Leek moth is a relatively new pest of leeks and onions, thought to be mainly concentrated in the south-east coast of the UK. However this pest has been recorded further inland and north. Leek moth has two generations during the summer. The first generation being seen May-June and the second generation, being more numerous and damaging in August-September. The larvae of this moth tunnels into the leaves, or flags of leeks, causing whitish-brown patches to develop. In severe cases leaves are likely to turn yellow where rotting occurs within them. The older larvae are likely to bore into leeks stems/onion bulbs.
Remove and destroy infected plants. When planting out cover the young leek plants with horticultural fleece to prevent the adult moths from laying eggs.
There is no chemical registered for the control of this pest on onions.