The Large White is common throughout Europe, North Africa and Asia to the Himalayas often in agricultural areas, meadows and parkland. It is a strong flier and the British population is reinforced in most years by migrations from the continent. Scattered reports of the Large White from the northeastern United States (New York, Rhode Island and Maine) over the past century are of a dubious nature and indicate either accidental transport or intentional release. Such introductions threaten to establish this agricultural pest in North America.
The wings are white, with black tips on the forewings of both males and females, the female also has two black spots on each forewing. The underside is a pale greenish and serves as excellent camouflage when at rest. The black markings are generally darker in the summer brood. Its wingspan is 5 to 6.5 cm.
The female lays batches of 20 to 100 yellow eggs on plants in the cabbage family and can be a pest on food crops. They seem to have a preference for cultivated varieties of Brassica oleracea such as cabbage and brussels sprouts. The caterpillars are yellowish green with yellow lines and black spots and feed in groups in plain view on the leaf surface. They gain protection from predators by possessing distasteful chemicals. It has once been reported that these chemicals were mustard oils from glucosinolates in the foodplants, but this appears to be a myth since Pieris caterpillars actually suppress the formation of mustard oils (see glucosinolate). Despite their chemical defence, large numbers are sometimes lost to the parasitic wasps such as Apanteles glomeratus, whose females attack clusters of newly hatched caterpillars. The chrysalis is also yellowish green with black spots. The species over-winters as a chrysalis. It has two broods in a year, the first is on the wing in May and June and the second in August.
Female upperside similar to that of the male but the irroration of black scales at the bases of the wings more extended; the black area on apex and termen of fore wing broader, its inner margin less evenly curved ; a conspicuous large, black spot in outer half of interspace 1, and another near base of interspace 3. On the hind wing the subcostal black spot before apex much larger, more prominent. Underside: similar to that of the cf but the apex of the fore wing and the whole surface of the hind wing light ochraceous yellow, not ochraceous brown ; the black discal spots on fore wing much larger. Antennae, head, thorax and abdomen as in the male.