Fall Webworms (Hyphantria cunea, family Erebidae) have been virtual no-shows over much of Ohio this season. Although, there are reports of highly localized populations with noticeable nests, trees in much of the state are largely free of silk webbing.
Indeed, I was finally able to take a few pictures earlier this week of nests in southwest Ohio. One nest was on their favored host, eastern black walnut (Juglans nigra), and the picture is used as the lead photo for this Alert. The other was on winterberry holly (Ilex verticillata ‘Winter Red’) demonstrating the moth’s wide host range.
Fall webworm moths are native to North America. Of course, native insect pest populations can oscillate dramatically from year to year from so-called “outbreaks” followed by population “crashes.” The hypothetical graphic below illustrates the rise and fall of fall webworms as well as other native plant pests and a few non-natives.
Population crashes are driven by several factors sometimes acting together. These include predators, parasitoids, and pathogens (the “3-Ps”), environmental conditions, and food.
Fall webworms showed their potential for creating massive mayhem in 2021 with a spectacular outbreak in Yellow Springs, OH. They were largely confined to eastern black walnut with large trees being completely encased in webbing. You can read about the outbreak by clicking on this hotlink: https://bygl.osu.edu/index.php/node/1848
Fall 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.
Two forms of the caterpillars are recognized based on the color of their head capsules, the color of the caterpillars as well as 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 in the literature as “races,” “color-forms,” or “biotypes.”
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.
Indeed, I’ve come across fall webworm caterpillars that shared morphological characteristics of both biotypes indicating that the two are of the same species and periodically interbreed. For example, the image below shows caterpillars with red head capsules but other colors more consistent with black-headed caterpillars.
Both biotypes produce communal nests involving caterpillars that hatched from multiple egg masses; however, 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.
Another difference between black-headed and red-headed caterpillars is associated with nest abandonment. Black-headed caterpillars tend to stay inside their nests until they are about to pupate. Then they commonly crawl from their nests to pupate in protected locations. Finding the caterpillars far from their silk nests can present an identification challenge.
Red-headed caterpillars may leave their nests in search of food if they defoliate their tree host before they complete their development. The first time I observed this was with the Yellow Springs webworm outbreak with caterpillars commonly found feeding outside of their nests on the leaves of understory trees after they stripped black walnut trees.
Fall webworm moths have at least two generations per season in Ohio. However, the generations commonly overlap to muddy the waters. There have been some indications in past years that a third generation may occur in the southern part of the state.
Despite its common name, first-generation caterpillars commonly appear in our state during astronomical spring (around March 19 to around June 21). Indeed, I looked at past BYGL reports dating back to 2006 and found that the date when the first BYGL report was posted on first-generation nests ranged from June 2 to June 28 with the average date being June 15.
The common name "fall webworm" is based on when we typically see the largest number of nests as well as the largest nests. The nests reach their zenith in the fall, both astronomical and meteorological. However, this does not mean fall webworms are crawling below our radar this season and will suddenly leap onto the scene in the fall. On the other hand, we never say “never” with insects.
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 on 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.
The Fall of Webworms
Fall webworms are native to North America. Thus, there’s a bevy of 3-Ps that have evolved a taste for the webworms. Indeed, the literature notes that there are over 50 species of parasitoids and at least 36 species of predators known to make a living on fall webworms.
We commonly see significant impacts of the 3-Ps on fall webworm populations. However, it’s not known whether the current widespread absence of webworms is due to natural enemies, environmental challenges, or a combination of the two.
If fall webworms are common in your neck of the woods, it’s important to note that insecticide applications to kill the webworms 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.
A more direct approach to managing fall webworms is to physically destroy nests if they are few and directly accessible. The following images illustrate the digital “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).