Life cycle and management of the onion fly

Onion flies lay their eggs near to or in the leaf sheaths of host plants. Larvae then burrow into the plants and feed on soft tissue, causing seedling death or marketability issues.

• Using implements that damage onion plants can increase the likelihood of infestation

Scientific name: Delia antiqua

Adult flies are pale grey, hairy and 5–7 mm long.

Eggs are elongate (1 mm long) and white. Females may lay 100–500 eggs during their lifetime.

Larvae are white maggots that reach 9–10 mm in length.

Pupae are oval, reddish–brown to dark brown and 6–7 mm long.

Sep–Apr: Overwinter as pupae in the soil.

May–Jun: The first generation of flies emerges in May/June. The first eggs are usually laid in batches towards the end of May in soil adjacent to host plants or in leaf sheaths.

May–Jun: Larvae burrow into the bases of plants, where they feed on soft tissue.

Jun–Jul: Pupation occurs in the soil near to the host plant.

Jul–Aug: The second generation of flies emerges in July/August.

Jul–Aug: Second generation of larvae.

There are usually two generations per year, but, in warm locations, there may be a partial third generation.

Onion fly maggots present in large numbers can lead to patchy crops. Older plants wilt and the foliage may discolour, dry out or start to decompose.

Larger bulb onions withstand attacks, but, eventually, the foliage dies. When bulbs are cut open, larval feeding damage is evident.

Non-chemical control

Onion fly is a highly localised pest and crop rotation may be an effective way of reducing the risk.

Natural controls are likely to include generalist predators, such as certain species of beetle, insect-pathogenic fungi and parasitoids (beetles and wasps).

Monitoring

Use white, blue or yellow water or sticky traps to monitor onion fly adults. Trap captures indicate when onion flies are active and how numbers change during the season.

Thresholds

None established.

Insecticide resistance

None confirmed in UK; however, insecticide resistance is present in North American populations.