Carrot root fly is a pest with vertigo. A physical barrier a couple of feet high around the crops that need protecting will do the job

I wouldn't worry about parsnips. As far as your cauliflowers is concerned, the old books suggested that roofing felt or similar could be used to make soft collars. The slits would fit around the stems and expand as the plant grows. Now carrots. If you have an allotments where others grow carrots you might have a problem. I would make sure that you sow your seeds thinly so you don't have to thin out the carrots and keep the roots earthed up. If you are gardening on a small scale, then the polythene barriers or covering with very fine netting could be considered. My old allotment site was surrounded with cow parsley on which the fly spent the winter as grubs feeding on the roots.

I have used bought collars on brassicas on ground that was susceptible to root fly and they did seem to work.(But following a tip from an old allotment gardener I also dipped the roots in lime when planting out) these things together did solve the problem. I'd have thought cardboard ones wold disintegrate in wet weather? But you could make your owm from plastic or vinyl.

I never tried a collar either but I heard if you plant alternative rows of carrots and onions, the carrots will keep away the onion flies and the onions will keep away the carrot flies.They help each other. It probably would work for parsnips too.