Wildlife Spotting

While walking in a state park I came across these strange creatures covering a young sapling. At first glance I took them for wholly aphids but I soon realized they were much larger and behaved differently than the wholly aphid I am used to. Their torsos seemed to not move at all, they did not walk around yet the fluffy white tufts that appear to be like tails moved back and forth and it sped up if they sensed movement as when I blew on them. Other times they sat perfectly still.