How to identify and deal with Cabbage root fly

Cabbage root fly

Delia radicum

White maggots eat the roots of young brassicas, which wilt blue-tinged and collapse. Fit collars around stems at planting and grow under insect mesh.

Insect Root feeder Serious SpringSummerAutumn

How to know it

A gray fly much like a small housefly, seen only if you are looking for it. The damage is done by white legless maggots up to 8 mm long, packed around and into the roots just below the soil surface.

What you'll see

A newly planted brassica that wilts in the sun and does not recover, with leaves tinged blue-purple.
Plants that lift out of the ground with almost no root left on them.
White maggots and brown tunnelling around the root collar when a failing plant is dug up.
Stunted, blue-green plants that sit and sulk while their neighbors grow away.
Tunnels in swede, turnip and radish roots, spoiling them for the kitchen.

Where it comes from

Two or three generations a year, the first flying from late April as the first transplants go out. Eggs are laid on the soil surface right against the stem; the maggots feed for three weeks, pupate in the soil, and the last generation overwinters as pupae in the bed.

Transplants are the vulnerable stage — a plant with an established root system usually survives what kills a new one. Warm springs bring the first flight forward, straight onto the newly planted crop.

Dealing with it

Fit a collar — a 12 cm disc of carpet underlay, cardboard or felt — snug around each stem at planting so the fly cannot lay against it.
Grow the crop under fine insect mesh from planting to harvest.
Firm transplants in well and water generously; a strong root system survives light attack.
Pull up and bin failing plants along with the maggots in their roots.
Plant a little later, after the first flight, where the pest is chronic.

Keeping it out

Rotate brassicas properly — the pupae are in the soil where the crop stood.
Raise plants in modules to a good size before setting them out.
Keep collars on until the plants are well established, then remove them.
Encourage ground and rove beetles with permanent edges and mulch; they eat the eggs at soil level.
Clear old brassica stumps and roots at the end of the season.

What it goes for

Roots & tubersSeedlings

All brassicas — cabbage, cauliflower, sprouts, broccoli, kale — plus swede, turnip and radish.

and 5 more in the catalog

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.
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