Garden pest control - Large Cabbage White Caterpillar (Description and control instructions)

Scientific name: Pieris brassicae

The Large Cabbage White Butterfly is less widely distributed in the world than the Small. The caterpillars will be found on cruciferous crops, eg. cabbage, swede, kale, throughout the summer. The adult, nearly all-white butterfly emerges from the pupa (chrysalis) in April and May. The adult male has black markings on the tips of its wings; the female has the same marks with the addition of two black spots in the middle of each wing. After mating the female lays 20 to 100 yellow eggs on the underside of cruciferous plant leaves.

After about 14 days the larvae (caterpillars) emerge then moult as they grow. They have well developed mandibles which they use with devastating effect on crops. This first generation pupates in June in a crevice or woody stem, emerging in July as the second generation which usually causes greater damage to crops in the garden, repeats the cycle and pupates to overwinter for the next year.

Apart from brassica crops they will use wallflowers and other cruciferous plants (four-petalled flowers), as alternate hosts. If the damage is extensive and the leaves look torn, the culprits may be pigeons. Slugs will also strip the leaves, but there will be a slime trail.

Treatments

• Inspect the leaves of cabbage and turnips in July when adults are seen, for eggs and small caterpillars - crush by rubbing with a finger. They are usually around the outer edges where the female probably has a better grip while she places them in clumps.
  
  
• Covering the target plants with horticultural fleece or fine netting should prevent the butterflies laying their eggs in the first place. Netting should be held away from the leaves and not draped over them otherwise the butterfly may reach them.
  
  
• Apantales glomeratus, a small wasp, is a biological control which lays its eggs in the caterpillar. These develop inside devouring the caterpillar - more suited to commercial growing.
  
  
• Another such control is a virus which attacks the caterpillars, turning them grey and causing death.
  
  
• Starlings are another predator. They are much maligned creatures accused of being filthy, due mainly to their invasion of city buildings to roost, but to agriculture and horticulture they are extremely useful. Their numbers in the countryside have been decreasing recently. The aerial display at dusk as they fly around in large flocks, is breathtaking.
  
  
• The caterpillars can be sprayed with Azinphos-methyl or derris with a wetting agent to keep the droplets on the waxy brassica leaves.