Onion fly & leek moth
Delia antiqua & Acrolepiopsis assectella
Maggots burrow into the base of onions, leeks, and garlic; leaves yellow and droop. Grow alliums under insect mesh and rotate to fresh ground each year.
How to know it
Two pests with one answer. Onion fly is a gray housefly-like fly whose white maggots tunnel into the bulb base. Leek moth is a small brown moth whose green caterpillars mine within the leaves, leaving windowed white patches and, in leeks, tunnels down into the stem.
What you'll see
- Young onions yellowing, wilting and pulling out of the ground with a rotted base.
- White maggots and a soft, foul-smelling rot at the base of the bulb.
- Whitish windowpane patches and pinholes along leek and onion leaves.
- Green caterpillars and frass inside the folded leaves of leeks.
- Secondary soft rot turning a light attack into an unusable crop.
Where it comes from
Onion fly runs two or three generations from May, laying at the base of the plants and overwintering as pupae in the soil. Leek moth flies in May and again in August, its caterpillars mining leaves for a few weeks before pupating in a fine net-like cocoon on the foliage. Both have expanded their range noticeably in recent warm decades.
Warm dry summers, and beds where alliums have grown repeatedly. Thinning and weeding release the onion scent that draws the fly, exactly as with carrot fly.
Dealing with it
- Cover the crop with fine insect mesh from sowing or planting until harvest — the single reliable control for both pests.
- Pull up and destroy affected plants promptly, maggots and all.
- Grow from sets rather than seed, which passes the vulnerable seedling stage quickly.
- Squash leek-moth caterpillars in their mines and remove the netted cocoons from the leaves.
- Do not compost infested bulbs or leaves.
Keeping it out
- Rotate alliums onto fresh ground every year — the pupae stay in the old bed.
- Avoid thinning where possible, and firm and water the soil after any disturbance.
- Clear all allium debris, including leek stumps, at the end of the season.
- Keep mesh sealed at the edges; both pests walk or fly in through any gap.
- Encourage rove and ground beetles, which take eggs and larvae at soil level.
What it goes for
Onions, shallots, leeks, garlic, spring onions and ornamental alliums.
Trouble it brings with it
Who hunts it for you
- Beneficial nematodesSteinernema & Heterorhabditis spp. Microscopic worms watered onto damp soil that hunt down soil-dwelling grubs and release a bacterium that kills them within days. Timing is the whole thing: warm, damp soil and a small target grub, which means late summer for most of what they are used against.
- Rove beetleStaphylinidae Slim, quick beetles with short wing cases that hunt in soil and litter, taking root-fly eggs and larvae, fungus-gnat larvae, and mites. Mulch and undisturbed ground keep them resident year-round. Grow to recruit it: Mint,Thyme
See also
The Cottage Garden Companion — the gardener's bestiary at cottagegardencompanion.com