Seedcorn maggot
Delia platura
Seed that rots in the ground and never comes up, with a small white maggot inside it. A cold-wet-soil problem, and the answer is to sow warmer.
How to know it
A legless white maggot 5–7 mm long, blunt at the rear and tapering to a point at the head, found inside a rotting seed or in the hollowed base of a failed seedling. The adult is a small gray fly like a slender housefly. Almost always the maggot is found after the fact, when you dig up a gap in the row to find out what happened.
What you'll see
- Poor, gappy germination in a row that was sown well.
- Seeds dug up and found hollowed out, rotten, with a small white maggot inside.
- Bean and pea seedlings emerging with the seed leaves eaten away and no growing point — snakeheads.
- Seedlings that come up, look sickly and then collapse.
- Worst in the coldest, wettest part of the bed and in any part that had manure or green matter dug in shortly before sowing.
Where it comes from
The flies are attracted to decaying organic matter in cool, damp soil and lay there. Eggs hatch in a few days and the maggots feed on the seed and the seedling before it can get away. Three to five generations a year, but only the first, in cold spring soil, usually matters — a fast-germinating seed in warm soil is simply gone before the maggot can reach it.
Cold wet soil in early spring, and beds with fresh manure, a recently turned cover crop or heavy undecomposed organic matter dug in. Sowing too early into a cold seedbed is the whole cause.
Dealing with it
- Resow, but wait for warmer soil. The remedy is entirely in the timing.
- Warm the bed with a cloche, fleece or black plastic for a fortnight before sowing again.
- Do not dig in fresh manure or a green cover crop within three or four weeks of sowing.
- Start the crop in modules and plant out established seedlings instead of direct sowing.
- Sow a little thicker in cold conditions and thin later.
Keeping it out
- Sow when the soil is genuinely warm enough rather than at the earliest calendar date.
- Incorporate manure and cover crops in autumn, not just before spring sowing.
- Cover the seedbed with fine mesh so the flies cannot reach the soil.
- Prepare a fine, firm seedbed so germination is fast and even.
- Use treated seed for beans and sweetcorn where the problem is chronic.
What it goes for
Sweetcorn, beans, peas, cucumbers, squash, onions and spinach — any large seed sown into cold ground.
Who hunts it for you
- Ground beetleCarabidae Fast, glossy night hunters that patrol the soil surface for slugs, cutworms, and root-fly larvae. Give them daytime cover — thick perennial edges, stepping stones, a log — and skip the tillage that wrecks their burrows. Grow to recruit it: Lavender,Mint,Thyme
- 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