Avoiding controversy and keeping to practicalities I would say that well over half of the young plants have been lost. I noticed elsewhere though that flea beetles were crowding onto the mesh covering brassicas but were unable to get through. So I think the treatment has to be mesh rather than netting. I had used butterfly netting on these plants rather than enviromesh. Replacing the netting with mesh at least prevented further waves of flea beetles.
For some reason there seem to be a lot more this year than usual at least in my patch. I wonder whether they have come off the nearby rape fields in greater quantities than normal.
Janet G