Two color races of fall webworm (Hyphantria cunea) coexist throughout the species’ range. The race with pure white moths and black-headed caterpillars predominates in the northern part of the range. The southern portion of the species range is dominated by the race with spotted moths and red-headed caterpillars. (Photo credit: Gerald J. Lenhard, Louisiana State University Agricultural Center, Bugwood.org)

By Meredith Swett Walker

In late summer and fall, extension agents and pest control professionals across North America get a version of this call: “It looks like a gang of hyperactive spiders have set up a commune in my pecan/Bradford pear/cottonwood tree. There are webs everywhere! How do I get rid of them?” More curious tree owners who inspect these webs will find that they do not contain spiders but rather masses of hairy caterpillars that twitch in synchrony when disturbed. They are the larvae of the moth Hyphantria cunea, better known as the fall webworm, and a new guide to managing them is now available in the open-access Journal of Integrated Pest Management.

Meredith Swett Walker

The good news is that, despite their voracious appetite for leaves, these caterpillars do not seem to cause significant harm to otherwise healthy trees. So, fear not for your landscaping. But fall webworm nests are unsightly and many property owners are anxious to eliminate them. Thankfully, this insect has been a target of pest control professionals since the late 1920s, and there are many effective control methods available. Louisiana State University entomologists Timothy Schowalter and Dennis Ring review these control methods, as well as fall webworm biology, in their JIPM profile of the species.

The fall webworm is a “remarkable species,” says Schowalter. The moth ranges from southern Canada, throughout the U.S., and into northern Mexico. Along with its broad geographic range, the fall webworm has an “equally wide breadth of plant hosts, indicating an extremely flexible physiological range of tolerance to … host chemistry,” says Schowalter. There are two color races of fall webworms which co-occur throughout its range in varying frequencies. The race with pure white moths and black-headed caterpillars is more common in the northern portion of the range, while in the southern portion moths with spotted wings and red-headed caterpillars predominate.

The species is native to North America and typically doesn’t cause problems in forests because native predators, parasites, and pathogens keep populations in check. However, in parts of Europe and Asia, where the species was accidentally introduced, it has become an aggressive invasive. Fall webworm infestations can reduce productivity in commercial orchards but are rarely a problem in well-managed orchards that are treated for other insect pests. The fall webworm is most frequently a problem for homeowners who find that their webs make ornamental trees less ornamental.

Fall webworm eggs are laid on the underside of leaves. These newly hatched larvae were found on a redbud tree in Baton Rouge, Louisiana, before they had constructed a web. (Photo credit: Timothy Schowalter and Dennis Ring)

Fall webworms will feed on virtually any hardwood tree as well as some conifers. Female moths lay their eggs on the undersides of leaves. After hatching, caterpillars build a web that encloses them and the leaves that they are feeding on. They enlarge the web to engulf more leaves as they grow. Fall webworm webs typically enclose the ends of branches, as opposed to branch bases like those of tent caterpillars. The webs help the caterpillars thermoregulate, and fall webworms like it hot. Researchers have found that inside the web, temperatures may reach 50 degress Celsius (122 Fareheit) and the caterpillars tolerate it just fine.

But the main role of the webs is presumably to protect the caterpillars from their many predators and parasites. Birds, spiders, and predatory insects like wasps will sometimes raid the web to eat the caterpillars inside. Webworms are also hosts for at least 50 species of parasitoids and are susceptible to microbes like Bacillus thuringiensis (Bt), baculovirus (NPV), and granulovirus (GV). These vulnerabilities create a range of options for control of this pest.

Fall webworm webs typically enclose the ends of branches, as opposed to branch bases like the webs of tent caterpillars. Webs protect caterpillars from predators and parasites and help them regulate temperature. (Photo credit: Timothy Schowalter and Dennis Ring)

The simplest method may be to rip open the web, exposing the caterpillars to their natural enemies like birds and wasps. Pruning off the webbed branches is also very effective, particularly if the infestation is caught before it gets too large. However, some tree owners may not like the gaps in foliage that pruning may cause.

While many of the biological control agents utilized in the control of the invasive gypsy moth are also effective on fall webworms, homeowners may prefer a more immediate solution. Schowalter and Ring list several synthetic insecticides that can be used to control webworms, but they say that microbial insecticides such as Bt are most cost effective. Both microbial and synthetic insecticides work best if applied when caterpillars and colonies are small.

But, if you’re willing to live with some webs, your trees can likely tolerate them as well. If not, Schowalter and Ring’s review will be an invaluable resource for pest management professionals, orchard managers, and extension agents working to control fall webworms.

Meredith Swett Walker is a former avian endocrinologist who now studies the development and behavior of two juvenile humans in the high desert of western Colorado. When she is not handling her research subjects, she writes about science and nature. You can read her work on her blogs Pica Hudsonia and The Citizen Biologist or follow her on Twitter at @mswettwalker.