Chewing insects that show up late in the season are annoying because they make our plants look bad. Luckily, they do no lasting damage. Leaves are the food factories of our plants. If chewing insects attack in May or June, the amount of food that plant can make for itself is greatly reduced, leading to stress and possible decline of the plant. Usually by the end of July or early August, the leaves have produced most of the food they will make for the season. So when caterpillars and other chewers show up in August or September the damage is mostly cosmetic. This means that there really is no reason to spray any kind of insecticide on late feeding chewers. Take the green approach and don't spray in this case.