|A leek moth damaged garlic bulb|
Onion leaf miner
Caterpillars eat into the leaves resulting in white-brown patches on the leaves where caterpillars have eaten internal tissues. Leaves may turn yellow and begin to rot in severe cases. Stems of leek, onion, shallots and garlic are tunnelled.
Symptoms are similar to that of the allium leaf miner larvae. Leek moth caterpillars are creamy-white with brown heads and short legs. Allium leaf miner larvae are white, headless maggots with no legs.
None of the pesticides available to amateur gardeners for use on leeks and onions is likely to give effective control.
Remove and destroy affected plants.
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.
When transplanting; consider covering leeks with fleece if you think this might be a problem.