Leek moth

What you are looking at

Hold a leek leaf up to the light. Leek moth caterpillars feed inside the leaf, grazing away the tissue between the two surfaces, so what is left is a pale elongated window with the outer skin still stretched over it. Several mines run down the same leaf and the blade dies back from the tip.

Peel the outer leaves off and the caterpillar is in the heart. It is pale green, up to about 13 mm long, with a brown head and faint darker spots along its body. Behind it are tunnels running down towards the base of the plant, packed with frass, and those tunnels are the route the soft rots take afterwards.

On the outside of a leaf you may find the pupa, and it is unmistakable. A loose, open white cocoon, spun like a piece of coarse netting with the brown pupa visible inside it. Nothing else on an allium in a British garden makes one.

Onions and shallots show the same mines and then tunnelling into the neck and the bulb. Garlic and chives get it too, and on chives the damage often reads as the clump failing for no visible reason.

A pest that has arrived recently

Leek moth was for a long time a local insect of the south coast and the Channel Islands. From the late 1990s it moved inland and northwards, and it is now found through most of England and Wales with records well into Scotland.

Gardeners who grew leeks for thirty years without meeting it are meeting it now, and older gardening books either do not mention it or describe it as a curiosity of Kent and Cornwall. Anyone searching for what is wrong with their leeks will find plenty of advice written before this became a mainstream problem.

Two generations, and which crops sit in the way

Adults are small brown moths about 15 mm across the wings, with a white triangle on the trailing edge of each forewing. They fly at dusk and are almost never seen.

The first flight runs from around April into June, from moths that spent the winter as adults in sheds, hedge bottoms and among standing alliums. Eggs go on the leaves, the caterpillars mine and then tunnel, and they pupate in those netted cocoons.

The second flight, in August and September, is the larger one and does most of the damage, because it lands on maincrop leeks that are sizing up and on onions approaching harvest. Those caterpillars feed into the autumn.

Salad onions and autumn-planted garlic often miss the worst of it by timing. Maincrop leeks stand through both flights and cannot dodge either, which is why they are the crop worth covering.

Mesh, and why it has to be done properly

An insect-proof mesh over hoops, on from planting and off at lifting, stops this pest. Three details decide whether it works.

The mesh has to be fine enough. Around 1.3 mm is the usual specification, and ordinary fleece or bird netting will not exclude a moth this size.

It has to be held clear of the leaves. Where the fabric rests on the foliage, a moth standing on top can lay through it onto the leaf beneath.

And the edges have to be sealed to the soil the whole way round, buried in a slit or weighted with a continuous line rather than pegged at intervals. This moth walks under a lifted edge as happily as it flies over a low one.

The fourth condition is rotation. Covering a bed that grew leeks last year seals the emerging moths in with the new crop and shuts out the wasps that would otherwise parasitise them.

What to do about it

Cover leeks and onions with 1.3 mm insect mesh over hoops from planting, sealed at the edges.

Rotate so that new alliums never follow old ones, and clear the last of the overwintering row before the new plants go out.

Lift and bin every leek stump, onion top and volunteer at the end of the season rather than composting them.

Cut mined leeks back to about 5 cm above the neck in midsummer and let them regrow.

Crush the caterpillars and the netted cocoons on any plant you handle.

Grow salad onions and autumn garlic where the flight periods allow, and save the mesh for the maincrop leeks that cannot avoid it.

What it gets mistaken for

Allium leaf miner punches a distinctive line of white dots down the leaf where the adult fly has fed, and it pupates as orange-brown grains inside the plant rather than in a cocoon outside it. It also flies in different months, roughly March to April and October to November, so a plant damaged in high summer is usually leek moth.

Onion white rot rots the base and the roots, coating them in white fluff dotted with hard black specks, and the leaves yellow from the tip because the plant has lost its roots.

Onion thrips silver the leaf surface all over without tunnelling.

Downy mildew on onions makes pale oval patches with a fine grey-violet furry growth on them in damp weather, and the patches have no caterpillar behind them.

Rust puts raised orange pustules on leeks and chives that leave powder on a finger.