Looks like Hemileuca maia, but I'm not sure about the food plant. It was on a hickory tree, but this specimen was on the underside of a large poison oak leaf, (seemingly) growing out of the tree trunk at eye level. However, I saw similar caterpillars higher on the tree. I would expect to see them on oaks (oak trees, not poison oak :P), but Wagner says later instars are less picky.