Delia antiqua Meigen Onion fly

The onion fly is a widely distributed and often serious pest of onion; leek and shallot are also attacked. Most important damage occurs in June and July, with seedlings and salad crops most at risk. Infested seedlings soon wilt, and the leaves appear flaccid and may turn brown; such symptoms often occur in patches within crops and may be followed by the complete collapse of plants. On older plants, the larvae (maggots) may burrow within and destroy the inner tissue of the actual bulbs; there may be as many as 30 larvae inside a heavily infested bulb. The oldest leaves protruding from infested bulbs may wilt, turn yellow and then whitish; such leaves are readily detached. In the British Isles, this pest is most important in central and eastern England.

BIOLOGY

Adult flies appear in May. Eggs are deposited on the neck or young leaves of onion plants or placed in the soil immediately adjacent to host plants, each female fly depositing up to 200 eggs in her life-time. The eggs hatch within a few days, and the larvae then attack their hosts. Larvae may move from one bulb or plant to another in order to complete their development, passing through three instars and becoming fully grown in approximately 3 weeks. Pupation occurs in the soil, a few centimetres from the surface, and a second generation of adults emerges 2-3 weeks later. In favourable situations, there may be three generations annually. The pest overwinters in the pupal stage.

DESCRIPTION

Adult 5-7 mm long; mainly grey, with numerous long black hairs; thorax yellowish-grey with four brownish, longitudinal stripes; abdomen grey with a dark stripe down the mid-line; wings yellowish. Egg 1.0-1.3 mm long, white, elongate-oval and ridged longitudinally. Larva up to 10 mm long, dirty white, tapered anteriorly and blunt posteriorly (Plate 9d); posteriorpapil-lae relatively prominent, the small hind-most (median) pair distinctly separated (Fig. 266a) (cf. bean seed fly, Delia platura, p. 199). Puparium 6-7 mm long, oval, dark brown.

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