Your situation is important - carrot fly is more likely on an allotment than in a small veg patch in a garden - according to a book I've got by Arthur J Symons called The New Vegetable Grower's Handbook ( Symons was the technical adviser of the Dig for Victory campaign during World War II). I grew Nantes early as a variety last year, planting them in early May in a bed that did have some direct sun but was also partly shaded by a high fence and a hedge. They were under my homemade poly tunnels for 6-7 weeks to start with as I live in a northerly area. They did really well and I had no probs with carrot fly, but I have a veg plot on my garden and all the other gardens around me are shrubs and flowers so I think this helped. I also picked them as bunches of smaller carrots and did not let them reach full maturity. To quote Symons:
"This pest is most likely to be met with in large gardens, on allotments, and in districts where lots of carrots are grown year after year. Where a small crop is grown in a private garden here and there it may never be met with at all."
He goes on to say: "The first step in keeping off the carrot fly is to keep down weeds and as far as practicable deprive it of its spring hiding-places. We have seen already that the disturbance of the soil when thinning is favourable to the operations of the carrot fly. You should therefore not only make the soil firm after thinning, but also water copiously in order to consolidate the earth around the remaining roots. Never thin carrots if the soil is dry. To do so would lead to the breaking of foliage and roots during thinning and the carroty odour is bound to attract a carrot fly or two."
He adds: "Growing onions alongside is said to deter the carrot fly which is confused by the stronger onion smell. Carrot fly is rare in organic gardens. Possibly, with so many odours arising from the garden compost used in such gardens, the smell from the carrot bed is too weak to attract it."
Hope this helps - I'm trying F1 Eskimo this year as a variety for carrots hoping to leave them in from October and just pulling them when needed. Has anyone out there tried this variety before and if so how did you get on?