Take action against carrot fly before they find your newly-germinated carrot seedlings and lay their eggs in the soil around their necks. Those eggs will hatch out into the larvae which do all the damage, eating holes in the roots which are then vulnerable to secondary infections and rotting, ruining your crop. If you can stop the adults laying their eggs in the first place, though, you’ve solved the problem.
Several varieties of carrot are resistant to fly damage from the outset: if you grow ‘Resistafly’ or ‘Flyaway’ carrots you’ll find you have little problem. For other types of carrot, though, you’ll need to protect your crop.
There's just one useful thing you need to know about carrot fly: they can't fly much above 50cm. So a 60cm-high barrier around the crop made of horticultural fleece or insect-proof mesh, available off the roll or in easy-to-use packs from our garden centre here in Bitton and Warminster, is all you need for season-round protection.
Stick 1m-long bamboo canes or wooden posts in the ground around your carrot bed, then staple your barrier onto them and tuck the bottom edge into the soil so that nothing can wriggle underneath. Job done: maggot-free carrots till the end of the year.