Fall webworm is appearing again in Illinois as very young, small webs. This is the start of the second generation in the southern half of Illinois and the only generation in the northern half of the state. These insects become quite noticeable in August and September with silk webbing up to 3 feet long enclosing the ends of branches and associated leaves. These leaves are eaten off by the larvae, leaving bare branches with dirty webbing attached. Although it is aesthetically unpleasing, this feeding is not very harmful to the health of the tree. Many species of trees are attacked, although walnut, hickory, pecan, crab apple, birch, and maple are the most common.
At this time, the most effective control entails pruning out the small webs that enclose the larvae. If insecticide sprays are used, use enough pressure to penetrate the water-resistant web. A succession of new colonies is normally produced from now through August and sometimes into early September. Scout weekly to identify and treat new colonies with their web nests.