These pupae of these moths overwinter in plant debris, and as temperatures rise in the spring, they start to become active.
In the UK there are two generations of caterpillars per year, one in spring and one in late summer.
The caterpillars are pale yellowish-green with brown heads and up to 12mm (½”) long.
Most damage is caused by the second generation, especially after warm, dry summers and the fact that there are more host plants around.
First generation damage generally appears in May-June, and the second generation damage in August-October.
The problem is recognized by the white or brown patches caused by leaf-mining caterpillars that bore into the stems of leeks and bulbs of onions.
Secondary rotting occurs within areas of the stem causing parts to become brown and slimy.
As damage becomes more extensive, leaves start to turn yellow with brown patches.
When the caterpillars are fully fed, they emerge and pupate inside net-like silk cocoons spun on the foliage.
These cocoons contain reddish-brown pupae.
There is no effective insecticide available to the amateur gardener.
Growing plants from the seedling stage under horticultural fleece will prevent the moth laying their eggs on the crop.
Clearing away plant debris after harvesting and digging the bed over may disturb the adults and pupae.
Check over-wintered leeks and remove and destroy any cocoons, caterpillars or leaf damage you may find.