In common with the Small White butterfly, this is seen by vegetable gardeners as a serious pest, and as the specific epithet implies its caterpillars feed on members of the cabbage family. The first adults emerge in April, and by May they can reach plague proportions on brassica-rich allotments. One or sometimes two further broods follow, so that Large White butterflies can be seen on the wing through to the middle of September.
As the name suggests, the Large White is on average larger that the Small White, but the size ranges do overlap a little. There is a large black half-moon-shaped patch on the edge of the forewing of a Large White, and the black extends further down the trailing edge that along the leading edge; the converse applies in the case of the Small White.
Males have creamy white upperwings with narrow black basal regions and a narrow black area on the lower leading edge of the forewing. They have a black spot on the hindwing but usually no spot (or just a small black smudge) on the forewing.
Females have a large black spot on the forewing plus one on the hindwing, and the black basal area is larger than that of the male. (Females are shown in the picture above and in the one on the left.)
In Britain the Large White is widespread and abundant in England and Wales, becoming more scarce towards the north of Scotland; it is also common and widespread throughout Ireland. Elsewhere this is a very common butterfly throughout mainland Europe, northern Africa and Asia.
The yellow skittle-shaped eggs are laid in groups of typically 20 to 60 on the underside or sometimes the upperside of a brassica leaf. They are particularly fond of cultivated cabbages, broccoli and sprouts. The larvae emerge from the eggs in about ten days, depending on air temperature, and the young hairy caterpillars, which are a yellowish green with yellow lateral lines and black spots, feed communally. Toxins that build up in the caterpillars deter most predators, so that these very visible caterpillars do not need to hide away inside a cabbage: they eat the outer leaves first and, if there are enough of them, they continue munching until nothing but the stalk is left.
When fully grown, the caterpillars wander off in search of a suitable site for pupation. The chrysalises can often be found underneath windowsills or eaves of garden sheds and houses. Chrysalises from the final brood overwinter, and the adults emerge in springtime to continue the process.