Dasineura brassicae Winnertz Brassica pod midge

Infestations of this potentially major pest occur on oilseed rape and other brassica seed crops but not on white mustard. The larvae feed within the developing pods and cause premature ripening and splitting, which may result in significant seed loss. Infested pods often swell (the so-called 'bladder pod' symptom) but this symptom is not expressed on all hosts. Midge damage is often concentrated on headlands and decreases markedly further into the crop.

BIOLOGY

Adult midges emerge in May. They are weak fliers, and the egg-laying females migrate downwind to seek host plants, usually within a few hundred metres of emergence sites. Eggs (up to

30 per female) are laid in batches in the pods of brassica plants in late May or early June. The adult females are unable to penetrate an unblemished pod wall and usually deposit their eggs through feeding or egg-laying punctures previously made by adults of the cabbage seed weevil, Ceutorhynchus assimilis (p. 154). Sites of other mechanical injury are also utilized; later in the season, for example, second-generation midges may deposit eggs through holes (exit holes) formed in pod walls by fully fed seed weevil larvae. Midge eggs hatch within a few days. The larvae then feed gregariously on the pod wall (Plate 6c), without damaging the central septum, causing premature ripening and splitting. (In prematurely split, bird-damaged pods, the septum is usually broken.) When fully grown, the larvae escape and fall to the ground. They then enter the soil and pupate, each in a small silken cocoon. A second generation of adults appears about 2 weeks later. Some larvae, however, do not pupate but remain in a state of diapause until the following or subsequent seasons. There are usually up to three or more generations per year, depending upon temperature, although any one crop is likely to sustain only two generations in a season.

DESCRIPTION

Adult 2 mm long, greyish-brown, with a pinkish (male) or red (female) abdomen. Egg minute, sausage-shaped and translucent. Larva up to 2 mm long, whitish; the younger stages are translucent.

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