These are tent caterpillars and very destructive. They can completely defoliate a branch or an entire tree. We get them in our young trees here; dh removes them ASAP to protect the trees (which took a lot of work to plant in addition to money for purchase!).
Edit: Sorry, I thought I had quoted the post I was responding to. I didn't mean the caterpillars in the original poster's pictures. Someone mentioned seeing caterpillars in white tents in the trees; those are the caterpillars I meant. Don't know what the others are, but they don't look good!!!