What Is Tunnelling Inside My Leek Stems?

Small caterpillars — typically 10–15 mm long, pale green or cream-coloured with a brown head — found tunnelling inside leek stems, leaf sheaths, or at the growing tip are the larvae of leek moth (Acrolepiopsis assectella). Leek moth is established across much of England and parts of Wales and has become an increasing problem in home gardens over the past decade. The damage it causes ranges from cosmetic to severe depending on timing, plant stage, and generation.

Identifying leek moth damage

Early signs are small irregular holes in outer leaves and characteristic frass (dark green or brown excrement) visible in the leaf sheaths or at the plant base. As larvae move deeper into the plant, the central shoot may show damage, distortion, or "dead heart" — where the central growing tip has been destroyed and the plant stops producing new growth. Plants with dead hearts may produce side shoots but will never develop a normal shank. This central damage is the most serious outcome of leek moth infestation. Pulling the outer leaves away from the shank often reveals the larva feeding inside, or the frass-lined tunnel it has left behind.

Life cycle and timing

Leek moth is active from April through to October in the UK, with two main generations in southern England (and sometimes three in warm years). Adult moths lay eggs on leek leaves; young larvae initially mine the leaves, then move progressively inward to the leaf sheaths and stem. Pupation occurs in a characteristic white silken cocoon on plant debris or in the soil. The autumn generation causes the most damage because leeks are at a critical stage and conditions are typically cooler and less favourable for rapid plant recovery.

Prevention and control

Fine mesh insect netting (1.3 mm or less) covering the crop throughout the growing season from transplanting onward is the most reliable prevention. Adult moths cannot lay eggs on covered plants. The netting must be well sealed at the edges. There is no approved chemical treatment for home growers. If netting is not in place and you find active larvae, remove them by hand — split the leaf sheath, extract the caterpillar, and assess the damage. Plants with intact growing tips will usually continue to develop; plants with destroyed growing tips should be removed.

Edibility of affected leeks

Leeks with leek moth damage are edible once the tunnelled areas are removed. Trim away any discoloured, frass-filled tissue — the undamaged shank below the infestation is clean and perfectly good to eat. Inspect each leek carefully when harvesting from an affected bed and trim generously.