I use armillatox see here;

http://www.thegardenersalmanac.co.uk/Data/Brassica%20planting/brassica%20planting.html

Do you grow your cabbages in pots before planting out, or sow seeds where they will grow?

I was told that in areas where root fly is a problem, the stronger the plant you put out, the less the chance of the fly being able to burrow into it.

I always grow in batches of 6 in 6 cell compartments anyway, plant out when 4 pairs of leaves, and use netting over a cloche to keep things out.

Though I will try the armillatox as well this year in open ground to see what happens.

An old gardener told me to dip the roots in lime when planting out and to surround each plant with a ring of it.
Or come to think of it, maybe that was for club root - sorry, old age clicking in again...
The collars do work though and only take a few seconds to apply. You can make your own too.