The caterpillars of the leek moth (Acrolepiopsis assectella) are mainly a pest of leeks and onions. They may also attack chives, garlic and shallots. In the UK there are two generations of caterpillars per year, one in spring and one in late summer. Most damage is caused by the second generation, especially after warm, dry summers.
Leaves of leek and onion develop whitish brown patches where caterpillars are feeding within the leaves. Young caterpillars tunnel into the leaves; older caterpillars tunnel down into the stem and bulb causing extensive damage. 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. Silken cocoons on the leaves may contain brown pupae.
The adult is a small, inconspicuous, brown moth. The caterpillars are yellow-green in colour with grey-brown patches and a yellowish brown head. Mature caterpillars are up to 13mm long. Caterpillars pupate in white silk cocoons found within the foliage. They contain the reddish-brown pupae.
Adult moths overwinter in plant debris. As temperatures rise in the spring, the moths start to become active and egg laying begins in April and May. Each female may lay up to 100 eggs on host plants. The caterpillars hatch about a week later and tunnel into the leaves to feed. They mine the leaves, leaving the outer skin intact, eating down through the outer leaves to feed on the inner leaves and growing points of stems. On onions, they mine the hollow leaves, sometimes boring down into the bulb. The caterpillars feed for about a month before crawling back up the leaves to pupate in cocoons.
The first generation of caterpillars causes damage in May and June. As these become adults and lay more eggs, a second generation of caterpillars emerge to cause damage from August to October. As populations rise through the summer, so plant damage increases through the growing season, being most noticeable in early autumn. Adult moths and pupae of the later generation hibernate in plant debris to overwinter, emerging in spring to start the cycle over again.
Clear away all plant debris at harvest. Dig over the soil to disturb overwintering adults and pupae.
Check leek plants for damage in the spring (see symptoms above). Remove and destroy any caterpillars and pupae visible on the leaves. Destroy severely infested plants.
Protect the crop, from seedling stage onwards, with horticultural fleece or environmesh to prevent adult moths from laying eggs.
Later plantings after May, may avoid the first generation of caterpillars.
Encourage predators – birds, bats, hedgehogs, frogs and beetles will eat adult moths, pupae and caterpillars.