Onion fly

The maggots of onion fly can damage the roots and bulbs of onions and some related plants, sometimes killing seedlings.

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Onion fly (Delia antiqua) on onion

Quick facts

Common name: Onion fly
Latin name: Delia antiqua
Plants affected: Onion, shallots and leeks
Main symptoms: Yellowing and wilting of leaves, damaged bulbs and seedlings killed
Caused by: Larvae of a fly
Timing: June and July

What is onion fly?

Adult onion flies are 5-7mm long and resemble small grey house flies. The larvae are white, legless and headless maggots that can reach 10mm in length. The maggots feed on the roots and bulbs making them inedible and can kill seedlings.

Symptoms

Seedlings affected by onion fly can be killed. On older plants outer leaves can yellow and wilt. The onion bulb can be destroyed as the larvae feed within it. This insect is most often encountered in central and eastern England and is most damaging during June and July.

Onion fly can be distinguished from allium leaf-miner and leek moth as it feeds on roots rather than within the leaves.

Bean seed fly can also be a problem on allium crops and can reduce seedling emergence (particularly of salad onion) dramatically.

Control

  • There are no insecticides available to home gardeners for controlling onion fly
  • Badly damaged plants found towards the end of summer should be lifted carefully and burnt to prevent maggots from completing their development.
  • Some protection may be gained by using insect-proof mesh
  • A mixture of pathogenic nematodes, sold as 'Fruit and Vegetable Protection', can be watered into the soil which may control the young larvae, this is available by mail order from biological control (pdf document) suppliers
  • Growing onions and shallots from sets, rather than seeds, can help plants to survive attacks by the first generation of larvae

Biology

Adult onion flies emerge from overwintered pupae from May onwards and deposit eggs at the neck on young leaves or in the soil adjacent to host plants. The eggs hatch within a few days and each female can produce more than 200 eggs. The white, legless and headless maggots can move from one plant to another through the soil to complete development. They reach 10mm long when fully grown which takes about three weeks. They pupate in the soil, unlike allium leaf miner which pupates within the foliage. The next generation of flies emerges after two to three weeks and there can be three generations a year.

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