Looks like that leaf cutting bee that attacks roses as well, they make nests with the pieces, saw one once.

Thanks Lizziebee, that will be what i saw then. Does it do any harm to the tree?

No, I don't think so,maybe they find the leaves to be the right texture, and easy to cut.

I found a few Vine Weevils on my Japanese Weevils over the past few days. They certainly make the same segmental chinks out of leaves. It could also be leaf cutting bees, my firs thougt is Vine Weevil though.

I think the bees though, as they do make those very round holes, whereas vine weevils holes are usually more irregular.

I'd say leaf cutter bees too and given that we want the bees to survive and the bees aren't actually harming the Acer I'd say live and let live.

Yes, let the bees live--they aren't doing as much damage as you might think, and they pay for it ten times over in pollination.