The maggots of onion fly can damage the roots and bulbs of onions and some related plants, sometimes killing seedlings.
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?
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.
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.
Bean seed fly can also be a problem on allium crops and can reduce seedling emergence, particularly of salad onion, dramatically.
- 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
The Royal Horticultural Society is the UK’s leading gardening charity. We aim to enrich everyone’s life through plants, and make the UK a greener and more beautiful place.