GRANTS, N.M. – Recently, a reader sent a photo of their tree which had some strange webbing wrapped around some of the branches. The tree appeared to be dying from the bug infestation, and the mass of webbing left neighbors asking, “How many spiders does it take to create that?” The answer was that no spiders created that, instead, worms did. New Mexico State University – Grants branch Extension Agent Chase Elkins said that the worms appeared to be Bag or Fall Worms. These creatures are harmless to the tree, arriving with the changing season to feast on dying leaves, largely leaving the wood alone.
After cocooning, the worms turn into a white moth.
At their worst, these moths coat the tree with their webbing and eat the leaves. They pose no threat to the tree or neighboring trees. While these worms do not have a particularly favorite tree, they do enjoy resting in apple, cottonwood, and elm trees.
Elkins said that the worms can be easily to remove, but are still an eyesore, “[They] can be washed off with high pressure water.”
NMSU’s Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist at NMSU-Los Lunas Agriculture Science Center. Thompson said, “The good news is that when the populations of fall webworms increase, the populations of predators that feed on them often increases. There is a good chance that the population of fall web worms will be much less next year. However, just to be safe, begin looking for signs of the fall webworm in late July and early August next year. If you see them developing, there are several insecticides you can use at that time to stop them. One of the effective insecticides is a natural product often duced by a bacterium (Bacillus thuringiensis). The toxin from the kurstaki strain of B.t. is specific to larvae of moths and butterflies (that includes fall webworm). It paralyzes the intestines of the larvae causing them to stop feeding and die. It will not harm other insects; especially beneficial insects that help protect your plants from other pests. It will also not harm you or your pets and livestock (and wildlife) when used according to the directions on the label. There are other strains of B.t. that produce toxins to control other insects such as mosquitos or beetles.”
The worms create the webbing in trees so they can protect themselves from predators, especially birds. Being multifunctional, the webbing also protects the worms from pesticides. “You can use strong jets of water from a power washer, a long pole, or baseballs and rocks to rip open the webbing so that some of the caterpillars fall out. That may expose some to predation by birds, but is more to make you feel like you are doing something,” Thompson said.
For now, webbing in trees is not dangerous but is an eyesore. If webbing persists into the spring, it will be a sign that another pest is attacking the tree.