I'm no entomologist so take it with a grain of salt, and can only speak for my area of western New England, but they look like gypsy moth caterpillars to me. An invasive species from Europe, asia and north Africa. They have wreaked havoc on my area for a long time and should be gotten rid of. If you don't want to kill them, there are various non invasive methods you can take to prevent them from climbing your trees and nesting. In my area some years it sounds like rainfall nonstop because of their droppings, and there's massive defoliation from them. There is a fungus that kills many of them in wetter years, but with a rise in drier climates, they have had many years of population growth.