WHAT IS USED TO CONTROL CABBAGE ROOT FLY

The best organic way is to use offcuts of old carpet, felt or something similar. Cut a small hole in the centre (for the cabbage stem to grow through) and a slit to it and place on the ground around the plant

You could also try planting with Nastertiums to deter the flies. This is used as a sacrificial plant, and can be destroyed when the pests have finished landing on it. (sorry, can't think of a better way to explain - maybe someone else can do it for me, lol!)