These are, interestingly enough, not moth larvae as you might expect. Rather, they’re the larval form of Craesus latitarsus, the Dusky Birch Sawfly, which looks like this in adulthood (photo from BugGuide.net):
I first encountered these creatures as tiny green caterpillars fringing the edges of leaves all over one particular tree, as seen in the top picture. (I don’t know my trees very well, but it must be a birch; these caterpillars prefer gray birch specifically.) The young tree stood in a grassy field, one of the areas where the NWR’s forests are working on taking back the paved lots that were once scattered throughout this area. While many of the leaves had some caterpillar presence, and a few were completely eaten, the tree still had plenty of foliage.
Was I ever in for a shock when I returned to that tree about ten days later. It had been nearly stripped of leaves, starting from the top down; the lower third or so still had some foliage, but was covered in the caterpillars you see in the lower two pictures. They had grown considerably larger- only a few could fit on each leaf- and taken on more of a yellow color. I was able to find a few other trees in the area that harbored a few of these larvae, but none with the same density and voraciousness of infestation.
By the next week, the tree was completely bare- not a single leaf or caterpillar in sight. Usually, these larva don’t completely defoliate the trees they infest, but I guess this was an exception! With the food supply exhausted, I assume they’ve now all retreated underground to pupate for the winter, as they do.