Woolly Bear Caterpillars: Poisonous? Can They Predict Winter?
A woolly bear caterpillar is the bristly larva of the Isabella tiger moth (Pyrrharctia isabella): black at both ends, rusty brown in the middle, about 1.5 inches (38 mm) long when grown, and harmless in the sense that it neither stings nor is venomous. Its stiff bristles can lodge in skin and cause irritation in some people, so let it walk onto a stick rather than your palm. It cannot forecast winter: the width of the brown band reflects the caterpillar’s age and growing conditions. It survives winter under leaf litter with up to about half its body water turned to ice.
Woolly bears are the caterpillars people notice in October, crossing roads and paths in a hurry. That timing explains almost everything about them, from the folklore to the sudden “why are there so many this year” question.
Woolly bear or a caterpillar that stings?
The real risk with a fuzzy caterpillar is mistaking a stinging species for a woolly bear. The banded woolly bear is easy to recognise once you have the pattern; several look-alikes are not.
| Caterpillar | What it looks like | Grown length | Handle it? |
|---|---|---|---|
| Banded woolly bear (Isabella tiger moth) | Dense stiff bristles, black at both ends, rust brown in the middle; curls into a ball | About 1.5 in (38 mm) | Does not sting; bristles can irritate. Use a stick or gloves |
| Yellow woolly bear, saltmarsh caterpillar | Similar bristly tiger moth larvae, cream, yellow, brown or black | Not given in the sources we used | Non-stinging, according to Texas A&M |
| Hickory tussock moth caterpillar | White with black tufts along the back and black “pencils” at each end | Up to 1.7 in (45 mm) | No: barbed hairs cause a rash in sensitive people |
| Puss caterpillar (southern flannel moth) | Teardrop of long silky hair, like a tuft of cotton, with a hairy tail | About 1 in (25 mm) | Never: venomous spines hidden under the hair |
| White flannel moth caterpillar | Yellow with a wide black back stripe and raised yellow tufts | Up to 1.25 in (32 mm) | Never: stinging hairs at the base of the tufts |
| Saddleback caterpillar | Green “saddle” with a brown spot, spiny horns at both ends | About 0.75 in (19 mm) | Never: urticating spines |
Compiled by The Pest Control Brief from the University of Arkansas Arthropod Museum, Texas A&M AgriLife, Penn State Extension, University of Kentucky Entfact-003 and NC State Extension.
Kentucky’s entomologists note that most caterpillar spines are for show and harmless, but that the stinging species carry hollow, venom-connected hairs. If the fuzzy caterpillar in front of you looks like a wig or a cotton ball rather than a bottle brush, read our puss caterpillar guide before anyone touches it.
What is a woolly bear caterpillar?
“Woolly bear” covers eight or more US species of tiger moth caterpillar, Iowa State and Ohio State say, all densely covered in bristles. The famous one is the banded woolly bear, larva of the Isabella tiger moth, in the family Erebidae (older sources say Arctiidae). Each of its 13 body segments, three thoracic and ten abdominal, carries tufts of long, stiff, barbed setae, the University of Arkansas Arthropod Museum explains: black on the front and rear segments, reddish brown on typically four to six in the middle. Some individuals are almost entirely black or entirely rust.
Ohio State lists the four common Ohio woolly bears: the banded woolly bear, the yellow woolly bear (Virginia tiger moth), the saltmarsh caterpillar and the all-black giant leopard moth caterpillar. When disturbed, a woolly bear rolls into a tight ball with its bristles pointing out, which is why it is also called the hedgehog caterpillar.
Where do woolly bear caterpillars live?
The Arkansas museum gives the range as throughout North America except northern Canada and Alaska; Iowa State says the US, Mexico and southern Canada. They live in gardens, meadows, pastures, old fields and road edges, feeding low on plantain, dandelion, clover, aster, grasses and other weeds.
Our analysis of iNaturalist research-grade records puts the banded woolly bear’s strongholds in the Northeast and upper Midwest. West Virginia leads with 26.05 records per 10,000 observations of anything, then Vermont (25.89), Iowa (23.40) and Minnesota (22.81). Oregon (17.35) is the western exception in the top 14. Texas has only 0.28 and Arizona 0.29. That fits Texas A&M, whose entomologist names the saltmarsh caterpillar and garden tiger moth caterpillar as the common woolly bears in Texas, not the banded one.
Are woolly bear caterpillars poisonous?
No. Woolly bears are not venomous and do not sting. Texas A&M AgriLife says these are not stinging caterpillars, and Ohio State says the bristles, though defensive, are not stinging (urticating) hairs.
The bristles are the catch. Ohio State notes that some people suffer severe localised reactions if the sharp hairs penetrate the skin, and Washington State University reports reactions ranging from allergy to rashes, including one child whose eye had to have a hair surgically removed. Two medical case reports describe infants who put a woolly bear in their mouths: an 8-month-old with airway swelling that needed urgent care in an operating room (Journal of Emergency Medicine, 2015), and a 19-month-old with hairs embedded in the lips, mouth and throat after eating half a caterpillar, observed in hospital for 48 hours (Wilderness and Environmental Medicine, 2023).
So the sensible rule is the one Washington State gives: teach children to let fuzzy caterpillars walk onto a stick rather than bare hands. Anyone with hairs in the eye or mouth, or swelling of the face or throat, needs a doctor or emergency care. Our guide to bites and stings explains why a rash alone is weak evidence of what caused it.
Do woolly bear caterpillars bite?
None of the extension sources we read describes a woolly bear biting a person. Caterpillars have chewing mouthparts for leaves, Texas A&M notes, but the woolly bear’s defence is the bristles: it curls up and waits. An itchy patch after handling one is far more likely to be bristles in the skin than a bite.
Can woolly bears predict winter?
No. The folklore says the more black, the harder the winter, and a wide brown middle band means a mild one; some versions read the 13 segments as the 13 weeks of winter, or the darker end as the colder half of the season. Every extension source we checked says the evidence does not support it.
| The folklore says | What extension entomologists say |
|---|---|
| More black means a severe winter | Young caterpillars are mostly black and gain rust-coloured rings with each moult (University of Arkansas) |
| A wide brown band means a mild winter | A wide brown band says the previous weather was mild, and earlier-developing caterpillars carry more brown (Clemson HGIC) |
| The bands are a forecast | Black varies with the caterpillar’s age and the moisture where it grew (Iowa State, Ohio State) |
| Any woolly bear will do | About eight species are called woolly bears and their colours differ; diet and instar change the bands too (Texas A&M) |
Compiled by The Pest Control Brief from the University of Arkansas Arthropod Museum, Clemson HGIC, Iowa State University Extension, Ohio State University Extension and Texas A&M AgriLife.
The 1948 Curran study
The legend’s scientific veneer comes from C. H. Curran, curator of insects at the American Museum of Natural History. In autumn 1948 he went to Bear Mountain State Park in New York, collected as many woolly bears as he could find in a day, compared their brown and black segments and gave a winter forecast to a New York Herald Tribune reporter. New York State Parks records that he continued for eight more years and was never able to conclude whether the woolly bear could predict the winter. Clemson’s verdict is that a study confined to one small area over a short period was not conclusive scientific proof. Ohio State adds a practical objection: forecasters reading a yellow woolly bear would predict no winter at all.
How woolly bears survive winter frozen
The banded woolly bear is unusual in spending the winter as a caterpillar, under bark, logs, rocks or leaf litter, while the yellow woolly bear and saltmarsh caterpillar overwinter in cocoons, Colorado State notes. It is freeze tolerant: ice forms in its body and it thaws out and walks away. Layne and Kuharsky (2000) showed that both cold and drying trigger production of cryoprotectants such as glycerol, and the Arkansas museum notes that caterpillars frozen in solid ice have been seen moving normally after thawing.
| Study | Test | Result |
|---|---|---|
| Layne and Blakeley, 2002 | Frozen at 27, 21 and 14°F (minus 3, 6 and 10°C) | 24 to 49 per cent of body water froze; caterpillars moved within minutes of thawing; 46 to 52 per cent went on to pupate |
| Layne and Peffer, 2006 | Frozen at 24°F (minus 4.5°C) for 1 or 6 weeks | 100 and 95 per cent survived the thaw, but the 6-week group reached the pupal stage about half as often |
| Yi and Lee, 2016 | After 18 weeks of cold acclimation, frozen at minus 4°F (minus 20°C) | All survived a week frozen or seven freeze-thaw cycles; none survived minus 112°F (minus 80°C) |
| Boardman and others, 2011 | Frozen at 7°F (minus 14°C) for up to 6 weeks, or at minus 22°F (minus 30°C) | Longer than 3 weeks frozen, or minus 30°C, killed all of them |
| Marshall and Sinclair, 2011 | Five short freezes against one long one | Repeated freeze-thaw raised mortality to almost 30 per cent |
Compiled by The Pest Control Brief from the published studies named, all on Pyrrharctia isabella.
One counterintuitive finding: Marshall and Sinclair (2012) found caterpillars wintering above the snow, colder by 1.3°C on average, used 18.36 mg less of their 68.97 mg fat reserve than those under snow, because being frozen shuts metabolism down. Freezing is not just survived; it saves energy.
What do woolly bear caterpillars turn into?
The Isabella tiger moth, a moth rather than a butterfly. After a brief spell of feeding in spring, the overwintered caterpillar spins an oval cocoon of silk woven with its own black and red bristles, usually in April or May, and pupates. Adults are ready to mate by June or July, with a wingspan of about 1.5 to 2 inches and dull yellow wings with black or grey spots, the Arkansas museum says, and they fly at night from June to September. Iowa State describes two generations of caterpillars a year, in May and August; the second is the one seen crossing roads in autumn.
Why are there so many woolly bear caterpillars this year?
Usually there are not more of them; they are just more visible. In autumn the second generation leaves its food plants to look for a sheltered spot to overwinter, and that is when they cross roads, paths and house walls. Texas A&M’s entomologist doubts there are necessarily more in any given year and points out that it depends on who is paying attention.
The iNaturalist record shows how concentrated that wandering is. Woolly bear records in October reach 477 on our index, nearly five times their usual share of everything people record, when moths and butterflies as a group are well past their July and August peak. The stinging look-alikes peak on either side: saddlebacks in September (422) and puss caterpillars in November (223).
How to get rid of woolly bear caterpillar?
In most cases, do nothing. Iowa State says woolly bears on garden plants rarely cause significant damage, and Washington State says the banded woolly bear is not known to feed extensively on yard or garden plants; it prefers plantain and other weeds. What damage there is comes so late in the season, Colorado State notes, that it rarely matters. Those on house walls are looking for shelter, not food.
- Caterpillars on a path, porch or garage floor: lift them on a stick or with gloves and move them to leaf litter or a log pile.
- Caterpillars on garden plants: Iowa State says special control is rarely required and handpicking is adequate.
- Spraying is not needed for a caterpillar that does this little damage. Leave some leaf litter and log piles if you want them to overwinter in the garden.
Caterpillar control in vegetable beds is a different question, covered in our piece on why pest control companies will not spray your garden.
Frequently asked questions
Is it safe to touch a woolly bear caterpillar?
It will not sting you, and Texas A&M says these caterpillars will not hurt you. But the stiff bristles can break off into skin, and Ohio State notes some people have severe localised reactions when they do. Washington State advises letting children move them on a stick. Never touch an unfamiliar fuzzy caterpillar: puss and saddleback caterpillars carry venomous spines.
What does a black woolly bear mean?
Nothing about the weather. Young banded woolly bears are mostly black and gain rust-coloured segments with each moult, so an all-black one is often simply younger or grew up in wetter conditions, according to the University of Arkansas, Iowa State and Ohio State. It may also be a different species: the giant leopard moth caterpillar is all black.
Do woolly bears freeze in winter?
Yes, and they survive it. Banded woolly bears are freeze tolerant and overwinter as caterpillars under leaf litter. Laboratory studies found that between about a quarter and half of their body water turns to ice depending on temperature, and acclimated caterpillars survived a week frozen at minus 4°F. Long freezes of more than three weeks at 7°F, or repeated freeze-thaw cycles, kill many of them.
Is a woolly bear a butterfly or a moth?
A moth. The banded woolly bear becomes the Isabella tiger moth, a night-flying moth with dull yellow wings with black or grey spots and a wingspan of about 1.5 to 2 inches. The caterpillar spins its cocoon in spring from silk woven with its own bristles, and adults fly from June to September, with a second generation later in summer.
Sources
- University of Arkansas Arthropod Museum, Woolly bear caterpillar (Arthropod of the Month).
- Iowa State University Extension and Outreach, Yard and Garden: Woollybear Caterpillar (2016).
- Ohio State University Extension, Buckeye Yard and Garden onLine: Woolly Bears on the Move.
- Texas A&M AgriLife Today, Woollybear caterpillar, with Wizzie Brown, Texas A&M AgriLife Extension (December 2023).
- Clemson Cooperative Extension Home and Garden Information Center, The Legend of the Woolly Worm (B. H. Smith, 2019).
- Washington State University Puyallup, Woolly Bears, PLS-54 (A. L. Antonelli, 2003).
- Colorado State University Extension, PlantTalk Colorado 1409: Woollybear Caterpillars.
- New York State Parks and Historic Sites, The Folklore of the Woolly Bear (S. Carver, 2015).
- Penn State Extension, Hickory Tussock Moth Caterpillar (M. J. Skvarla).
- University of Kentucky Entomology, Stinging Caterpillars, Entfact-003.
- NC State Extension, Saddleback Caterpillar.
- Layne, J. R. and Kuharsky, D. K. Triggering of cryoprotectant synthesis in the woolly bear caterpillar. Journal of Experimental Zoology 286: 367 to 371, 2000.
- Layne, J. R. and Blakeley, D. L. Effect of freeze temperature on ice formation and long-term survival of the woolly bear caterpillar. Journal of Insect Physiology 48: 1133 to 1137, 2002.
- Layne, J. R. and Peffer, B. J. The influence of freeze duration on postfreeze recovery by caterpillars of Pyrrharctia isabella. Journal of Experimental Zoology A 305: 570 to 575, 2006.
- Boardman, L., Terblanche, J. S. and Sinclair, B. J. Transmembrane ion distribution during recovery from freezing in the woolly bear caterpillar. Journal of Insect Physiology 57: 1154 to 1162, 2011.
- Marshall, K. E. and Sinclair, B. J. The sub-lethal effects of repeated freezing in the woolly bear caterpillar. Journal of Experimental Biology 214: 1205 to 1212, 2011.
- Marshall, K. E. and Sinclair, B. J. Threshold temperatures mediate the impact of reduced snow cover on overwintering freeze-tolerant caterpillars. Naturwissenschaften 99: 33 to 41, 2012.
- Yi, S. X. and Lee, R. E. Cold-hardening during long-term acclimation in a freeze-tolerant woolly bear caterpillar. Journal of Experimental Biology 219: 17 to 25, 2016.
- MacKinnon, J. A. and others. Oropharyngeal edema in an 8-month-old girl after woolly bear caterpillar exposure. Journal of Emergency Medicine 49(5), 2015.
- Halmo, L. S. and others. A nonsurgical approach to management of lepidopterism due to woolly bear caterpillar ingestion. Wilderness and Environmental Medicine 34(2), 2023.
- The Pest Control Brief analysis of iNaturalist research-grade records, pulled October 2026.
