It took me a while to identify this caterpillar. My first reference is "Moths & Caterpillars of the North Woods" by Jim Sogaard. This is a great book with matching photos of most moths with their larval forms. Although the white-marked tussock is supposed to be common in the eastern United States and Canada it was not included in "Moths & Caterpillars of the North Woods". So I had to do some searching on the Internet and that is where I found several pics that helped to ID it. This caterpillar seems to feed mostly on tree leaves, like birch, oak, apple cherry, fir and spruce. There are typically two broods each year in the northern part of their range with the second brood over wintering in the egg form.