One of the commonest butterflies in Ireland is the Large white butterfly (Pieris brassica) or Bánóg mhór in Irish which is widespread during the months May to September. The females have two black spots on the upper sides of each of their forewings and dark tips on these forewings. (1)
They lay eggs between April and June on cabbage plants or nasturtiums. These bright yellow eggs are in neat clusters on the leaves. (2a) (Some gardeners deliberately sow Nasturtiums among cabbages to divert Cabbage whites from laying their eggs on the cabbages). (2b; 2c)
When these hatch the caterpillars begin devouring the leaves depositing little green balls of frass or excrement as they gorge on them. (3a) They develop warning colours of black and yellow as they do so. (3b) They feed for about a month before turning into chrysalids (pupae).
Some never reach this stage because a parasitoid wasp called Cotesia glomerata lays her eggs in them. These hatch into tiny grubs that eat the host from the inside out. Before the caterpillars pupate, they kill their host and emerge as grubs that spin cocoons around themselves. (4) They emerge as wasps after about a week.
Those Cabbage white caterpillars that are not parasitized make silken mats and then wind single strands of silk around themselves which are attached to these mats. They remain motionless for about twenty-four hours. Meanwhile, their skins split down the middle and the chrysalids form. These chrysalids hang by little hooks off the silken mats on walls, under window ledges and fences. They remain as chrysalids for periods of either two weeks or six to seven months, depending when they pupated. The ones pupating in August and September remain as chrysalids until the following spring.