Onion fly

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

Onion fly (<EM>Delia antiqua</EM>) on onion
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

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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.

The fly belongs to the family Anthomyidae, it is a diverse group of flies with over 100 species Britain. Whilst some fed on roots others are leaf miners such as the beet leaf miner. Most do not cause damage to garden plants. More information on this fly family is available from the Anthomyidae recording scheme.

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.

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Onion fly (Delia antiqua) on Onion (Allium cepa)
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Onion fly adults (Delia antiqua) on Onion (Allium cepa)
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Onion fly (Delia antiqua) on Onion (Allium cepa)
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Onion fly adults (Delia antiqua) on Onion (Allium cepa)
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Onion fly (Delia antiqua) on Onion (Allium cepa)
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Management

  • Encourage predators and other natural enemies in the garden such as birds, hedgehogs and ground beetles. Some ground and rove beetles are efficient predators of root feeding maggots 
  • 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. They are not specific to onion fly and may also affect other insects in the soil and so should be used in a targeted manner
  • Growing onions and shallots from sets, rather than seeds, can help plants to survive attacks by the first generation of larvae
  • There are no insecticides available to home gardeners for controlling onion fly

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 and edible parts. The next generation of flies emerge after two to three weeks and there can be three generations a year.

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