Fall Webworm (Hyphantria cunea, family Erebidae) nests are becoming noticeable in southwest Ohio. Their arrival seemed to be unusually late this season until close inspections revealed the nests are housing the red-headed biotype. But more on that later.
The webworms feed on the leaves enveloped by their silk nest unless they run out of leaf-food. Early instar caterpillars feed as leaf skeletonizers with later instars consuming all leaf tissue except for the petioles and coarse veins.
As caterpillars grow in size, they expand their nest by casting silk over an increasing number of leaves to accommodate their expanding appetites. The caterpillars may leave their nests in search of food if they defoliate their tree host before they complete their development.
Hairs on other caterpillars are sometimes used as defensive tools. However, the hairs on fall webworms are primarily used to help them remain suspended inside their silk nests. You can see this in the following picture. Note that the hairs fold back as the caterpillar appears to "swim" through the nest.
The common name "fall webworm" is based on when we typically see the largest nests. Female moths tend to lay their eggs on or near the nests from which they developed with webworm nests becoming larger with each generation. I’ve often wondered if fall webworm silk includes an oviposition stimulant as has been documented with mimosa webworm (Homadaula anisocentra). However, I’ve found no published research describing such an investigation.
We typically see two generations in Ohio: sometimes three. Thus, the nests reach their zenith in the fall (both astronomical and meteorological) which accounts for the common name.
Fall webworm is native to North America and ranges across the U.S. as well as north into southern Canada and south into northern Mexico. It was accidentally introduced into eastern Europe in the 1940s and is now found in most European countries. Likewise, the moths were also unintentionally introduced into Japan, the Koreas, and parts of China where it’s referred to as the “American White Moth.”
Given its wide geographical range in North America, it's not surprising that variability has developed within the species. Two forms of the caterpillars are recognized based on the color of the caterpillars as well as the color of their head capsules and the small bumps from which the hairs arise, called “tubercles,” that run in longitudinal lines along the top of the caterpillar’s body. The two forms are variously referred to as “races,” “biotypes,” or “color-forms.”
The caterpillars referred to as the “red-headed biotype” have red to reddish-orange head capsules and tubercles that range from orangish-yellow to dark red. The caterpillars are most often tawny-colored.
The “black-headed biotype” has black head capsules and black tubercles. The caterpillars are pale yellow to yellowish-green; however, they sometimes appear black with starkly white hairs.
The two biotypes were once considered separate species with the black-headed biotype called H. textor and the redheads H. cunea. They are now considered the same species, H. cunea, with the biotypes representing variability within the species.
Ohio is home to both the red-headed and black-headed biotypes. However, the historical distribution is the reverse of what’s been reported throughout the U.S. Numerous scientific papers note that the black-headed biotype is primarily found in the northern U.S. and the red-headed biotype dominates the south.
However, historically, northeastern Ohio was the dominion of red-headed fall webworms. The central and southwestern parts of the state were where we found black-headed webworms. Indeed, I never found red-headed webworms in Greater Cincinnati before 2016.
The distribution of two types of webworm caterpillars is significant because many of the differences between the biotypes that supported the “two species view” remain evident. Those differences include body coloration, the time of the season when overwintered eggs hatch, nesting behavior, and to some extent, host preferences.
In other words, it’s not only important to report what we’re observing with fall webworms, but it’s equally if not more important to note which biotype is being observed. For example, the time of the season when we first notice fall webworm nests is strongly associated with the biotype.
Overwintered eggs of the red-headed biotype tend to hatch much later than the eggs of the black-headed biotype. The lag time may be as long as 4 weeks. Before 2016, the vast majority of my fall webworm alerts were focused on southwest Ohio, so black-headed caterpillars were the primary subject. I performed a quick analysis of 14 BYGL Alerts reports on fall webworms and found that the average date for the first report was June 2.
However, the dominant fall webworm in my part of the state is now the red-headed biotype. Indeed, I’ve yet to find a black-headed nest this season. I have no explanation for this change. But the shift in biotypes provides a plausible explanation for the “delay” this season in the appearance of fall webworms compared to previous seasons.
Both biotypes produce communal nests occupied by caterpillars from multiple nearby egg masses. However, black-headed fall webworm nests appear to include caterpillars from only a few egg masses, if not just a single egg mass. Consequently, they tend to produce small, wispy nests that envelop only a dozen or so leaves, but it is common for several of these small nests to be found on the same branch.
Red-headed fall caterpillars are far more cooperative; their communal nests may include caterpillars from a large number of egg masses. Thus, they can produce some truly spectacular multilayered nests enveloping whole branches or even entire small trees. Thus, the red-headed biotype is the more damaging of the two owing to the caterpillar's ability to produce massive nests.
Red-headed caterpillars also have a greater propensity to venture outside of their protective nests to feed on the leaves of other trees once they have completely defoliated their original host. Jim Chatfield (OSU Extension Emeritus) has reported this wanderlust behavior for years with red-headed caterpillars in northeast Ohio. However, the first time I saw this in southwest Ohio was last year during a spectacular fall webworm outbreak in Yellow Springs (see BYGL Alert, August 16, 2021, “Extraordinary Fall Webworm Outbreak,” hotlink https://bygl.osu.edu/index.php/node/1848
After the red-headed caterpillars had completely enveloped and defoliated a large black walnut tree (Juglans nigra), they ventured onto the leaves of an understory eastern redbud (Cercus canadensis) where they openly fed without shrouding the leaves in silk. I have never observed this behavior with black-headed caterpillars.
On the other hand, fully mature black-headed caterpillars commonly leave their nests to go on a crawl-about before pupation. They may be found in peculiar places on unusual hosts far from their nests. I once photographed black-headed fall webworms on Zinnia spp. in a greenhouse.
Red-headed caterpillars remain in their silk nests throughout their development unless they run out of food. Otherwise, they don't leave their nests until they are ready to pupate, and even then, they don't crawl far. They pupate inside thin cocoons in bark crevices or leaf litter beneath the tree where they fed as caterpillars.
The Fall of Webworms
The scientific literature notes that fall webworm caterpillars have been observed feeding on over 400 species of trees. They primarily focus their attention on hardwoods; however, they may also feed several conifer species.
High webworm populations seldom occur in forests. That’s because research has revealed that the caterpillars prefer to locate their nests in full sun. Thus, they are most commonly observed as a pest of woody ornamentals and shade trees in landscapes.
However, the vast majority of the damage occurs in mid-to-late summer after established trees have acquired and stored enough carbohydrates through photosynthesis to support next season’s new growth. Despite the tree’s appearance, the caterpillars cause no significant harm to the overall health of healthy, established trees.
On the other hand, newly planted trees may be at risk, particularly from the red-headed biotype. Heavy defoliation by both biotypes can affect fruit sizing on fruit trees.
Insecticide applications are problematic. Most are stomach poisons and penetrating the dense silk nests to deposit the insecticide onto the enveloped leaves is a challenge.
Of course, insecticides may also kill bio‑allies that help keep population densities in check. Fall webworms are native to North America and there are over 50 species of parasitoids and at least 36 species of predators known to make a living on fall webworms. The following image shows the cocoons of an unidentified parasitoid wasp hanging in fall webworm webbing
Paper wasps (Polistes spp.) are well-known predators as well as pollinators. They provide an ecological twofer.
The wasps have powerful mandibles used to grind up wood fibers that are mixed with their saliva to extrude the paper-like material used for their nests and referenced in their common name. They also use their wood-grinding mandibles to macerate soft-bodied insects like caterpillars and sawfly larvae. They bring the pulverized meat mass back to their nests to provide protein to the developing wasp larvae.
The following image shows the native paper wasp, Polistes fuscatus, grinding up a catalpa hornworm (Ceratomia catalpa). Unfortunately, the hornworm also harbored the cocoons of the parasitoid wasp, Cotesia congregata. Nature has no favorites.
In a paper published in 1977, the authors described a P. fuscatus diving into the silken threads of a fall webworm nest to seize a caterpillar. The wasp was unable to navigate the strands of webbing holding a wiggling intact caterpillar, so it reduced the victim to a meatball before exiting to fly away with its protein prize.
I have commonly found two other predators in fall webworm nests: an unidentified ground beetle (family Carabidae) and the two-spotted stink bug (Perillus bioculatus, family Hemiptera). In 2019, I came across a large fall webworm nest with no caterpillars, but it was full of fat ground beetle larvae!
Physically destroying first-generation nests will prevent or at least reduce the development of the larger, more destructive nests produced by later generations. If nests are few and easily accessible, the most effective control option is to apply digital management. The following images illustrate the “3-Step Fall Webworm Control Technique.” Thus far, no populations have become resistant to this handy pest management tactic.
Oliver, A.D., 1964. Studies on the biological control of the fall webworm, Hyphantria cunea, in Louisiana. Journal of Economic Entomology, 57(3), pp.314-318.
Morris RF. 1963. Synonymy and color variation in the fall webworm, Hyphantria cunea Drury (Lepidoptera: Arctiidae). Canadian Entomologist 95: 1217-23.
Morris, R.F., 1964. The Value of Historical Data in Population Research, with Particular Reference to Hyphantria cunea Drury1. The Canadian Entomologist, 96(1-2), pp.356-368.
Schaefer, P.W., 1977. Attacking wasps, Polistes and Therion, penetrate silk nests of fall webworm. Environmental Entomology, 6(4), pp.591-591.
Schowalter, T.D. and Ring, D.R., 2017. Biology and management of the fall webworm, Hyphantria cunea (Lepidoptera: Erebidae). Journal of Integrated Pest Management, 8(1).