Webworms seem to be the worst in memory, but no worries
According to a recent informal survey of just about anyone and everyone who lives in the Ozarks, webworms are the worst in memory. You can almost hear them collectively chewing (some cue the Jaws theme song) their way into the first day of fall (Sept. 23 officially). While these tiny caterpillars are not on every tree, they are widespread and seemingly all-consuming, likely because of extremely wet conditions in many areas of the Ozarks and indeed the Midwest.
Fall web worms are different from the black-headed spring variety of web worm, or even tent worms that also emerge in spring as a white moth in their adult stage, which typically emerges in late April and May.
Appearing in late summer, these unsightly masses of of useless silk are home to Hyphantria cunea. Similar to the eastern tent caterpillar, which prefers to spin its equally unattractive nests in the crotches of tree branches rather than at the ends, fall webworms are caterpillars about an inch long with pale yellow coloring. They will spend the duration of their larval state inside their nests, feasting on the encased leaves. As the leaves are devoured and dry out, the nest will be expanded to accommodate fresh foliage. Almost any caterpillar is a voracious eater, but we usually see them one at a time instead of by the thousands in one giant colony.
The fall webworm feeds on just about any type of deciduous tree, munching on the leaves until the branches or even the entire tree (see below) are defoliated. Worldwide, there more than 600 species of web worms, who together they seem willing to chow on almost any deciduous leaf. In the U.S., pecan, walnut, American elm, hickory, fruit trees, and some maples are preferred hosts; in some areas persimmon and sweetgum are also readily eaten. Further west, alder, willow, cottonwood and fruit trees are commonly used.
Don’t confuse web worms and bag worms. The latter spin their “bags” as cocoons on juniper, arborvitae, spruce, pine, and cedar, and occasionally attack deciduous trees (those that lose their leaves each fall). Indeed, bagworms can be deadly to a tree if not treated. But that’s another column.
Meanwhile, webworm “tents” will soon break apart, dropping future generations of moths to the ground where the pupae will overwinter in bark and debris. Adults will emerge in late spring, laying hundreds of eggs on the underside of leaves, and the cycle begins again. If you’re a feeder of birds, you will be less likely to have the infestations as they are a high-protein food source for those birds that prefer worms, caterpillars and insects to seeds. Wasps will also eat the tiny caterpillars.
The caterpillars mature in about six weeks. During that time, some smaller trees may be covered entirely with multiple nests as the caterpillars dine aggressively. In some cases defoliation may be extreme, but lasting damage to trees is rare. If only the web and worms had some value. If you’re a classroom teacher in the sciences, you may find that your students will enjoy and learn from watching these tiny critters in action, just as other moths and butterfly larva feed on host plants in the butterfly house at the Springfield Botanical Gardens.
Eventually the nests break apart on their own as webworms prepare to overwinter and leaves may regenerate. Removing them will quickly restore beauty to the landscape and reduce future outbreaks.
The challenge in removing web worms is reaching them, which is nearly impossible when they are high in trees. If you can reach them, use a rake or long pole to pull down the webs and destroy the webworms by hand. The pest can also be eliminated using biological methods by tearing a hole in the delicate sack and allowing natural predators like yellow jackets, paper wasps and birds to feast on the caterpillars. The webs are not predator-proof, so with a little help, Mother Nature will unleash her armies.
Insecticides will also control webworms; a light coating of appropriate insecticidal spray may be applied to the nest. As the caterpillars move within the nest, they will come into contact with the insecticide without the need to overspray the site.