I have just lost all my Purple Sprouts to root fly. So went digging on google to find out all I can about the fly. I came across this method that is used by Organic growers to combat the dreaded fly.
I think I will give it a go
http://www.gardenorganic.org.uk/organicveg/innovations/show_innov.php?id=1 (http://www.gardenorganic.org.uk/organicveg/innovations/show_innov.php?id=1)
I think i will stick to my collars.