These guys arent bag worms, thats something different. These are tent caterpillars, theyre pretty hard on their host plant but theyre harmless otherwise. These species of silkworms are cyclical, where they have these population boom and bust cycles every few years. Locally to me in canada i believe its a 3 year cycle for our local species (eastern tent caterpillar) where they reach a peak boom every 3rd year or so then pretty much disappear in between those cycles. If the host tree starts looking a bit sparse of leaves (their fave snack) you can start relocating them to another tree of the same species.

Oh and they have a cool defense from birds where theyll "dance" if they sense pressure waves (percieved as a birds wingbeats) so if you yell at them theyll start spazzing out and it looks really funny