You may be referring to fall webworm. http://extension.umd.edu/hgic/fall-webworm-trees
There are predators and parasitoids that attack and kill fall webworm. You can open the webbing with a pole pruner and expose the contents to birds. The webworms rarely consume enough leaves to affect growth. Larvae are or will be getting ready to pupate and no chemical control is recommended. If this is not what you are noticing send us photos.