Carrot fly is a major problem for gardeners not only growing their own carrots but can attack the rest of the carrot family, celery, parsnip, parsley. The main signs to look out for are leaf discolouration and holes or tunnels in the carrot.
One of the best organic methods of getting rid of carrot fly is companion planting with onions, my dad and his dad before him would always plant their carrots alongside their spring onions and firm the soil around the plants to prevent the fly from laying eggs.
Another method is covering young plants with fleece to contain the crop and prevent flies getting close, obviously this method doesn’t work as well as the season goes on because it gets far too warm to continue using fleece.
Simple checking preferably in the evening when the adult flies are less active to remove signs of infestation and any damaged plants to reduce the smell of carrot which attracts the flies. This is also a good time to thin out your plants and destroy any unused plants to prevent rotting on your plot and therefore providing a home for flies to lay eggs.
There are also a range of less susceptible varieties of carrots which I haven’t tried myself as I am a lover of the Autumn King variety at the moment but always worth a try if your plot appears particularly susceptible, maybe chat to other plot owners if on an allotment to find out if carrot fly is a problem on the site?