Phaedon cochleariae F Watercress beetle

This beetle was formerly a common, widely distributed pest of brassica crops (Brassicaceae), including cabbage, cauliflower, mustard and watercress. Attacks on vegetable crops are generally of no importance, although the adults and larvae may be a problem in watercress beds, particularly during the late spring and summer. Most significant damage occurs on brassica seed crops, especially white mustard. Adults and larvae feed on the leaves of host plants and sometimes cause extensive defoliation; they also strip away the outer tissue of the stems and this has a deleterious effect on flowerhead development. Feeding also occurs directly on the buds and young pods, the pest being capable of causing considerable loss of yield. Nowadays, however, attacks tend to be sporadic and this insect is of only minor importance.

BIOLOGY

Adults emerge from hibernation in April and May. They then attack the foliage of host plants. Eggs are eventually deposited in small cavities bitten into the leaf surface (Plate 4d). The eggs hatch about 2 weeks later. Larvae pass through three instars and are fully fed in about 3 weeks. They then drop to the ground to pupate a few centimetres below the surface. New adults emerge about 12 days later, usually in June and July. These adults give rise to a further generation of larvae that produce adults in August and September. In favourable situations, when the rate of development is enhanced, there may be three generations in a year.

DESCRIPTION

Adult 2.5-3.5 mm long, bright metallic-blue, oval and somewhat bulbous; thoracic disc distinctly punctured; elytra each with eight fine longitudinal lines of punctures (Fig. 219). Egg 1mm long, oval, yellow. Larva up to 6 mm long; body brownish-yellow to dark grey; pinacula black and prominent; head black; thoracic legs black, marked with yellow (Plate 4e).

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