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 can be affected in a similar manner by larvae of 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

Non-pesticide control

• The female moths can be prevented from laying eggs by covering susceptible plants with insect-proof mesh. Crop rotation should also be practised to prevent potential build-up of moth populations under the mesh. The mesh should be kept in place for the entire growing season. 
• Look for the white, net-like silk cocoons on the foliage and squash them
• Encourage natural enemies in gardens, predators such as ground and rove beetles are likely to consume some pupae
   

Pesticide control

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

Leek moth has two generations during the summer with larvae feeding on the plants in May to June and August to October. The second generation is the more numerous. 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 exit 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.