Q What is leek moth?

A Once localised to the south and east coasts, this pest is now becoming far more widespread. The adult leek moth, Acrolepiopsis assectella, is small and nondescript, but it's the caterpillars you need to look out for, as they do the damage.

Caption: Leek moth caterpillars are about 10mm long

Q Where does leek moth occur?

A Most books refer to leek moth as a localised problem, only occurring on the south and east coasts, but it’s now spreading north and west, and has been recorded in north Wales. It's widely distributed in Europe.

Q How do I recognise leek moth?

A The caterpillars are yellow-green with legs at the front and fleshy pro-legs at the back, they have a dark head and dark raised spots with hairs. They grow from 1mm, when newly hatched, to 10mm. The pupa is 10mm long and wrapped in a cream-coloured silk cocoon. The moth is grey-brown, 8-9mm long and barely distinguishable from many other similar small moths.

Q What plants does leek moth attack?

A As you might expect, leek moth is best known as a pest of leeks, but it can also attack chives, garlic, onions and shallots.

Q When do leek-moth attacks occur?

A There are two generations of caterpillars a year in the UK, in spring and late summer. The first attack is often mild, especially after a cold winter. The second one is the most damaging, and hot summers tend to favour the pest and make the problem worse.

Q What damage does leek moth cause?

A The young caterpillars burrow between the surfaces of the leaf, like a leaf miner, creating brownish-white patches. As they get bigger, they bore down into the shaft of the leek, or the bulb of the onion, causing more substantial damage and allowing in rotting fungi and bacteria. At this stage, the vegetables tend to develop yellow leaves and go slimy.

Caption:  The worst damage is seen in late summer

Q How serious is leek moth?

A It's rarely a problem for commercial growers as treatments for other pests are likely to control it. But for gardeners and organic growers, it can make growing leeks and onions very difficult, unless effective preventive measures are taken.

Q Are there any early-warning signs of leek moth?

A Check for the brownish-white patches caused by early mining activity in May and August. Remove any caterpillars you find and cut back badly damaged foliage. Leeks can often recover and still produce a reasonable crop; onions are less likely to. You might consider using leek-moth traps. Similar to codling-moth traps, these use a pheromone lure inside a tent-shaped trap with a sticky base. Their main value is to warn when the moths are about, so you can be alert for hatching caterpillars. However, traps will also reduce the number of surviving adults, which may reduce damage. For traps, see gardening-naturally.com.

Q Could I mistake leek moth for anything else?

A Allium leaf miner, a new pest now spreading outwards from the West Midlands, can create a similar effect by damaging leaves and encouraging rot. However, the leaf-miner larvae are like maggots - off-white with no legs or obvious head - and the pupae are orange-brown with no silk cocoon.

Onion-fly damage can also lead to rotting at the base - look out for yellowing leaves and clusters of maggots in the base of the onion or leek.

Q What should I do with plants affected by leek moth?

A If you can remove the caterpillars when they are still at the mining stage, then the plants should survive, and leeks in particular can overcome quite severe damage of the upper parts to provide at least some crop.

Once the lower part of the plant is attacked it will probably be killed by secondary rotting.

Undamaged parts can still be eaten, but make sure any caterpillars and pupae on discarded leaves are destroyed. Badly affected plants are best consigned to your local council's composting scheme, or deeply buried.

Q What is the life cycle of leek moth?

A There are two generations a year. The adult moths overwinter in plant debris, become active in spring and lay eggs in April and May.

The caterpillars hatch and feed on the leaves for about a month, gradually working their way down towards the base of the plant. Once mature, they crawl back up to the leaves to pupate.

These pupae then hatch and the second generation of caterpillars cause damage from August to October then pupate. Most of these pupae hatch before winter, though some may not do so until spring.

Q How do I prevent leek-moth attacks in the future?

A The most effective way to keep this pest off your crops is to cover them. In cold weather you can use horticultural fleece, but in summer this can make crops too hot and does not allow much rain to penetrate, so insect-proof mesh is better.

Fleece and insect-proof mesh are available in garden centres and from Gardening Naturally (0845 680 0296; gardening-naturally.com), The Organic Gardening Catalogue (01932 253666; organiccatalogue.com) and Wondermesh (01561 377946; wondermesh.co.uk).

Other control methods include clearing away all plant debris after harvest and digging the soil to disturb and expose overwintering adults. There are no pesticides approved for amateur use for this pest.

Stems can be covered with eggs, so, unless the stems are efficiently destroyed, the eggs can hatch and infest new crops. Carrots, parsnips and parsley, left over in the spring, harbour many willow-carrot aphids. These overwintered root vegetables must also be destroyed before May, by thorough composting.