Leek moth

What is leek moth?

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 miner;

• Leek moth caterpillars are creamy-white with brown heads and small legs
• Larvae of the leaf-mining fly are white, headless maggots with no legs

Control

Non-chemical control

The female moths can be prevented from laying eggs by covering susceptible plants with horticultural fleece, or an insect-proof mesh such as Ultra-Fine Enviromesh. Look for the white, net-like silk cocoons on the foliage and squash them.

Chemical control

None of the pesticides available to amateur gardeners for use on leeks and onions is likely to give effective control.

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.