Plants affected
Carrot, parsnip, parsley and celery.
Symptoms
Rusty brown scars ring carrot roots, making them inedible,
and susceptible to secondary rots. When the roots are cut
through tunnels are revealed, often inhabited by slender creamy
yellow maggots up to 9mm long.
Cause
The maggots hatch from eggs laid by the carrot fly. Newly hatched larvae feed on the fine roots but later bore into the tap roots. The brown scars are where tunnels near the surface have collapsed. Two or three generations of carrot fly can occur between May and September, with the pest overwintering as larvae or pupae.
Prevention
Sow sparsely to avoid thinning the seedlings. Female carrot flies are attracted by the smell released when surplus plants are removed.
Late sown carrots (after mid-May) avoid the first generation of this pest; similarly carrots harvested before late August avoid the second generation.
Protect vulnerable crops by surrounding them with 60cm (2ft) high barriers made of clear polythene to exclude the low-flying female flies or cover the plants with a horticultural fleece, such as Enviromesh. It is essential to practice crop rotation with these methods, otherwise adult carrot flies may emerge within the protected crop from overwintered pupae in the soil.
Choose carrot cultivars that are less susceptible to carrot fly, such as 'Fly Away', 'Maestro', 'Resistafly' and 'Sytan'.
Chemical control
None of the insecticides currently available to amateur gardeners is approved for use against this pest.