I've noticed it, too. However, I've also noticed that my big-toothed aspen does not seem to have an infestation of aphids this year. Could the caterpillers be the reason why?

I bet they're gypsy moth caterpillars. Seems like there getting worse in NJ every year :(

Star Ledger article

If the ones I have were gypsy moth cats, they'd be devouring the tree leaves and that's not happening. The caterpillars that I have are all black.

If one or both of you could take a picture or two, I'd suggest posting them over on the Butterfly Forum. Those guys are great at ID'ing cats.