The “Fall” in Fall Webworm (Hyphantria cunea, family Erebidae) remains accurate, with the red-headed biotype producing a dramatic localized outbreak in Toledo. LaRae Sprow (Natural Resources Supervisor, Metroparks Toledo) and Josh Brenwell (GIS Manager, Metroparks Toledo) are reporting heavy webbing and defoliation of preferred hosts in the Oak Openings area.
Fall webworm is native to North America. The caterpillars must survive a bevy of predators, parasitoids, and pathogens (the 3-Ps). Research has revealed that over 50 different parasitoids and 36 species of predators help to keep fall webworms in check.
Normally, these enemies keep populations below damaging levels. For example, I’ve only observed sporadic nests this season in the southwest part of the state.
However, occasionally, fall webworms surpass the collective impacts of the 3-Ps to produce dramatic outbreaks. Fortunately, the outbreaks are usually highly localized and short-lived, with their enemies catching up to “crash” the populations.
I posted BYGL Alerts describing outbreaks last year in and around Springfield, OH (Clark County) [ https://bygl.osu.edu/index.php/node/2038 ], and in Logan County (OH) [ https://bygl.osu.edu/index.php/node/2559 ]. In 2021, I posted an Alert reporting a dramatic outbreak in the Village of Yellow Springs, OH (Green County). Virtually every black walnut (Juglans nigra) was encased in webbing [ https://bygl.osu.edu/index.php/node/1848 ].
These outbreaks had three things in common:
1. Each outbreak was produced by the redheaded biotype, but more on that later.
2. A repeat performance was not observed the following season.
3. The impacts on the overall health of their tree hosts appeared negligible, as illustrated in the images below.
Feeding Behavior and Life Cycle
Fall webworm has at least two generations per season throughout Ohio. However, four generations may develop in the southern U.S., and there is observational evidence that the moths can squeeze out a third generation in the southern part of our state.
Fall webworms spend the winter in egg masses laid on tree stems. First-generation eggs hatch in the spring. The caterpillars feed in communal nests with members arising from a few to many overwintered egg masses, depending on the biotype.
The caterpillars feed on the leaves enveloped by their silk nest, producing progressive damage. Early instar caterpillars consume the epidermis and mesophyll between the veins, gradually breaking through to skeletonize leaves.
Later instars consume all of the leaf tissue except for the petioles and coarse veins. As the caterpillars develop, they expand their nest by casting silk over an increasing number of leaves to accommodate their expanding appetites.
Female moths tend to lay their eggs on or near the nests from which they developed. Consequently, first-generation nests are expanded by the second generation to become most evident late in the season. The common name "fall webworm" is based on when we typically see the largest nests. They reach their zenith in the fall, both astronomically and meteorologically.
Impacts Depend on the Biotype
Two forms of fall webworms are recognized. They are variously referred to as biotypes, races, or color-forms. The two biotypes differ in the timing of their spring egg hatch, nesting behavior, and host preferences. Most importantly, the two biotypes have significantly different impacts that affect management decisions.
The two biotypes are named “black-headed” and “red-headed” based on the color of the caterpillar’s head capsules. They also have different colored “tubercles,” which are the small bumps from which the hairs arise that run in longitudinal lines along the top of the caterpillar’s body.
Black-Headed Biotype
Caterpillars in this biotype have black head capsules and black tubercles. The caterpillars are pale yellow to yellowish-green. However, they sometimes appear black with starkly white hairs, particularly the last instars.
Black-headed fall webworm eggs hatch much earlier in the spring compared to eggs that give rise to red-headed webworms. The nests appear to include caterpillars from only a few egg masses, if not just a single egg mass.
Consequently, the black-headed webworms tend to produce small, wispy nests that envelop only a dozen or so leaves. It’s common for several of these small nests to be found on the same branch. However, black-headed fall webworms seldom produce heavy damage, even during “outbreak” years.
The black-headed caterpillars tend to remain in their nests throughout their larval development, unless they run out of food. This seldom occurs on large trees but may occur on small trees. However, they may leave their nests as they near pupation. It’s not unusual to find individuals wandering about in search of a pupation site. The wandering caterpillars may be mistaken for other moth caterpillars.
Red-Headed Biotype
Caterpillars in 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.
Red-headed fall webworm caterpillars are far more cooperative than their black-headed cousins. Their communal nests may include caterpillars from a large number of egg masses. Consequently, they can produce some truly spectacular multilayered nests enveloping whole branches or even entire small trees.
The red-headed biotype is the more damaging of the two biotypes owing to the caterpillar's ability to produce massive nests. They are also most likely to produce outbreaks. Red-headed webworms were the culprits behind all of the aforementioned outbreaks in 2021 and 2025, as well as the current outbreak in Toledo.
Red-headed caterpillars will readily abandon their nests if their food is exhausted. They may be found feeding singly or in groups on leaves that are not enveloped in webbing, which can lead to misidentification. For example, the fall webworms pictured below could be mistaken for Spongy Moth (Lymantria dispar dispar). Of course, those non-native caterpillars show up in the spring.
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, meaning the generational times between the two biotypes are not synchronous. It’s speculated that this could create reproductive isolation between the two biotypes that sustains their separate existence.
However, we occasionally observe caterpillars that are clearly “hybridized” between the two biotypes. They are not true hybrids since they’re the same species, but the caterpillars share traits associated with both biotypes.
The Bottom Line: Should We Worry About Fall Webworm?
Fall webworm caterpillars may feed on more than 600 species of trees and shrubs, but some hosts are more equal than others. Black walnut seems to be webworm candy for both biotypes. All of the previous outbreaks in Ohio involved this native hardwood species. LaRae and Josh noted that black walnut is also receiving a lot of attention from fall webworms in Toledo.
They noted that other hosts commonly targeted in the outbreak include maples (Acer spp.), honeylocust (Gleditsia triacanthos), and black locust (Robinia pseudoacacia). Interestingly, although oaks (Quercus spp.) are included in the fall webworm host range, they are not being heavily impacted in Oak Openings. Indeed, while I’ve taken numerous pictures of fall webworm nests on oaks, I’ve never observed large populations like those I have observed on black walnut.
Although fall webworm may feed on a wide range of tree hosts, including some conifers, the reason it doesn’t rise to the same level of concern compared to other general defoliators is in the name. The vast majority of fall webworm defoliation occurs late in the growing season. The loss of photosynthesis occurs after trees have produced and stored enough carbohydrate to support next year's production of new leaves and stem growth.
This means insecticide applications are not required to protect the overall health of infested trees. In fact, widespread applications of standard insecticides may remove two important members of the 3-Ps: predators and parasitoids, that are responsible for producing population crashes during outbreaks. Crashing outbreaks prevents the recurrence of this and other native defoliators year after year.
Selected References
Hattori, I. and Ito, Y., 1973. Status of black-headed and red-headed types of Hyphantria cunea (Drury) (Lepidoptera: Arctiidae): II. External characters of the two types and their hybrids. Applied Entomology and Zoology, 8(3), 172-182.
https://doi.org/10.1303/aez.8.172
Ito, Y. and Warren, L.O., 1973. Status of black-headed and red-headed types of Hyphantria cunea (Drury) (Lepidoptera: Arctiidae): I. Biology of two types and results of crossing experiment. Applied Entomology and Zoology, 8(3), 157-171.
https://doi.org/10.1303/aez.8.157
Ito, Y. and Hattori, I., 1975. Status of black-headed and red-headed types of Hyphantria cunea (DRURY) (Lepidoptera: Arctiidae): III. Distribution of various types and the discussion on the relationship among them. Applied Entomology and Zoology, 10(3), 189-202.
https://doi.org/10.1303/aez.10.189
Oliver, A. D. (1964a). A behavioral study of two races of the fall webworm, Hyphantria cunea (Lepidoptera: Arctiidae) in Louisiana. Annals of the Entomological Society of America, 57(2), 192-194.
https://doi.org/10.1093/aesa/57.2.192
Oliver, A. D. (1964b). Studies on the biological control of the fall webworm, Hyphantria cunea, in Louisiana. Journal of Economic Entomology, 57(3), 314-318.
https://doi.org/10.1093/jee/57.3.314
Morris, R. F. (1963). Synonymy and Color Variation in the Fall Webworm, Hyphantria cunea Drury (Lepidoptera: Arctiidae) 1. The Canadian Entomologist, 95(11), 1217-1223.
https://doi.org/10.4039/Ent951217-11
Schowalter, T. D., & Ring, D. R. (2017). Biology and management of the fall webworm, Hyphantria cunea (Lepidoptera: Erebidae). Journal of Integrated Pest Management, 8(1), 7.
https://doi.org/10.1093/jipm/pmw019
Tadić, M.D. (1963). Natural enemies of fall webworm (Hyphantria cunea DR.) in North America. Entomophaga 8, 245–252.
https://doi.org/10.1007/BF02377530
