Identification and symptoms caused by the leek moth

Caterpillars of the leek moth mine into the leaves of leeks, eventually penetrating the central leaves of leeks, causing severe damage and marketability issues. There are likely to be two generations of this species in the UK, with larvae feeding in May/June and August/September.

• Mainly distributed in warm locations towards the south of the UK, especially around the coast

Scientific name: Acrolepiopsis assectella

Adult moths have a wingspan of 15 mm. The forewings are slender and narrow and have a variable brown colour, with paler scales near the apices. There is a conspicuous white triangular mark halfway along the rear margin of each forewing. The rear margins of the forewings are fringed with pale-coloured hairs.

Eggs are oval-shaped, white, iridescent and about 0.4 mm in diameter.

Fully grown larvae are 13–14 mm long. It has a brown or yellow head and a yellowish-green body, with inconspicuous grey-brown patches, especially around the spiracles. There are yellow plates on the first and last segments.

Pupae are about 6 mm long and brown.

Apr: Adult moths emerge in April and lay eggs (up to 100 per female) singly on foliage towards the base of the plant.

May–Jun: The larvae feed during May and June. At first, they mine the leaves, leaving the epidermis intact, but then bore through the folded leaves to feed near the centre.

Jun: The larvae pupate in flimsy silken cocoons attached to the host plants.

Jul–Aug: Adults (second generation) emerge and lay eggs.

Aug–Sep: Larvae (second generation) feed.

Sep–Mar: Pupae overwinter.

When young larvae mine the leaves of leeks, it leads to patches of papery, necrotic tissue. Older larvae make ‘shot holes’ in folded leaves. Severely damaged leaves sometimes rot and, if extensive, the plant dies. 

Non-chemical control

To prevent reinfestation, it is necessary to practice good crop rotation and locate crops away from previously infested soil. Crop debris can be destroyed to kill pupae. Natural enemies, including parasitic wasps, can attack leek moth larvae.

Biopesticides based on Bacillus thuringiensis (Bt) can control the larvae. The use of insect-pathogenic nematodes appears to be effective.

Monitoring

Catch male moths with pheromone traps.

Thresholds

None established in the UK.

Insecticide resistance

None known.