Fall webworm caterpillars come in two distinct color forms that share tree branches across much of eastern North America: a red-headed morph with a brownish body and a black-headed morph with a grayish-to-black body. Both are larvae of the moth Hyphantria cunea, which in its adult stage is a relatively plain white moth that most people walk right past. The silky webs these caterpillars spin in tree canopies are far more conspicuous than either the larvae or the adults, and they’re commonly mistaken for a different pest altogether.
Two Caterpillar Color Morphs in One Species
If you’ve spotted webworm caterpillars in the wild, the first thing you probably noticed was head color. The red-headed morph has a distinctly reddish or orange-red head capsule paired with a body that ranges from brown to tan to black. The black-headed morph has a jet-black head capsule and a body that runs from pale gray to solid black.1PLoS ONE. Crowd-sourced observations of a polyphagous moth reveal evidence of allochronic speciation varying along a latitudinal gradient Both forms are covered in fine, wispy hairs along the body, which gives them a fuzzy or bristly look. The hairs can be pale on lighter individuals or dark and almost tufted on the black morph, sometimes with small raised bumps (called tubercles) running in rows down the back.
Fully grown caterpillars of either morph reach roughly an inch to an inch and a half in length. Young larvae are much smaller and paler, and telling the morphs apart is harder in the earliest stages. As they feed and molt through successive instars, the color contrast sharpens. By the time the caterpillars are big enough to notice with the naked eye from the ground, the head-color difference is usually obvious, especially if you peel back part of the web to look.
These two morphs aren’t random color variation. They differ in biology too, and some researchers think they may be in the early stages of splitting into separate species. The black-headed morph tends to emerge from winter dormancy at least a month ahead of the red-headed morph and typically squeezes in more generations per year in areas where both coexist.2PubMed Central. r/K-like trade-off and voltinism discreteness: The implication to allochronic speciation in the fall webworm, Hyphantria cunea complex (Arctiidae) That timing gap means the two forms often don’t interbreed even though they share the same trees.
What the Webs Look Like and Why They Differ
The webbing is usually what draws your eye before you ever see the caterpillars inside it. Fall webworm webs are built at the tips of branches, and they expand outward as the colony grows. A fresh web might enclose just a cluster of leaves at the end of a twig. Give it a few weeks, and it can balloon to cover several branches and a foot or more of canopy. At heavy infestations, multiple webs may merge, and entire sections of a tree can be draped in silk.3Journal of Integrated Pest Management. Biology and Management of the Fall Webworm, Hyphantria cunea (Lepidoptera: Erebidae)
The two color morphs build noticeably different webs. Black-headed larvae spin thin, loosely woven silk that looks almost translucent in direct sunlight. Red-headed larvae produce a thicker, denser web that’s more opaque and can look like a grayish-white bag engulfing the branch end.3Journal of Integrated Pest Management. Biology and Management of the Fall Webworm, Hyphantria cunea (Lepidoptera: Erebidae) If you see a web that looks flimsy and almost see-through, you’re likely looking at a black-headed colony. A heavier, more conspicuous tent-like structure usually belongs to the red-headed form.
The webs serve a practical purpose beyond just shelter. They trap solar heat, giving the caterpillars inside a warmer microclimate than the surrounding air. Webworms can’t generate body heat on their own, but the web’s three-dimensional silk structure captures daytime warmth efficiently enough to create a thermal advantage for the colony.4Journal of Thermal Biology. Temperature profiles inside webs of the fall webworm, Hyphantria cunea (Lepidoptera: Arctiidae): Influence of weather, compass orientation, and time of day That heat boost helps the larvae digest food faster and develop more quickly, which partly explains why webworms can chew through leaves at such an impressive rate.
What the Adult Moth Looks Like
The adult fall webworm moth is easy to overlook. It’s a medium-small moth with a wingspan of roughly one to one and a half inches, and in most populations the wings are pure white. Some individuals, particularly in the southern part of the range, carry small dark spots scattered across the forewings, but even the spotted form is mostly white. The body is stout and furry-looking, covered in dense white scales, and the legs may show faint yellow or orange banding in some individuals.
If you’ve ever seen a plain white moth sitting on a porch light in late spring or summer and thought “that’s just a moth,” there’s a good chance it was a fall webworm adult. They’re nocturnal and strongly attracted to light, but during the day they rest with their wings folded flat over the body, blending in against light-colored surfaces. Compared to the dramatic webs their larvae build, the adults are remarkably unremarkable. There’s no bold patterning, no striking color, and no large eyespot. It’s one of the reasons the species gets far less visual attention in its adult form than as a caterpillar.
Females tend to be slightly larger than males and have a noticeably plump abdomen when carrying eggs. After mating, a female lays a mass of several hundred tiny, pale green or yellowish eggs on the underside of a host leaf, often covering the cluster with hairs shed from her own body. The egg masses look like a small fuzzy patch on the leaf’s underside, and they can be easy to miss unless you’re specifically flipping leaves over to look.
How to Tell Fall Webworms from Tent Caterpillars
This is the confusion that trips up most people. Both fall webworms and eastern tent caterpillars build conspicuous silk structures in trees, but the two species look different at every life stage and build their webs in completely different locations on the tree.
The placement of the web is the quickest visual clue. Fall webworm webs sit at the ends of branches, enclosing leaves and twig tips. Eastern tent caterpillar webs sit in the crotches where branches fork, closer to the trunk.3Journal of Integrated Pest Management. Biology and Management of the Fall Webworm, Hyphantria cunea (Lepidoptera: Erebidae) If you see a web wrapped around the outer tip of a limb, it’s almost certainly a fall webworm. If the web is nestled in the V of two branches closer to the trunk, you’re looking at a tent caterpillar.
The caterpillars themselves look quite different. Tent caterpillar larvae have a bold pattern of blue, white, and black stripes running lengthwise along the body, often with a distinct white stripe down the center of the back. Fall webworm larvae, as described above, are either reddish-headed with a brown body or black-headed with a grayish body, and they lack that vivid striped pattern. The timing is also off: tent caterpillars appear in spring (March through May in most areas), while fall webworms are mostly a late-summer and autumn pest, showing up from July onward. Seeing web-building caterpillars in August or September virtually rules out tent caterpillars.
Sod Webworms Are a Completely Different Insect
The name “webworm” gets applied to a handful of unrelated caterpillars, and the one that causes the most confusion with fall webworms is the sod webworm. These are lawn pests in the family Crambidae, not tree-dwelling caterpillars, and they look nothing like fall webworms once you get a close look.
Tropical sod webworm larvae, for instance, start out cream-colored as young caterpillars and gradually turn greenish in later stages. Their head capsule is a dark yellowish-brown rather than red or black. A distinguishing mark is two pairs of brown spots on each body segment, running the length of the caterpillar’s back. Mature tropical sod webworm larvae top out at roughly three-quarters of an inch to one inch long.5Journal of Integrated Pest Management. Tropical Sod Webworm (Lepidoptera: Crambidae): a Pest of Warm Season Turfgrasses When disturbed, they curl into a defensive C-shape and can scoot forward or backward surprisingly fast.5Journal of Integrated Pest Management. Tropical Sod Webworm (Lepidoptera: Crambidae): a Pest of Warm Season Turfgrasses
The habitat difference is the dead giveaway. Sod webworms live in lawns, feeding on grass blades at soil level. They spin small silk tubes in the thatch layer, not conspicuous aerial webs in tree canopies. Their adult moths are small, drab tan or buff-colored, and tend to fly in a zigzag pattern low over the grass at dusk. If your problem is brown patches in the lawn and you’re finding small greenish caterpillars in the turf, you have sod webworms. If your problem is a big silky bag at the end of a tree branch, you have fall webworms. The two share a common name and almost nothing else.
When and Where Fall Webworms Show Up
Fall webworms are native to North America and can be found across the continent from southern Canada to Mexico. Their name is slightly misleading because in warmer climates they can produce multiple generations per year, with webs appearing as early as late spring and persisting through October. In cooler northern areas, there’s often a single generation that shows up in late summer.
The species is also one of the more successful insect invaders on the planet. It was accidentally introduced to Europe and Asia in the mid-twentieth century and has spread aggressively. Modeling studies suggest that the fall webworm’s potential range in Europe could cover more than half the continent, excluding only Scandinavia and northern Russia.6PubMed Central. Niche and Range Shifts of the Fall Webworm (Hyphantria cunea Dury) in Europe Imply Its Huge Invasion Potential in the Future In China, where it arrived in the late 1970s, it has become a significant pest of urban and roadside trees.
Part of the reason for this success is diet flexibility. Fall webworm larvae have been recorded feeding on over 600 host plant species worldwide.7PubMed Central. Fall Webworm Host Plant Preferences Generate a Reduced Predation Enemy-Free Space in Its Interaction with Parasitoids In North America, common targets include hickory, walnut, persimmon, sweetgum, cherry, crabapple, and pecan, but webworms will set up on almost any broadleaf deciduous tree if conditions are right.8PubMed. Trade-offs in host choice of an herbivorous insect based on parasitism and larval performance They rarely attack conifers or evergreens.
How Much Damage They Actually Do
The webs look alarming, and a heavily infested tree can be almost completely stripped of its leaves. At high population densities, defoliation can reach 100% across areas stretching several square kilometers.3Journal of Integrated Pest Management. Biology and Management of the Fall Webworm, Hyphantria cunea (Lepidoptera: Erebidae) That sounds catastrophic, but the timing works in the tree’s favor. Because fall webworms feed mostly in late summer and early autumn, the tree has already completed most of its growing season by the time defoliation hits. A healthy, established tree that gets stripped by webworms will almost always leaf out normally the following spring. The aesthetic damage is severe; the long-term health damage is usually minimal for otherwise vigorous trees.
Young, stressed, or newly transplanted trees are more vulnerable. Repeated defoliation over multiple years can weaken them, and trees already dealing with drought or disease may not bounce back as reliably. In managed landscapes and orchards where appearance or fruit production matters, control measures make more sense than in a wild woodland setting where the webs are mainly a cosmetic nuisance.
For a single web on a small ornamental tree, the simplest approach is physical removal. You can prune off the webbed branch tip or use a stick to open the web, which exposes the larvae to birds and parasitic wasps. Spraying with Bt (Bacillus thuringiensis) is effective on young caterpillars if you can penetrate the web. The key is timing: once the caterpillars are large and the web is dense, they’re harder to reach and closer to finishing their feeding anyway.
Skin Reactions from Handling Webworms
Most people who prune webworm-infested branches or accidentally brush against the caterpillars don’t have any trouble. But some individuals react to the fine hairs on the larvae. A documented case of fall webworm dermatitis in the southeastern United States described acute hive-like skin reactions after direct contact with the caterpillars, including localized pain, itching, and some systemic symptoms. The reaction resolved within a week with standard antihistamine and corticosteroid treatment.9Academia. Fall Webworm Dermatitis
Fall webworm hairs aren’t venomous in the way that some stinging caterpillars (like saddlebacks or puss caterpillars) are. The reaction is typically a mechanical irritant response rather than an envenomation. Wearing gloves and long sleeves while pruning infested branches is usually sufficient protection. If you do develop a rash or hives after contact, over-the-counter antihistamines and topical hydrocortisone are the standard first-line treatment, but see a doctor if swelling spreads or you develop difficulty breathing.
The Pupal Stage Nobody Sees
Between the caterpillar and the moth sits the pupal stage, and it’s the one phase of the life cycle that almost nobody encounters by accident. When fall webworm caterpillars finish feeding, they leave the web and crawl down the tree to pupate in leaf litter, loose soil, or under bark at the base of the trunk. The pupa itself is a dark brown, roughly oval capsule about half an inch long, often loosely wrapped in a thin silk cocoon mixed with shed larval hairs and debris. It’s inconspicuous and well camouflaged against the forest floor.
In regions with a single generation per year, the pupa overwinters and the adult moth emerges the following spring or early summer. In warmer areas with two or more generations, the summer pupa may emerge in as little as two to three weeks. The depth of dormancy differs between the two color morphs: black-headed populations enter a shallower dormancy and break out of it earlier, while red-headed populations enter a deeper winter dormancy and emerge later.2PubMed Central. r/K-like trade-off and voltinism discreteness: The implication to allochronic speciation in the fall webworm, Hyphantria cunea complex (Arctiidae) That staggered emergence is one of the clearest behavioral differences between the morphs, and it means the two forms often peak at different times even in the same neighborhood.
If you’re trying to find pupae for identification, look in the leaf litter directly below a tree that was webbed the previous summer. Gently raking aside a few inches of decomposing leaves in late winter or early spring sometimes turns them up. They’re unremarkable to look at, just a smooth brown shell, but finding them confirms that the adults will be flying from that tree’s vicinity in the coming season.