It could be a small beetle as they do that type of damage. There are also some leaf diseases that cause round dead spots that then fall out of the leaf. It these are present you should see some other spots that have not yet fallen out of the leaves. I would not worry about either problem now. Fall is coming and there is not need to spray. The loss of leaf area is minor and the trees can take a lot of loss without any detrimental effects. Also the leaves will soon be falling anyway.
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