Late last week, I came across a nest of early instar, first-generation Fall Webworms (Hyphantria cunea, family Erebidae) on a boxelder (Acer negundo) in southwest Ohio. We see two generations in Ohio, sometimes three in the southern part of the state.
The common name "fall webworm" is based on when we typically see the largest nests. They reach their zenith in the fall, both astronomical and meteorological.
However, fall webworm has been conspicuous by its absence for several years in my part of the state. This native moth has a long history of dramatic year-to-year swings in population density. Outbreaks are commonly highly localized and usually followed by years with low populations owing to a cadre of enemies that can cause the fall of fall webworms.
Last year, I posted a BYGL Alert on September 13, titled “Fall Webworm Outbreak,” about a highly localized fall webworm infestation in Logan County that was photo-documented by Jim Downs (OSU Extension, Field Specialist, Forestry). The webworms were confined to woodlots surrounding farmland, so there was no impact on landscape trees. You can see more pictures and read the Alert by clicking this link: https://bygl.osu.edu/node/2559
The last significant outbreak in southwest Ohio was in Yellow Springs (Greene County) in 2021. The population density was nearly apocalyptic and highly impactful, as nearly every black walnut (Juglans nigra) in the community, both large and small, was heavily infested. I posted an Alert on August 16, 2021, titled “Extraordinary Fall Webworm Outbreak.” Click this link to see more pictures: https://bygl.osu.edu/node/1848
Will there be a tsunami of webworms this season, or just a ripple? It’s too early to know.
Two Biotypes? Or Two Species??
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.
Whether the biotypes represent variations within the same species or different species was long debated. However, it’s now accepted that they are the same species.
Both biotypes have bodies covered in hair. However, the hairs on black-headed webworms remain sparse, but appear to lengthen as they develop through instar stages, while the red-heads have long hairs throughout their development.
Hairs on other caterpillars are sometimes used as defensive tools. However, the hairs on fall webworms seem to primarily aid the caterpillars in remaining suspended inside their silk nests. You can see this in the following pictures. Note that the hairs fold back as the caterpillar appears to "swim" through the nest.
Fall webworm caterpillars may feed on more than 600 species of trees and shrubs. Host trees are predominantly hardwoods, but fall webworm also may feed on several conifer species such as baldcypress (Taxodium distichum L.).
While both biotypes are commonly found on eastern black walnut, red-headed webworms appear to have a particular taste for this native tree. Black walnut was the primary host in both the Logan County and Yellow Springs outbreaks.
Both biotypes feed in communal nests with members arising from a few egg masses to many. They 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, 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.
Black-Headed Biotype
Black-headed biotype caterpillars have black head capsules and black tubercles. The caterpillars are pale yellow to yellowish-green; however, they sometimes appear black with starkly white hairs.
Black-headed fall webworm eggs hatch much earlier in the spring compared to eggs that give rise to red-headed webworms. The webworms that I found last Thursday were the black-headed biotype.
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. 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.
Red-Headed Biotype
Red-headed biotype caterpillars 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 red-headed fall webworm caterpillars are far more cooperative compared to 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 the outbreaks last year in Logan County and in 2021 in Yellow Springs.
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.
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, thereby sustaining 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.
Hysterical History and General Mayhem
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.
I never found red-headed webworms in Greater Cincinnati before 2016. However, groups of redheads began to appear in western Hamilton County and gradually became the dominant biotype in southwest Ohio.
However, over the past few years, black-headed webworms have once again become the dominant biotype in southwest Ohio. In fact, all of the nests that I have found in recent years in southwest Ohio contained the black-headed biotype.
Management: The Fall of Fall Webworm
It’s important to tackle fall webworm early in the season. The number of nests, nest size, and the associated damage increase with each generation.
Nature Finds a Way:
Fall webworm 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.
These natural enemies play a significant role in fall webworm populations rising and falling dramatically from year to year. For example, the population “crash” in Yellow Springs was substantial, with no nests being evident in 2022.
Insecticides:
Applications of traditional insecticides such as pyrethroids (e.g., bifenthrin, permethrin, cyfluthrin, etc.) as well as other broad-spectrum insecticides such as acephate, are effective. However, the applications can kill beneficial bio-allies that keep the webworms in check.
So-called biorational insecticides, which have a limited effect on non-targeted organisms, have less of an impact. These include products based on the active ingredients azadirachtin, chlorantraniliprole, spinosad, and tebufenozide. The naturally occurring bacterium, Bacillus thuringiensis Kurstaki (Btk), only kills caterpillars; however, it is most effective on early instars.
The 4-Step Surefire Webworm Annihilation Technique (SWAT)
Fall webworm caterpillars tend to congregate within their nests at night for thermal regulation and remain packed together during cool mornings. This handy behavioral trait makes the webworms, along with their unsightly nests, susceptible to digital removal. It is a highly satisfying approach and very effective. The SWAT method is demonstrated with the following images. Thus far, no webworm populations have become resistant.
NOTE: Small webworm nests at the ends of branches can be pruned from trees and destroyed if the pruning doesn’t affect the overall desired tree form. Otherwise, it’s best to use SWAT.
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
