Delia platura Meigen A bean seed fly

This fly is an increasingly important pest of French bean and runner bean, and various other vegetable crops, including Brussels sprout, cabbage, cauliflower, lettuce, onion, pea, potato and radish; infestations also occur on sunflower and certain ornamentals. The larvae often hollow out the seeds, which then fail to germinate; they also cause death or severe stunting of seedlings. Damage is particularly severe when germination is slow. In trashy conditions (especially following crops such as mustard, oilseed rape, parsnip and sugar beet), larvae may attack germinating cereal plants. On potato, the larvae will also act as vectors of the bacterial disease known as 'black leg'.


Adults are active from May onwards. Eggs are deposited in the soil, usually in the presence of decaying organic matter. The eggs hatch within a few days, and the larvae then tunnel inside germinating bean seeds, young bean stems and other suitable hosts. They feed for approximately 1-3 weeks and then pupate in the surrounding soil. New adults appear 2-3 weeks later and, after mating, females initiate a further generation; each fly deposits about 50 eggs. There are usually three to five generations each year.


Adult 6 mm long, greyish-brown; wings 5mm long. Egg 1.0 x 0.3 mm, elongate-oval, white. Larva up to 8mm long, white, relatively robust; head only partly retracted into the prothorax; posterior papillae relatively prominent, the very small hind-most (median) pair contiguous and inconspicuous (Fig. 266c) (cf. onion fly, Delia antiqua, p. 197). Puparium 4-5 mm long, oval, reddish-brown.

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