Delia antiqua

Appearance

The onion fly has an ash-grey body and resembles a housefly. The male has a longitudinal stripe on the abdomen which is lacking in the female. The legs are black, the wings transparent and the compound eyes brown. The eggs are white and elongated and are laid in groups on the shoots, leaves and bulbs of host plants and on the ground nearby. The larvae are white and cylindrical and hatch in from three to eight days. Each batch tends to keep together and collectively create large cavities in bulbs. More than fifty maggots may feed on one bulb, sometimes originating from eggs laid by several different females. The larvae moult three times, feed for about twenty days and reach a length of about one centimetre. The pupa is brown, ringed and ovoid and measures 7 millimetres long. Pupation occurs in the ground with the pupal phase from the spring generation lasting two or three weeks. Late generation pupae overwinter in the soil.

Distribution

The onion fly is found in North America, Western Europe, Russia, Central Asia, China, Japan and Korea but is absent from deserts. In the far north of its range it is univoltine but further south there may be two, three or four generations in one season.

Behavior

The onion fly has an ash-grey body and resembles a housefly. The male has a longitudinal stripe on the abdomen which is lacking in the female. The legs are black, the wings transparent and the compound eyes brown. The eggs are white and elongated and are laid in groups on the shoots, leaves and bulbs of host plants and on the ground nearby. The larvae are white and cylindrical and hatch in from three to eight days. Each batch tends to keep together and collectively create large cavities in bulbs. More than fifty maggots may feed on one bulb, sometimes originating from eggs laid by several different females. The larvae moult three times, feed for about twenty days and reach a length of about one centimetre. The pupa is brown, ringed and ovoid and measures 7 millimetres long. Pupation occurs in the ground with the pupal phase from the spring generation lasting two or three weeks. Late generation pupae overwinter in the soil.