Key takeaways
- The fly comes twice a year, in March to April and October to November
- Cover crops with fine insect mesh before each flight starts, not after you see damage
- Rows of tiny white dots on the leaves are the first sign
- Leeks, onions, garlic, shallots and chives are all at risk
- Bin or burn infested plants, as the pests can survive a home compost heap
- There's no spray approved for home gardeners, so mesh and a good tidy-up are your tools
Allium leaf miner is a tiny fly that has become a real headache for anyone growing leeks, onions and garlic. Its maggots tunnel through the leaves and hide in the stems. Often you won’t know until you cut a leek open at harvest and find it full of little brown cases.
There’s no spray for home gardeners, but don’t worry. Fine mesh at the right time, plus a good tidy-up, keeps your crops safe.
What is allium leaf miner?
It’s a small grey-black fly, about 3mm long, with a yellow or orange patch on its head. It’s smaller than a housefly and easy to miss. The female pricks the leaves to feed, leaving neat rows of white dots, and lays her eggs in some of them. The maggots then tunnel down into the plant.
How to spot allium leaf miner damage
The first sign is rows of tiny white dots running along the leaves. These are the feeding marks left by the female fly.
The adult fly is only about 3mm long. Look for the orange-yellow patch on its head.
Soon after, the maggots hatch and tunnel through the leaves, leaving wiggly trails. They’re small and pale. After a couple of weeks of feeding they head down into the stem or bulb to change into adults.
Rot often follows. The tunnels let in other problems. Onions can go soft and smelly at the base, and leeks turn slimy between the layers. This rot can spoil more of the crop than the maggots do.
Don’t confuse it with other pests. Onion thrips leave silvery streaks, not rows of dots. Leek moth caterpillars leave droppings and silky webbing, which allium leaf miner doesn’t. The RHS allium leaf miner page has more photos to compare.
When allium leaf miner strikes during the year
The fly comes twice a year, and timing is everything. Your crops only need protecting during these two flights. Miss the start and netting afterwards won’t help.
Spring flight, March to April. The adults hatch out as the soil warms up. It starts a bit earlier in the south and later in the north and Scotland.
Autumn flight, October to November. The young from spring grow up over summer and fly again in autumn.
Over winter. The autumn brood spends winter tucked inside plants and old debris, then flies the next spring. That’s why clearing old plants matters so much.
| Stage | Spring | Autumn |
|---|---|---|
| Flies about | March to late April | October to mid-November |
| Maggots feeding | April to May | October to December |
| Resting inside plants | Over summer | Over winter |
| Next flight | October | March next year |
The big mistake: putting mesh on once you see white dots. By then, eggs are already laid. The mesh must go on before the flies arrive.
Small dark brown cases tucked between the layers of a cut leek. Bin or burn any leek like this.
How to protect crops with insect mesh
Cover your crops with fine insect mesh before each flight starts. The holes need to be no bigger than 0.8mm. Ordinary butterfly netting is far too coarse to stop a fly this small. Enviromesh, Bionet and Wondermesh are the usual UK brands.
When to cover:
- Spring: get the mesh on by early March in the south, mid-March in the Midlands and late March further north. Leave it on until the spring flight is over.
- Autumn: cover by late September everywhere, and keep it on until late November.
How to fit it:
Hold the mesh up on hoops so it doesn’t rest on the leaves. Bury the edges in the soil or weigh them down well. Flies will find any gap at ground level. Our crop rotation planner can help you plan beds that are easy to cover.
Fine mesh over hoops keeps the flies off. Make sure the edges are tight to the soil.
Good mesh lasts several seasons if you store it out of the sun between uses. For a bed of leeks, it soon pays for itself.
Other ways to keep allium leaf miner down
Tidy up after harvest. This is the next most important job after netting. Check every stem as you lift it. If you find brown cases, don’t compost the plant, as home heaps rarely get hot enough to kill them. Bag it for the bin or burn it. On a busy allotment, clearing all onion-family debris by mid-December removes next spring’s flies.
Plant leeks out later. Leeks transplanted from late June miss the worst of the spring flight. They’ll be a bit smaller, so treat this as a backup rather than a replacement for mesh.
Why allium leaf miner is so common now
It’s a fairly new arrival to the UK, and it has spread fast. A few things help it along.
Milder weather. Warmer springs and autumns give the fly a longer season.
Few natural enemies. In its home range in Europe, tiny parasitic wasps keep it in check. They haven’t built up here yet, and there’s no biological control you can buy for it.
Ornamental alliums. Popular garden alliums like ‘Purple Sensation’ can host the fly without showing much damage. If you companion plant with ornamental alliums near your veg beds, they may keep the fly around.
The answer is simple, if not glamorous: mesh during both flights and a careful clear-up after harvest.
Allium leaf miner calendar
| Month | What’s happening | What to do |
|---|---|---|
| January | Pests resting in old plants | Clear any old onion-family debris |
| February | Spring flight getting close | Get your mesh and hoops ready |
| March | Spring flies hatch | Cover crops before they arrive |
| April | Spring flight at its peak | Keep mesh tight, check for gaps |
| May | Maggots resting in stems | Uncover once the spring flight ends |
| June | No flies about | Safe to plant leeks without cover |
| July | Pests resting | Clear spent garlic and onion leaves |
| August | Pests resting | Check stored onions for rot |
| September | Autumn flight getting close | Cover crops by late September |
| October | Autumn flight starts | Keep all onion-family beds covered |
| November | Autumn flight ends mid-month | Keep mesh on until late November |
| December | Pests resting for winter | Bin or burn infested plants |
For everything else to sow and plant through the year, see our UK vegetable planting calendar.
Common mistakes with allium leaf miner
Covering too late. The most common slip. If you can see white dots, the flies have already been. Mesh goes on in early March and late September.
Using the wrong netting. Butterfly netting won’t stop a fly this small. Buy mesh sold as insect mesh, with holes no bigger than 0.8mm. Household fly screen breaks down in the sun quickly, so skip it.
Composting infested plants. Home compost heaps don’t get hot enough to kill the pests. Bin or burn anything with brown cases inside.
Trusting crop rotation. The flies travel a long way, so moving the bed doesn’t help much. Rotation is good for soil diseases, but mesh is what stops this pest.
Now you know how to beat allium leaf miner, read our guide to growing leeks in the UK for sowing, planting out and blanching tips.
Frequently asked questions
What does allium leaf miner damage look like?
Look for rows of tiny white dots on the leaves. These are where the female fly has fed and laid eggs. Later you’ll see wiggly trails as the maggots tunnel in. Badly hit leaves turn brown and papery. Cut a stem open and you may find small dark brown cases tucked between the layers.
When does allium leaf miner fly in the UK?
It flies twice, in spring and again in autumn. The spring flight runs from March to late April. The autumn flight runs from October to mid-November. Spring attacks hit onions, garlic and young leeks. Autumn attacks mostly hit leeks and autumn-planted garlic. The south usually sees it a little earlier than the north.
Does enviromesh stop allium leaf miner?
Yes, fine mesh is the best way to stop it. Choose insect mesh with holes no bigger than 0.8mm. Put it on before the flies arrive: early March for spring and late September for autumn. Seal the edges well with pegs or by burying them.
Can I eat leeks with allium leaf miner damage?
Yes, once you’ve peeled off the damaged layers. Strip the outer leaves until you reach clean, healthy layers underneath. The pests aren’t harmful to you. A badly hit leek has less to eat, but the clean part tastes normal. Bin or burn the peelings so any pests inside don’t survive.
Does crop rotation prevent allium leaf miner?
No, rotation on its own won’t stop it. The flies can travel a long way, so moving your onion bed down the garden makes little difference. Rotation is still good for soil diseases. The real fix is mesh during the flight times, plus binning or burning infested plants.
Are there any chemical sprays for allium leaf miner?
No, there’s nothing approved for home gardeners. Your tools are fine mesh, good timing and a careful tidy-up after harvest. Those three together do the job.
Which allium crops does allium leaf miner attack?
It goes for all of the onion family. Leeks suffer most because they’re in the ground through both flights. Onions, garlic, shallots, chives and spring onions are attacked too. Ornamental alliums and wild garlic nearby can also host the fly.
Our team produces practical UK gardening guidance from source research, contributor records and established horticultural practice. Editorial standards are overseen by Matt W.
