Early in the year, my Japanese acer which is in a large pot, looked ill. Many of the leaves seemed to be partially eaten; in fact, some stalks were quite bare. It reminded me of when my gooseberries were attacked by sawfly so I sprayed the acer with the same chemical. It picked up and did well all summer. I then went away for five weeks ...end of August to end of September. Upon my return, the acer was completely bare of leaves and there is evidence of 'nibbling'. It's a lovely pink and green leaved tree normally and I don't know how to help it.
I would use Neem Oil to treat the pest issue.
Neem Oil is organic and safe for people, pets and bees.