Bunch of carrots, totally free from the carrot Fly damage so often a problem
There are no chemical controls available to the amateur gardener that will treat or prevent Carrot Fly. However, there are cultivation techniques that will avoid the worst of the problem.
The carrot fly is normally active between the months of May to September, and are attracted by the scent of carrots - naturally. Anything that can be done to prevent or lower the scent, will have a beneficial effect.
Very early sowings of carrots - February to March - will lessen the affects of the worse attacks. Together with this, a sowing in June will help for it will be between generations of the Carrot Fly.
It is thought that the thinning of carrot seedlings helps to release the scent of the carrot root - attracting the Carrot Flies. After thinning you carrot seedlings - best done in evenings as the carrot flies are less active then - make sure that you firm the soil well. The carrot fly lays its eggs in loose soil!
You can also sow your carrot seeds very thinly in order to prevent the need for thinning. Crops can also be protected with horticultural fleece, which will keep the carrot fly away. Another way, is to erect a 2feet high polythene 'fence' around your carrot area. The carrot fly females are low flying, and unlikely to fly over the top of the plastic screen.
Make sure that you sow your carrots in a different area in subsequent years, for the adult carrot flies will emerge from the overwintered larvae in the soil.