I think many of us are obsessed these days with killing creatures simply because they exist. However, we do need to control ants, slugs, weevils, etc., etc., and caterpillars. I don't know what harm woodlice do but I do not want them entering the house so I kill them. If.pests are destroying a valuable plant or eating salad crops, etc., should we let that happen? So, Nikki if you grow your Acer for its appearance then check out what these caterpillars are and do what you want to do. It should not be made into a huge moral issue for you.If caterpillars are feeding on nettles leave them alone. Anyway, most of us now grow plants esp for butterflies, etc. and are aware of needing to strike the right balance.