Have you noticed masses of webbing on the ends of tree branches in your yard or along the roadsides? They are the work of the fall webworm, a species of caterpillar native to our region. Fall webworm outbreaks occur every year in our area and are most noticeable in late summer and fall. The good news is fall webworms rarely cause serious damage and in most cases there is no reason to do anything about them.
Where Do They Come From?
Fall webworms are native to much of North America and are one of the few insect pests that has been introduced from our continent to other parts of the world. The caterpillars that are currently feeding on trees in our area hatched from eggs laid by adult fall webworm moths, which are snow white and approximately 1 ½” long. These caterpillars will feed for four to six weeks, then leave the host tree to spin a cocoon in which they will spend the winter. Next spring adult moths will emerge from these cocoons and mate, after which the females will lay eggs, beginning the cycle all over again.
Fall webworms are sometimes confused with Eastern tent caterpillars, which only occur in the spring and are most common on wild cherry trees. Eastern tent caterpillars form their webs near the trunk of a tree, usually where a branch meets the trunk. Fall webworm webs are formed at the ends of branches and do not appear until mid summer. Fall webworms are also sometimes incorrectly referred to as bagworms, a species of caterpillar that feeds on cedars, arborvitae and other conifers. Bagworms do not make large masses of webbing. Instead, each caterpillar spins its own sack of webbing and plant leaves, in which it hides while feeding.
What Will They Eat?
Fall webworms have one of the widest host ranges of any insect and are capable of feeding on just about any deciduous tree species. In our region, sweet gum, persimmon, and pecan are favorites. This year fall webworms have been noticed in higher numbers and on a wider range of trees than usual, including dogwood, wax myrtle, redbud, and bald cypress.
The mass of webbing spun by fall webworms is known as a nest. Each nest can contain hundreds of webworms. The webworm caterpillars within a nest all hatched from the same mass of eggs laid by a female fall webworm moth. The caterpillars feed together for several weeks, expanding the web as needed. Nests can expand to three feet across or more. Fall webworms feed within their nest until they reach full size, at which time they crawl out of the nest, and usually away from the tree, to form a cocoon. Caterpillars from one nest will not crawl to other trees to form new nests.
Will They Hurt My Tree?
While the webbing and debris created by fall webworms looks alarming, their feeding activity rarely causes serious injury to trees. Webworms only damage tree leaves and do not kill the branches upon which their nests form. These branches will grow new leaves next year so there is no need to cut branches out of a tree to remove the nests. Nests will naturally weather away during winter months.
Established trees can tolerate losing a considerable amount of foliage, particularly in late summer and fall. The injury caused by fall webworm feeding is considered cosmetic, only affecting the appearance of the tree, not the tree’s health. In most cases there is no need to do anything about fall webworms. The exception is young, recently planted trees which can be completely defoliated by webworms. In this situation it is usually beneficial to treat or physically remove webworms before significant leaf loss occurs.
What Can Be Done?
Fall webworms have many natural enemies, including spiders, birds, and parasitic insects. Pulling webs open with a stick exposes the caterpillars to predators and will help reduce their numbers. Nests can be removed by pruning out the branch upon which they are found. This should only be done if caterpillars can be seen actively feeding in the nest and if the branch can be pruned without disfiguring the tree. Removing the web is not necessary for tree health. Removing empty nests does not prevent future outbreaks because this insect overwinters in a cocoon in the soil, not in the webbing.
In the case of young trees that are in danger of being completely defoliated, insecticides can be sprayed on the leaves to control this insect. Any product labeled to control caterpillars will work. Products containing neem oil, B.t., or spinosad are organic options. When treating, first check if caterpillars are still present in the webs. Often, by the time nests are noticed the caterpillars have already left, making treatment unnecessary and ineffective. If caterpillars can be seen inside the webbing, target treatment to the foliage directly adjacent to the nest. There is no need to spray the entire tree. Because most insecticides are highly toxic to bees, wait until late evening to spray when bees have returned to their hive. Never spray when bees are active in the area.
Learn more about fall webworms from these Extension fact sheets:
- NC: https://www.ces.ncsu.edu/depts/ent/notes/O&T/trees/note46/note46.html
- IFAS (Florida): http://edis.ifas.ufl.edu/in878