Plants affected
Leeks, onions and shallots (not ornamental Allium).
Symptoms
White or brown patches develop on leek and onion leaves where internal tissues have been eaten by the leaf-mining caterpillars. The larvae also bore into the stems of leeks and bulbs of onions. Badly infested plants are often killed by secondary rots.
NB Leaf mines on leeks, onions, shallots and ornamental Allium species in the West Midlands are likely to be caused by the allium leaf-mining fly,
which was first discovered in Britain in 2003. See www.defra.gov.uk/planth/pestnote/allium-info.pdf
Biology
The leek moth has two generations with damage occurring May-June and August-October. The second generation is more abundant. The caterpillars are pale yellowish-green with brown heads and up to 12mm long. When fully fed the caterpillars emerge and pupate inside net-like silk cocoons spun on the foliage. Adult moths emerge in early autumn and overwinter in sheltered places. Previously a coastal problem, leek moth is now found further inland in southern and eastern England.
Control
No effective insecticide is available to home gardeners for use on leeks and onions. Growing plants under fleece, such as Enviromesh, will prevent the moths laying eggs. Keep leeks watered in summer to ensure strong growth; larger plants are more tolerant of damage and can survive to produce usable crops.