Carrot rust fly

Two main flights, in spring and again in late summer to autumn, October and February to April.

Carrot rust fly is a small fly whose maggots tunnel into carrots, parsnips, celery and parsley, leaving rusty brown channels through the roots. The flies find crops by smell, so prevention and a good barrier matter far more than any spray.

How to identify

• Glossy black flies 5 to 8mm long with yellow legs around the crop
• Rusty brown tunnels and channels through carrot and parsnip roots
• White cottony maggots in the channels when a root is split
• Stunted plants with reddish, bronzed or yellowing foliage
• Forked, distorted or bulbous roots

How to prevent

• Surround the bed with a fine mesh barrier at least 60cm high, since the flies fly low
• Sow thinly so you avoid thinning, which releases the carrot scent that draws flies
• Rotate carrots and parsnips so larvae in the soil do not carry over
• Plant near onions, garlic or leeks to help mask the carrot scent
• Lift carrots before late autumn rather than leaving them to be tunnelled over winter

How to control organically

• Cover the crop with fine mesh or cloth as the only reliable control, since no organic spray works on the maggots in the root
• Thin in the evening, water well after and remove all thinnings from the garden
• Sow resistant varieties such as Resistafly where available
• Lift and dispose of badly infested roots rather than composting them
• Time later sowings to dodge the main flight periods