Your trees are safe. Those are fall webworms, not tent caterpillars. Whereas tent caterpillars arrive in the spring and make nests in tree crotches, fall webworms build web colonies at branch extremities in late summer/early fall. Fortunately, fall webworms are the caterpillars of a native tiger moth and co-exist with native trees. There are more than 75 species of predators and parasites that have evolved to keep webworm populations under control. Break up any nests you can reach with a long pole, so the predators can get to the webworms. You can prune off low webs but, obviously, you're removing part of your tree. Only small young trees really need this attention. Having carried on photosynthesis all summer, mature trees can tolerate losing a few leaves to caterpillars this late in the season.