Larvae of this generally common pest often cause damage to broccoli, Brussels sprout, cabbage, cauliflower, horseradish and other vegetable brassica crops. They devour the leaves and also contaminate plants with frass. During the daytime, the cryptically coloured larvae often lie stretched out along the veins of the leaves and, although often on the upper surface, are easily overlooked.
Adults of the spring generation occur mainly in March and April, and are very active in sunny weather. They deposit their eggs singly on both sides of the leaves of host plants. Larvae of the first generation eventually pupate to produce a second generation of adults in mid-summer. Their larvae, which feed in late summer and early autumn, are usually more numerous than those of the earlier generation, especially if a summer immigration of adults has occurred.
Adult 40-45 mm wingspan; forewings of male mainly white with tip and base more or less black, and a subcentral black spot (the latter often absent in spring specimens); forewings of female similar to those of male but with two subcentral spots and a blackish dash on the hind margin; hindwings mainly white above, with a blackish mark on the anterior margin; mainly dusky yellow below. Egg 1.3 mm long, yellow, spindle-shaped and ribbed longitudinally. Larva up to 30 mm long; body velvet-green, finely speckled with black, with a yellow line along the back (absent in early instars) and yellow markings along each side (Plate 14c). Pupa 20 mm long, pale brown to grey or pale greenish, with black markings.
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