs that arise from fleshy protuberances along the body.

Pupae are grey-green and attached to a vertical or overhanging surface by a silken girdle.

Nov–Apr: Pupae overwinter away from the host plant.

May–Jun: Adults (first generation) emerge from pupae and lay eggs in batches of 20–100 on the undersides of leaves.

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

Aug: Adults (second generation) emerge from pupae and lay eggs.

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

The second generation is the most damaging to brassica crops.

Non-chemical control

Several polyphagous predators attack this pest, including some birds and large beetles.

Natural mortality of larvae can be high thanks to a species of parasitic wasp (Cotesia glomerata) and because of infection by a baculovirus. The small, bright yellow cocoons of the parasitic wasps can often be seen clustered alongside dead or dying larvae from which they have emerged. 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). Control by inundative releases of egg parasitoids (Trichogramma spp.) and the use of fungal pathogens has not been evaluated in the UK.

Eggs are laid in batches and the larvae feed gregariously, so it is relatively easy to spot plants damaged by large white butterfly. The pest can be controlled by either hand-picking them or avoiding the affected plants at harvest.

Monitoring

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

Thresholds

None established.

Insecticide resistance

None known.