From what I've heard, putting a 6"x6" barrier like a piece of old carpet, or rubber mat (or even cardboard, I'm sure) snugly around the base of each plant as you put them in the ground supposedly stops the flies laying their eggs close enough for the maggots to reach the roots. I've tried it, but I don't actually know if it worked or not: my cabbages were fine, but then, I've only ever had attacks on my turnips (which were just too labor intensive to put a barrier on each). Good luck.
I've heard the suggestion for row covering for the first 6-8 weeks of growth. I get these on almost everything, but seldom enough to kill a plant, just mangle it and slow it down. It makes my radish bulbs mostly useless, turnips usually about 1/3 useless. But my radish still grows huge with some great greens, and I still have 2/3 of the turnip to eat...
I will try to get to a planting with a cover this year and see how it goes..
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