They are very bad this year. The very same question was asked this week and the only reply was from Kildermorie who suggested
"Putting some black bags around the bushes then shake the plants or using a hose to wash them off then roll up the bags and bin them. I have also noted lots of green caterpillars recently! Birds do eat them".
Good luck.

Same problem - skeletal brocolli and kale! From what I can discover, the only real way to deal with it is to keep the butterflies away to begin with - fine mesh around the brassica patch from about May/June onwards?

To deal with it at this stage you'd have to get rid of the visible caterpillars *and* the un-hatched eggs - an almost impossible task!

I've given up for this year - next year Death To Cabbage Whites!!