This species is a notorious and cosmopolitan pest of vegetable brassicas. Less-significant infestations also occur on brassica seed crops, including oilseed rape. Larvae, when numerous, cause significant leaf damage. Attacked leaves become riddled with holes and the remaining tissue often turns brown; infestations are of particular importance on cabbage and cauliflower, where the larvae often invade the heart leaves. Seed heads of swede and turnip are also liable to be destroyed. In the British Isles, attacks tend to be sporadic and are of greatest significance in hot, dry summers when large numbers of adults invade the country from continental Europe.
Pupae overwinter, and adults emerge in the following year, from May onwards. There are usually two main flight periods, from May to June and from August to September. Adults are most active at night but are readily disturbed during the daytime. Females lay large numbers of eggs, typically in groups of two or three. The eggs hatch about 10 days later. At first, the larvae mine within the leaf tissue but they then feed externally on the underside of the expanded leaves, sheltered by flimsy silken webs. Development, which includes four larval instars, takes approximately 3-4 weeks. Each larva then pupates in an open, net-like cocoon (7-10 mm long) spun on the foodplant. Adults emerge about 2 weeks later. Although in the British Isles there
tend to be two (occasionally three) main generations, the arrival of immigrants from continental Europe (where there may be six or more generations annually) may result in overlapping broods of larvae.
Adult 11-16 mm wingspan; head and thorax ivory-white; forewings light brown to dark brown, with an irregular creamish to ivory-white band on the hind margin (Fig. 274) (forming a characteristic series of three diamond-shaped marks when both forewings are held over the back in repose); hindwings grey, with a long fringe of hairs. Egg 0.5 x 0.25 mm, elliptical and pale lemon-yellow. Larva up to 12 mm long; body pale yellowish-green; head pale brown with darker markings; pinacula large and pale; body hairs black and prominent; prothoracic plate inconspicuous, with scattered blackish markings; anal claspers elongate and clearly visible from above. Pupa 6-8 mm long, at first pale green but soon turning brown.
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