It's hard to identify the caterpillar from this angle, but it's not causing the buttonbush any harm. This late in the season, leaf damage (both from insects and any infections) is not concerning and can be ignored. Many areas have already experienced at least one overnight frost, so deciduous plant foliage will be browning and shedding very soon. When the leaves fall, you can either let them stay as habitat or rake them up if you're worried about a disease re-infecting the plant next year. (Though in that event, leaf removal is not a foolproof method of prevention, and plant diseases can be highly weather-dependent.) No treatment is needed at this point (nor would it provide any benefit applied now), and we expect the plant will leaf-out normally and grow well next year as long as its roots remain healthy.

Miri

Thank you!

You're welcome!