Leek moth

Leek moth is mainly a problem in southern England but it is spreading north. The caterpillars feed within the foliage and stems or bulbs of leeks, onions and related vegetables.

Leek moth

Quick facts

Common name Leek moth
Latin name Acrolepiopsis assectella
Plants affected Leek, onion, shallot, garlic
Main symptoms White patches on leaves, with young plants rotting and dying. Small caterpillars may be seen in the plant tissues
Caused by Caterpillars of a small moth
Timing April-October

What is leek moth?

The adult leek moth is an inconspicuous very small (5-6mm) brown moth. Its larvae (caterpillars) feed on leeks and similar crops.

Symptoms

Damage from leek moth caterpillars appears as;

  • White patches developing on the foliage where the caterpillars have eaten the internal tissues
  • Tunnels in the stems of leeks and bulbs of onions, shallots and garlic
  • Affected plants often develop secondary rots and young leek plants may be killed

Note that these plants are also attacked in a similar manner by larvae of a fly, known as the allium leaf-mining fly (allium leaf-miner);

  • Leek moth caterpillars are creamy-white with brown heads and small legs. They do not usually pupate within plant tissues
  • Larvae of the leaf-mining fly are white, headless maggots with no legs. Cylindrical brown 3mm long pupae are likely to be found embedded in the stems and bulbs

Control

Non-chemical control

The female moths can be prevented from laying eggs by covering susceptible plants with insect-proof mesh. Crop rotation should also be practiced to prevent potential build-up of moth populations under the fleece. Look for the white, net-like silk cocoons on the foliage and squash them.

Chemical control

None of the pesticides currently available to home gardeners for use on leeks and onions will give effective control of leek moth.

Biology

Leek moth has two generations during the summer with larvae damaging the plants;

  • From the first generation in May-June
  • And the second generation in August-October
  • The second generation is the more numerous and damaging

Initially the caterpillars mine the foliage but the older larvae bore into the stems and bulbs.

When fully fed, the caterpillars are 11 mm long. They come out of the plant and pupate within net-like silk cocoons that are spun on the foliage.

Adult moths emerge in autumn and overwinter in sheltered places.

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