Anyone who has grown carrots will know the damage carrot root fly can cause. The fly’s maggots can kill off seedlings and also burrow into the root of mature plants leaving brown unsightly tunnels behind. Carrot root fly also affects parsnips and even celery.
Using a fleece is probably the best way to stop the root fly, especially when they are most active in April/May and July/August. However, the fleece can crush the foliage if it’s not supported. If you need to remove the fleece, do so late in the evening when the root fly is less active.
Carrot root fly generally flies close to the ground, so putting up a vertical barrier will keep them off your carrots. You can also try growing your carrots in a tall raised bed or in barrels.
Carrot root fly is attracted by the smell of carrots and its foliage. Interplanting with onions or leeks disguises the smell. I’ve personally had success in growing my carrots in a square foot bed next to onions.
Using a combination of all three methods is probably the best way of preventing carrot root fly. If carrot root fly is a problem in your area, you can also try resistant varieties such as Flyaway.