Growing up, I never noticed these. (But then, the older I get, the more I realize there were many things I didn’t notice in my younger years.)
It was when I moved north that I first noticed web worms in the trees as autumn approached. After we moved back to Georgia, I began to see them everywhere, beginning in late summer. This year they appeared in July and are still going strong.
Web worms are actually moth caterpillars. Although they can defoliate tree limbs, they do no real damage to their host trees. They just look terrible. Over the winter months the “webs” or nests will weather away.