Cabbage root fly
White legless maggots eating the fine roots and tunnelling into the swollen root of a brassica, leaving the plant blue-tinged, stunted and wilting in sunshine even though the soil is moist. On radish, turnip and swede the tunnels run through the part you were going to eat. It is the commonest reason a transplanted brassica sulks and dies in June while everything around it grows away.
Caused by: Delia radicum · also called root maggot, brassica root fly, maggots in the roots
What brings it on
Eggs laid in the soil surface against the stem, in two or three generations from late April onwards. Transplants are hit hardest because the fly finds a lone stem in bare soil easily, and the first generation coincides almost exactly with the traditional main planting-out date.
source
Sourced guidancehigh confidence
- Royal Horticultural Society: Cabbage root fly
- Agriculture and Horticulture Development Board — AHDB: Brassicas — cabbage root fly
Checked 2026-08-13
Usually seen: April, May, June, July, August, September
Preventing it
Ordered by how much difference each measure actually makes, not by how often it is repeated.
- A physical barrier is the whole of the control that works: insect-proof mesh over the bed from the day of sowing or planting, sealed at the edges, or a 12–15cm collar of card or roofing felt laid flat on the soil around each transplant so the fly cannot reach the stem base.
source
Sourced guidancehigh confidence
- Royal Horticultural Society: Cabbage root fly — non-chemical control
- Garden Organic — Garden Organic (Henry Doubleday Research Association): Brassica collars
Checked 2026-08-13
- Rotate brassicas onto fresh ground and lift and destroy the roots of finished plants, since the pupae overwinter in the soil where the crop stood.
source
Horticultural consensusmoderate confidence
- Agriculture and Horticulture Development Board — AHDB: Brassica rotation and pest carry-over
Checked 2026-08-13
Dealing with it once it is there
- Nothing reaches the maggots once they are inside the root. A wilting plant that turns out to be infested should be lifted and removed with its roots rather than watered, and the bed protected before the next sowing — which is the only decision still available.
source
GrowingHQ editorial positionhigh confidence
The management field would otherwise imply a rescue that does not exist for a root already tunnelled.
- GrowingHQ editorial synthesis — GrowingHQ
Checked 2026-08-13
Widely repeated, and not supported
Recorded so GrowingHQ can address them. These are not alternative approaches — they are things the evidence does not back.
“A wilting brassica in moist soil needs more water.”
Watering a plant whose roots have been eaten adds to the waterlogging and hastens the secondary rot. Lift one plant and look at the roots before deciding anything about water.
Horticultural consensusCommonly said, not supportedmoderate confidence
- Royal Horticultural Society: Cabbage root fly
Checked 2026-08-13
What it affects
On sprays and treatments
GrowingHQ does not tell anyone what to apply, at what rate, or how often. Pesticide approvals differ by country, change without notice, and the product label is legally binding — it is also the document you are holding. What is described here is cultural and physical control, which is what a home grower can act on without a label in front of them.