Hey, What’s That Web in My Tree?
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Collapse ▲You have probably noticed masses of webbing on the ends of tree branches in your yard, your neighbor’s 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, however they have been noted to be unsightly.
Fall webworms in a tree.
Where Do They Come From?
Fall webworms are native to much of North America and are one of the few insect pests that have 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 1½ inches 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.
Tent caterpillars nest in a tree.
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 Do 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. When small, fall webworms only eat the leaf surface, causing the remaining part of the leaf to turn brown. 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.
Bag worms nest in a tree.
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 or Kill My Tree?
As I mentioned before, the webbing and debris created by fall webworms looks unsightly, 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. 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 or long pole exposes the caterpillars to predators and will help reduce their numbers. You can use caterpillar controlling insecticides containing carbaryl or Bacillus thuringiensis. Removing empty nests does not prevent future outbreaks because this insect overwinters in a cocoon in the soil, not in the webbing.
For more information about gardening, contact your local Franklin County Cooperative Extension Office at 919-496-3344. You can also visit the website at franklin.ces.ncsu.edu.