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Garden pests

Allium leaf miner

Phytomyza gymnostoma · Insect

Also called allium leafminer (USA and Canada), onion leafminer (USA).

Illustration

A small fly whose white maggots tunnel down the leaves of leeks, onions, garlic and chives into the stem and bulb, leaving twisted plants that often rot. Cover crops with fine insect mesh through its two egg-laying seasons, spring and autumn, and grow them in fresh ground each year.

How to spot it

• A neat line of tiny white dots running along the leaf, often near the tip: punctures where the female fly has fed and laid eggs.
• Twisted or distorted leaves, and wilting plants.
• White to yellowish maggots with no legs and no head, up to about 8 mm (⅓ in) long, and reddish-brown pupae about 3 mm (⅛ in) long. Peel a leek or onion apart to find them between the leaf layers and in the stem or bulb.
• Plants that rot, mainly in autumn, as fungi and bacteria get in through the tunnels.
• The adult: a greyish fly about 3 mm (⅛ in) long, with a yellow patch on the head and yellow 'knees'.

Easily confused with

Where it's a problem

• United Kingdom: First found in Britain in 2002, and now widespread in England and parts of Wales. The flies lay eggs in March and April and again from late summer to November; the autumn generation usually does the most damage.
• Europe: Native to Europe, and recorded in most European countries, from Portugal and Spain to Poland, Ukraine and Türkiye.
• United States: First found in the Americas in 2015, near Lancaster, Pennsylvania, and has since spread through the mid-Atlantic and north-eastern states. Leeks are hit hardest; onion bulbs are rarely attacked.
• Canada: First found in Canada in 2025, in Ontario's Niagara region, and in Perth, Ontario, in 2026. It is a regulated pest there: report suspected finds to the Canadian Food Inspection Agency (CFIA).
• Australia: Not found in Australia, and listed by Plant Health Australia as an exotic threat to onions. Report a suspected find to the Exotic Plant Pest Hotline on 1800 084 881, and don't move samples until an expert has checked them.

What to do

1. Cover leeks, onions, garlic and chives with fine insect-proof mesh before the flies are about (in the UK and north-eastern USA, roughly March to May and September to November). Bury or weigh down the edges so nothing can creep under.
2. Grow alliums in a different bed each year: in the same ground, flies can hatch from pupae in the soil under the mesh.
3. Where it's common, time crops to miss the flies: plant leeks and onions out after the spring flight, and lift them before the autumn one.
4. Plant away from last year's allium beds and from clumps of chives where you can: the flies don't seem to travel far.
5. Check leaf tips often for lines of white dots, and peel back the leaves of any twisted or wilting plant to look for maggots and pupae.
6. Clear away all old allium plants and debris at the end of the season, so pupae have nowhere to spend the winter.

In Australia, report it rather than treating it. Elsewhere, if you use an insecticide, choose one labelled for allium leaf miner and follow the label and local rules.

Vegetables it affects

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Sources

• RHS: Allium leaf miner
• Cornell University IPM: Allium leafminer
• Canadian Food Inspection Agency: Allium leaf miner
• EPPO Global Database: Phytomyza gymnostoma, distribution
• Plant Health Australia: Exotic threats of onions, allium leaf miner (fact sheet)
• RHS: Onion fly

Facts checked Wednesday 21 October 2026. General guidance only: follow your local council, agency and product label.