and, more to the point, how do I deal with it and will I get any heads forming?
There's a lot of small moth-like insects (white fly?) but I can't see any flea beetles (unlike on the rocket, which is very pretty indeed, but growing it in a bathtub and keeping it wet seemed to keep them to reasonable levels)
I can't see caterpillars. I netted the young cabbages to deter the pigeons and have just added a second layer of netting to exclude the butterflies - but something must have got in!
I've just planted out the broccoli, so I'd like to sort out this problem before I lose that...
What is making my cabbages holey?
I almost lost my entire crop of brassicas this year to cabbage white fly - on underside of the leaves, small dusty white patches, tiny white moth-type insects fly off if you tap the leaves. Unless you rid your leaves of them you will very shortly get to see a whole lot of caterpillars... If you are organic a dilute spray of water and Ecover (washing up liquid) will do the job, but you need to keep at it as they are determined little blighters... Once obliterated (keep monitoring for returns) they should recover and deliver a crop, though they can look a little shabby if not dealt with early on. Net for pigeons essential in my experience! Good luck...