???
Jim,
the fly is active in 'June'? If you plant early the plants will be strong enough to resist an attack. Anyway, I puff around the base of each plant with 'Sybil' - HORROR - it's been banned by the European Thought Police.
If 'derris' is still allowed - use that. Good luck!
It is said that an inch of rhubarb in the bottom of the hole does the trick, never tried it tho.
A little washing up liquid mixed with water and then sprayed onto your brassicas should do the trick.
Mick,
Cabbage ROOT fly here!!!!!
:D