a gardening program suggested wood ash round the base, when you shake the bush they can't climb back up.
BTW, my hens won't touch them.
I used to use mild washing up liquid solution, they would all start wriggling so you could see them, then drop them in a bowl of soapy water, however it is kinder to make it a quick end, we may hate them, but it isn't their fault they are sawfly caterpillars

Look for pin pricks in the leaves. hopefully you can catch the whole batch under one leaf.