Posted: 20/08/2016 at 13:12
If you've got nothing else you're desperate to grow in that area of garden, I'd leave them there - pick off every single caterpillar and keep checking for any late hatchings. Use debris netting to protect the plants and make sure it's higher than the plants so the butterflies can't lay eggs on the leaves through the tiny holes.
If the autumn weather is kind to us they'll put on some more growth, and as the broccoli spears grow out of the joints between the leaves and the main stem, you may well get a reasonable crop.
I once had some sprouting broccoli on an allotment that were so damaged by horrendous weather and the depredations of wood pigeons that I was going to pull them up, but being a busy young mum I just didn't get around to it.
The neighbouring allotment holders' broccoli was in a similarly terrible state, and they all pulled theirs up.
By the time of the Village Horticultural Show in April, my sprouting broccoli had recovered so well that it was cropping well and won second prize in the show - it was the only entry 'cos it was the only broccoli in the village