As far as I know carrot fly does not attack radishes. I think cabbage root fly though will do this. Are your carrots ready yet? Can you see if they are ok? Growing rads and carrots under fleece is the answer. Or, growing near runner beans. On one side and a low hedge on the other...as I do.... Usually! This year I got complacent with one batch of carrots nd decided not to fleece and, hey presto!, I have some carrot fly. I use fleece to cover all veg seeds til they are 2 or 3 inches high, eg beet root, spring onions, lettuce, peas and carrots