I have a little Japanese Maple which I have dwarfed, going on about 5 years now, and it gets attacked almost every summer by something that eats a remarkably circular half-circle out of the leaves.

The eaten-out area is always some fraction of a half-circle and the chomped edges are remarkable for the smoothness and curviness of the bite marks.

Wondering what's doing this, and how to deal with it. It doesn't affect the surrounding plants, it's located in a Japanese style mixed-planting with lots of plants around it, all of which are doing well (including this one, except for the leaf eating).