The best time to control these is before they infest the rotos, once in the roots they are very difficult to do anything about. These are the larva of a small fly that lays its eggs in the soil near the plants roots and when the larva hatch they invade those roots. Floating row covers will help keep the adult flies from the plants as will some kind of impervious barrier (some sources say a roofing shingle) around the plant as a collar. Some control of the larva might be obtained with parasitic nematodes.
But if these larva have invaded the plants roots there is not much you can do except destroy the plants.

May be too late now. I have more trouble with my early brassicas than with the one's planted after mid April. So the early transplants get coverd with a row cover. Tom