What do the early stages look like?
Eggs are laid in large packed groups. They turn primrose-yellow and are easy to see on the upper (or more often) the underside of leaves. They hatch in about a week.
The first caterpillars are a pale yellow-green with prominent shiny black heads and about 2mm in length. As with most UK butterflies, the larvae have four moults, therefore five stages (instars). They become brighter in colour and larger with each stage, finally reaching a length of about 40mm. They also develop black tubercules sprouting white hairs all over their bodies and these become much larger and more noticeable as the caterpillars develop. Late-stage large white caterpillars have a striking appearance.
The pupa or chrysalis is usually found on a fence, wall or tree, attached by a silk pad at the end of the abdomen and a silk girdle around the waist. It is an attractive green-golden colour with black spots.
What do the caterpillars eat?
As all vegetable-growers know, large white caterpillars are very fond of cultivated varieties of Brassica oleracea, such as cabbage, broccoli, brussel-sprouts etc, which accounts for their other common name (cabbage white) and their reputation as a garden pest. Female adults lay eggs in large batches so these vegetables can be quickly covered in caterpillars. However, they will also feed on a huge variety of wild plants belonging to the Brassicaceae family as well as some others such as nasturtium Tropaeolum majus and wild mignonette Reseda lutea.1.
What happens in winter?
Large whites overwinter as a chrysalis. Although the resting stage for the first brood only lasts a couple of weeks, it persists for months in the second brood, lasting from autumn until spring. The chrysalises are often high up on the ground and attached by a silk pad and silk thread round the thorax to trees, fences or buildings.
Flowers they take nectar from
Adults feed primarily on bluebells,
buddleja, bugle,
common fleabane,
dandelions, devil's-bit scabious, field scabious, hemp-agrimony, knapweeds,
ragged-robin,
ragworts, thistles, wild marjoram and wild teasel.
2.
How are they doing?
Large white butterflies are still extremely abundant and the population seems fairly stable at present with only a slight change in numbers reported between the 2015 State of UK butterflies report (30% decline since 1976)3. and the 2022 version (32% decline since 1976).4. Although still widespread, their distribution has decreased by 18% since 1976. In some years the populations can increase dramatically due to mass migration from mainland Europe.
Other interesting facts
The caterpillars accumulate mustard oil from their foodplants and this is thought to make them unpleasant in taste or even poisonous and give them some protection from predators such as birds. However they are still vulnerable to parasitism by an ichneumon wasp Cotesia glomerata, which deposits its eggs inside young caterpillars. The fly larvae feed on the insides of their host, finally emerging from its skin when they and the caterpillar are fully grown.