White Maggots at the Base of My Leeks

Small white maggots — typically 5–8 mm long — found in or around the rotting base of leek plants are almost certainly the larvae of onion fly (Delia antiqua). The same pest that attacks onions and garlic affects all alliums, including leeks. The larvae burrow into the base of the plant, feeding on the stem and root tissue and creating entry points for bacterial soft rot. A wilting, yellowing leek plant that pulls out of the ground easily, with a soft, maggot-filled base, has onion fly damage. The timing of attack, the lifecycle, and the prevention measures are the same as for onions — but leeks are at risk across a longer season because they are in the ground from spring through to winter.

The onion fly lifecycle on leeks

The adult onion fly (similar in appearance to a small grey housefly) is active from April through to August, with two to three generations per year. Females lay eggs at soil level near the base of allium plants. Eggs hatch in five to seven days and the larvae burrow into the plant base, feeding for two to three weeks before dropping into the soil to pupate. The first generation (April–May) is most damaging to young transplants. Leeks transplanted in May or June without protection are vulnerable to the first and second generations of fly.

Physical exclusion

Fine mesh insect netting (1.3 mm mesh) placed over the bed immediately after transplanting and sealed well at the edges prevents adult flies from reaching plants to lay eggs. This is the most reliable organic control measure. Maintain the netting through at least late July when the main egg-laying period ends. Remove for weeding and replace immediately. There is no approved insecticide for home garden use against onion fly larvae.

Dealing with active infestations

Remove and destroy all affected plants immediately to prevent pupation in the soil nearby. Inspect neighbouring plants by gently clearing soil from the base. Apply netting over the remainder of the bed immediately to prevent further egg-laying. On previously affected beds, disturb the soil thoroughly after harvest to expose pupae overwintering in the soil to birds and frost.