Carrot fly: how to recognise and treat it

Pest

Symptoms

Rusty-brown scars and tunnels ringing the tap roots, making them inedible.
Slender, creamy-yellow maggots up to 9 mm long inside the tunnels when the root is cut.
Damaged roots are prone to secondary rots, in the ground and in store.
Usually nothing shows above ground — the damage is found at harvest.

Water on insect-killing nematodes against the young larvae — Steinernema feltiae in spring, S. carpocapsae in summer, or a mixed product sold for fruit and vegetables.
Peel off the damaged parts before using the roots.
Lift carrots as soon as they are ready, a whole block at a time, and remove infested parsley, celery and other related plants nearby.

No insecticide is recommended for home gardens; protect the crop with complete netting, sowing dates and rotation instead.

Reference information, not a guarantee. Read and follow the label of any product: dose, safety and the days to wait before harvest.

Cover the crop completely with insect-proof netting from sowing — a full cover works better than a low barrier — and rotate the bed so flies do not emerge under it.
In Britain, sow after mid-May to miss the first generation and harvest before late August to miss the second.
Sow thinly so that you do not need to thin; the smell of pulled seedlings attracts egg-laying females.
Grow less susceptible cultivars such as 'Fly Away' or 'Resistafly', and do not leave carrots in the ground over winter.

Figures checked against these sources.