As our summer kicks off to a humid, intense start, a pest that has been absent since 2019 seems to be returning with a vengeance. Despite their name, Fall Webworms (Hyphantria cunea) tend to appear in early summer, increasing in number and activity, sometimes persisting until fall. These defoliating caterpillars create intricate web networks that expand as generations progress. The caterpillars consume the leaf tissue between the veins within the safety of their webbing, leaving behind frass (insect waste) throughout their webbed home, as shown in the photos above.
Fall Webworms will uniquely attack almost any tree, and can appear in pecans, mulberry, oaks, and even sycamores.
The damage caused by these worms is generally just aesthetic, and they rarely pose a true threat to your tree’s health or safety. However, in great numbers, they can turn your tree into a ghastly skeleton. In this PSA, I wanted to discuss these caterpillars and how they relate to Austin’s Urban Forest currently.
For general information about Fall Webworm, see this article from Texas A&M AgriLife Extension:
https://texasinsects.tamu.edu/fall-webworm/
These Caterpillars are on my Tree, Should I Do Something?
While Fall Webworms are rarely considered a major pest, they do defoliate trees and cause stress and loss of energy. Most healthy trees will have no issue with this, but if your tree is already struggling from any of the environmental disasters of the past four years (such as Winter Storm Uri, Winter Storm Mara, or extreme drought), this additional stressor may be quite damaging. If you have an already stressed tree, it would be worth taking action to prevent webworms from adding further stress.
Another reason to take action is if you find them particularly gross or are concerned with the aesthetics of your tree. If the sight of these webs bothers or disgusts you, it may be worth addressing the issue. One of the main reasons to own trees is to enjoy being their steward and protector. Seeing something you care about being eaten can be disturbing. If you notice these infestations starting and know it will bother you, it is better to deal with it early to prevent the entire tree from being covered in webworms.
What can be done to get rid of them?
Small Tree Solutions
For small trees, the solution is simple. Prune out the infected branches and dispose of them in your compost bin. If the webbing is on a branch you do not want to prune, use a high-pressure hose sprayer to blast the web apart and send the caterpillars flying. If you use only water, the caterpillars may find their way back to the tree and try again. Keep spraying the webs off until birds and lizards finish off the caterpillars.
Medium Tree Solutions
For medium sized trees, the best solution may be to prune out the infected branches. You can do this yourself or hire a service to climb through the canopy and remove the infected branches.
Medium-sized trees with severe infestations are also where chemical spray treatments become a good option. These treatments use water mixed with selective pesticides, fired at high pressure to burst open the webs and kill the webworms inside. There are many different spray treatment options that are organically derived or environmentally friendly, such as Bacillus thuringiensis or Acelepryn. These treatments are relatively difficult to DIY, so it is recommended to reach out to an arborist for assistance. Make sure extra care is taken if you are treating pecan trees, as some foliar treatments are not labeled for nut-bearing trees.
Large Tree Solutions
For large trees, pruning can often become cost-prohibitive. Eliminating webworms in large trees typically requires a spray treatment, soil injection, or trunk injection. Soil injections of acephate products like Lepitect can kill the active infestation and provide 30 days of protection from webworms. Trunk injections of emamectin benzoate are the most expensive and technically difficult option but will likely provide protection for the entire year. All of these options are effective for eliminating webworms and keeping your trees healthy and looking great.
TL/DR:
Fall Webworms are unsightly and can cause aesthetic damage to trees. While they rarely cause significant harm, it may be worth addressing them if your tree is already stressed or if you want to maintain its appearance.
Author: Andrew Anstrom
Board Certified Master Arborist
