Leek moth tunnels into leeks, onions, shallots and garlic, but no Scottish record was found: it is mainly a southern English pest that is spreading north. Here is what it looks like, so you can recognise it if it turns up, and the controls that work if it has.
What does it look like?
Adults: Small, inconspicuous brown moths, 5-6mm.
Caterpillars: Creamy-white with brown heads and small legs, reaching 11mm when fully grown.
Is this pest active now?
April to October, with larvae feeding in May to June and again in August to October; the second generation is the more numerous. These are UK dates, and no Scottish dates were found.
May to June: First generation
August to October: Second generation, the more numerous
These are UK-wide timings, and Scottish onset may differ.
Symptoms
White patches on foliage where caterpillars consume the internal tissues; tunnels in stems and bulbs; secondary rots developing; and young plants sometimes killed outright.
Life cycle
Two generations a year, with larvae feeding on plants in May to June and August to October. The second generation is the more numerous. Caterpillars mine foliage before boring into stems and bulbs, pupate in silk cocoons on the foliage, and adult moths overwinterKeeping a plant alive and intact through the cold months, whether left outdoors, moved under cover, or stored dormant. Full definition → in sheltered locations.
Why this matters in Scotland
No Scottish record of leek moth was found. It is mainly in southern England and spreading north (RHS).
Where the northward spread has reached is not mapped in any source found, so watch for white patches on the leaves. If it turns up, the mesh and cocoon-crushing measures apply here as anywhere.
Controls: what works
Insect-proof mesh covering plants throughout the growing season
Cover the crop with insect-proof mesh through the growing season, and practise crop rotationMoving each crop family to different ground each year, on a multi-year cycle, so pests and diseases that build up in soil don't get a repeat host. Full definition →. Mesh laid over ground that already holds overwintering moths would trap them in with the crop.
A physical barrier works regardless of whether the moth has reached your area yet.
Encourage natural predators: ground and rove beetles
Search for and crush the white silk cocoons on foliage
Natural predators of leek moth
These 2 creatures eat leek moth, and each has a page on how to get more of them in your garden: what to plant, or what to order and when.
Myths about leek moth
“Leek moth only attacks leeks, so onions, garlic and shallots are safe from it”
Leek moth caterpillars feed across the allium family, not just on leeks. Onion, shallot and garlic are affected as well as leek; leek moth populations turn up in onion and garlic crops, and chives are also a host. A bed of onions or garlic sitting next to netted leeks is not automatically protected just because the netting was aimed at the leeks.
Our read Two independent sources, one UK and one North American, list the same wider allium host range, which is a stronger basis than either alone.
What to do: Cover onions, garlic and shallots with the same insect-proof mesh used for leeks, not just the leek bed, and rotate all alliums together rather than treating leeks as the only crop at risk.
2 sources
- RHS: Leek mothNames leek, onion, shallot and garlic as plants affected by leek moth caterpillars.
- University of Vermont Extension: Leek Moth Damage, Monitoring, and DetectionReports leek moth populations found in onions, garlic and leeks, with shallots and chives also listed among the alliums it attacks.
“Putting out pheromone traps will trap enough leek moths to control the infestation”
Pheromone traps for leek moth are built and marketed as a monitoring tool: a lure draws in adult males so a grower can see when moths are flying and time other controls accordingly. University of Vermont Extension frames trap catches purely as a trigger for timing pesticide applications, not as a way of removing enough moths to bring numbers down on their own. Perennia's Nova Scotia extension programme is explicit that its traps are for monitoring only, used to record weekly counts and identify peak flight periods rather than to suppress the population directly.
Our read Neither source claims that trapping meaningfully reduces leek moth numbers; both use the traps to decide when to act, not as the action itself.
What to do: Use a pheromone trap to find out when moths are flying in your garden and time mesh installation or hand-picking around that, rather than relying on the trap itself to bring numbers down.
2 sources
- University of Vermont Extension: Leek Moth ControlDescribes pheromone traps as identifying peak moth flights so that other control measures can be timed 7 to 10 days after adults are caught.
- Perennia (Nova Scotia Vegetable Blog): Monitoring for Leek MothStates its pheromone traps are for monitoring only, used to record when leek moths are active and identify peak flight timing rather than to reduce their numbers.
“You only need to net leeks once you spot the first signs of moth damage”
Mesh works by physically stopping the adult moth from reaching the crop to lay eggs; once caterpillars are already mining the leaves, a barrier over the top does nothing to remove them. Keep insect-proof mesh in place for the entire growing season rather than adding it once damage appears. Row covers work the same way: put them on before the first spring flight of adults and leave them on, lifting them only briefly for fieldwork.
Our read Mesh is prevention against egg-laying, not a treatment for an active infestation, which is the opposite of how the myth frames it.
What to do: Put mesh over leeks, onions and garlic before growth starts in spring and leave it on for the whole season, weighting or pegging the edges down so moths cannot walk in underneath.
In Scotland: Leek moth is mainly in southern England and spreading north, and no Scottish record was found, so this is good practice rather than a response to a confirmed local outbreak.
2 sources
- RHS: Leek mothRecommends covering susceptible plants with insect-proof mesh kept in place for the entire growing season to stop egg-laying females reaching the crop.
- Perennia (Nova Scotia Vegetable Blog): Monitoring for Leek MothAdvises row covers go on before the first leek moth flight in spring and stay in place, describing this as one of the most effective controls.
Frequently asked questions
What should I do first if I find leek moth in my garden?
The first thing to try: Insect-proof mesh covering plants throughout the growing season. Then: Encourage natural predators.
Will leek moth kill my plants?
It can. The caterpillars mine the leaves and tunnel into stems and bulbs, and young plants can rot and die. Mesh over the crop is the main protection.
When should I watch for leek moth?
April to October, with larvae feeding in May to June and again in August to October; the second generation is the more numerous. These are UK dates, and no Scottish dates were found.
Sources
Show the source
- RHS1: adult and caterpillar identification, affected alliums, damage, two generations with May-June and August-October timings, overwintering, southern England distribution and northward spread, comparison with allium leaf mining fly, mesh with rotation, ground and rove beetles, crushing cocoons