However seeing the almost stripped tree they were feeding on in person was still a shock:
I had tentatively identified them as codling moth larvae, which didn’t quite make sense since these typically grow and feed inside fruit, not on leaves. My revised identification is redhumped caterpillar. I just sent an email to Maine Cooperative Extension to see what they think.
The bright side is that only one tree appears affected, and there seems to be a fair chance that it will recover.