# Pyrrharctia isabella
**Pyrrharctia isabella** (J. E. Smith, 1797), the Isabella tiger moth, is a moth of the family [Erebidae](https://www.edgepedia.org/erebidae) whose larva, the banded woolly bear (or woolly worm), is a black-and-rust caterpillar famous in North American folklore as a forecaster of winter weather.[1](https://itis.gov/servlet/SingleRpt/SingleRpt?search_topic=TSN&search_value=117310) • [2](https://www.weather.gov/arx/woollybear) The species occurs across the United States, Canada, and Mexico, and its caterpillars are among the most recognizable insects on the continent each autumn.[2](https://www.weather.gov/arx/woollybear)
| Key fact | Detail |
|---|---|
| Scientific name | *Pyrrharctia isabella* (J. E. Smith, 1797); family Erebidae, subfamily Arctiinae; synonyms *Isia isabella*, *Phalaena isabella*[1](https://itis.gov/servlet/SingleRpt/SingleRpt?search_topic=TSN&search_value=117310) • [4](https://explorer.natureserve.org/Taxon/ELEMENT_GLOBAL.2.115923/Pyrrharctia_isabella) |
| Common names | Banded woollybear, black-ended bear, woolly worm, weather worm (larva); Isabella tiger moth (adult); isia isabelle (French)[1](https://itis.gov/servlet/SingleRpt/SingleRpt?search_topic=TSN&search_value=117310) • [3](https://content.ces.ncsu.edu/banded-woollybear) • [4](https://explorer.natureserve.org/Taxon/ELEMENT_GLOBAL.2.115923/Pyrrharctia_isabella) |
| Range | United States, Canada, and Mexico[2](https://www.weather.gov/arx/woollybear) |
| Caterpillar size | Mature larvae a little over 1 inch (about 2.5 cm) long, covered with stiff hairs[3](https://content.ces.ncsu.edu/banded-woollybear) |
| Adult wingspan | 4.5 to 6.5 cm (about 1¾ to 2¼ inches)[13](https://www.butterfliesandmoths.org/species/Pyrrharctia-isabella) • [11](https://ipm.missouri.edu/MEG/archive/2014/v20n11.pdf) |
| Cold tolerance | Freeze-tolerant final-instar caterpillar; supercooling point of winter-acclimated animals between −6 and −8 °C; reported survival to −90 °F (about −68 °C)[10](https://www.sciencedirect.com/science/article/pii/S0022191011001181) • [11](https://ipm.missouri.edu/MEG/archive/2014/v20n11.pdf) |
| Conservation status | NatureServe global rank G5 (secure); population trend appears relatively stable[4](https://explorer.natureserve.org/Taxon/ELEMENT_GLOBAL.2.115923/Pyrrharctia_isabella) |
## Description
The adult is a rather nondescript moth compared with many boldly striped tiger moths: orange-yellow to yellowish brown, with small black spots on the body and wings, and distinctive lines of dark spots along the top of the abdomen.[5](https://mynpn.usanpn.org/npnapps/species/Pyrrharctia/isabella) The forewing is pointed and marked with faint brownish antemedial, median, and postmedial lines and a broken black reniform spot; the hindwing is usually paler and flushed with pinkish orange in the female.[13](https://www.butterfliesandmoths.org/species/Pyrrharctia-isabella) The wingspan is 4.5 to 6.5 cm.[13](https://www.butterfliesandmoths.org/species/Pyrrharctia-isabella)
The caterpillar is densely covered with bristly hairs that are black at both ends of the body and light reddish brown in the middle.[8](https://yardandgarden.extension.iastate.edu/article/1994/11-11-1994/wooly.html) Mature larvae are a little over an inch long.[3](https://content.ces.ncsu.edu/banded-woollybear) The color pattern changes with age: the caterpillar molts six times before becoming an adult, and with each molt the coppery-brown mid-section becomes progressively longer as black hairs are replaced with orange ones.[11](https://ipm.missouri.edu/MEG/archive/2014/v20n11.pdf) • [3](https://content.ces.ncsu.edu/banded-woollybear) Younger larvae often show more black than mature, last-instar woolly bears.[5](https://mynpn.usanpn.org/npnapps/species/Pyrrharctia/isabella)
## Life cycle and overwintering
The species is a generalist feeder whose caterpillars consume many plants, including asters, birches, clover, corn, elms, maples, and sunflowers.[4](https://explorer.natureserve.org/Taxon/ELEMENT_GLOBAL.2.115923/Pyrrharctia_isabella) • [13](https://www.butterfliesandmoths.org/species/Pyrrharctia-isabella) In autumn the caterpillars wander extensively, sometimes in large numbers, seeking a hibernaculum, a concealed spot under bark, leaf litter, or a log where they spend the winter.[3](https://content.ces.ncsu.edu/banded-woollybear) • [5](https://mynpn.usanpn.org/npnapps/species/Pyrrharctia/isabella) • [12](https://phys.org/news/2014-12-wise-woolly-caterpillar-wintry-weather.pdf) Apparently only nearly mature caterpillars hibernate.[5](https://mynpn.usanpn.org/npnapps/species/Pyrrharctia/isabella)
After overwintering, the caterpillars feed in spring, form a cocoon from their own hairs together with silk, and pupate; the yellow-orange moth emerges about two weeks later and is found from early April through September.[11](https://ipm.missouri.edu/MEG/archive/2014/v20n11.pdf) The moths hide in trees and fly at night; after several days they mate, lay eggs, and die.[12](https://phys.org/news/2014-12-wise-woolly-caterpillar-wintry-weather.pdf)
## Cold hardiness
Unlike most insects, the woolly bear overwinters as a freeze-tolerant caterpillar, surviving ice formation in its body rather than avoiding it.[10](https://www.sciencedirect.com/science/article/pii/S0022191011001181) Winter-acclimated caterpillars have a supercooling point between −6 and −8 °C and accumulate glycerol as the main cryoprotectant, a natural antifreeze that protects cells from damage by ice crystals.[10](https://www.sciencedirect.com/science/article/pii/S0022191011001181) • [12](https://phys.org/news/2014-12-wise-woolly-caterpillar-wintry-weather.pdf) Cold acclimation raises glycerol to levels exceeding 300 mM in the hemolymph (the fluid acting as insects' blood).[14](https://onlinelibrary.wiley.com/doi/pdf/10.1002/jez.a.300)
Laboratory measurements quantify the trade-offs. Tissue ice content rises as freeze temperature drops: 24.4% of body water froze at −3 °C, 40.2% at −6 °C, and 48.7% at −10 °C, values below what hemolymph osmolality alone would predict, because glycerol at 200 to 300 mM colligatively reduces the amount of freezable water.[9](https://pubmed.ncbi.nlm.nih.gov/12770036/) Caterpillars resumed locomotion within minutes after thawing, but mortality accumulated over the following weeks, reaching 52.2% in the −10 °C fast-thaw group; pupation rates in test groups ranged from 45.7% to 52.4%, and adult emergence exceeded 60% of pupae in the −3 and −6 °C groups.[9](https://pubmed.ncbi.nlm.nih.gov/12770036/) About 90% of individuals survive being frozen at −12 °C for 35 hours, but repeated or prolonged freezing increases mortality, as does exposure to lower temperatures.[10](https://www.sciencedirect.com/science/article/pii/S0022191011001181) Extension sources report that caterpillars in natural shelters can survive temperatures as low as −90 °F.[3](https://content.ces.ncsu.edu/banded-woollybear) • [11](https://ipm.missouri.edu/MEG/archive/2014/v20n11.pdf)
## The winter-weather folklore
According to folklore, the amount of black on the caterpillar in autumn varies proportionately with the severity of the coming winter where it is found: longer black bands mean a longer, colder, snowier winter, a wider middle brown band means a milder one, a dark head end means a severe start to winter, and the caterpillar's thirteen body segments supposedly correspond to the thirteen weeks of winter.[2](https://www.weather.gov/arx/woollybear) • [11](https://ipm.missouri.edu/MEG/archive/2014/v20n11.pdf) A further version holds that caterpillars crawling southward are escaping a bitter winter.[11](https://ipm.missouri.edu/MEG/archive/2014/v20n11.pdf)
There is no known factual basis for these beliefs.[5](https://mynpn.usanpn.org/npnapps/species/Pyrrharctia/isabella) The National Weather Service notes that the width of the banding indicates the current or past season's growth rather than the severity of the upcoming winter.[2](https://www.weather.gov/arx/woollybear) Entomologists attribute year-to-year variation in banding to age and wetness: older caterpillars have more black than young ones, and caterpillars that grew in a wetter fall have more black hair than those from dry areas.[8](https://yardandgarden.extension.iastate.edu/article/1994/11-11-1994/wooly.html) A better growing season, particularly an early spring, gives caterpillars a head start on feeding and growing, so the warmer the past spring, the wider the brown stripe.[7](https://extension.missouri.edu/news/can-the-woolly-bear-caterpillar-predict-the-weather) • [12](https://phys.org/news/2014-12-wise-woolly-caterpillar-wintry-weather.pdf) Reviews of the claim find little reliable or repeatable evidence linking winter severity to stripe breadth.[12](https://phys.org/news/2014-12-wise-woolly-caterpillar-wintry-weather.pdf) One proposed biological kernel for the tale is that black integument absorbs more solar energy in ectothermic animals, a link between melanism and environmental conditions that researchers have begun testing directly.[6](https://papers.ssrn.com/sol3/papers.cfm?abstract_id=6140918)
**Origin of the legend.** The myths circulated for centuries but spread dramatically after a story in the *New York Herald Tribune* in 1948, when Dr. Howard Curran, an entomologist at the [American Museum of Natural History](https://www.edgepedia.org/american-museum-of-natural-history), counted the brown bands of 15 caterpillars at [Bear Mountain State Park](https://www.edgepedia.org/bear-mountain-state-park), New York, and made a winter prediction from them.[11](https://ipm.missouri.edu/MEG/archive/2014/v20n11.pdf)
## Cultural role and status
The caterpillar anchors two annual events: a Woollybear Festival each fall in [Vermilion](https://www.edgepedia.org/vermilion), Ohio, and a Woolly Worm Festival in Banner Elk, North Carolina.[3](https://content.ces.ncsu.edu/banded-woollybear) Because the caterpillars are virtually harmless to people, feed on weeds, and are noticed mainly in autumn as they search for overwintering shelter, insecticide control is unnecessary.[3](https://content.ces.ncsu.edu/banded-woollybear) Handling can cause a dermatitis-type reaction in some people, though the setae lack the substance behind the painful stings of some other caterpillars.[11](https://ipm.missouri.edu/MEG/archive/2014/v20n11.pdf)
NatureServe ranks the species G5, secure, describing it as widespread, abundant, known from many localities, and not apparently declining; more than 12,000 research-grade observations were recorded across its range between 2010 and 2019.[4](https://explorer.natureserve.org/Taxon/ELEMENT_GLOBAL.2.115923/Pyrrharctia_isabella)
## References
1. [ITIS Report: *Pyrrharctia isabella* (J. E. Smith, 1797), TSN 117310](https://itis.gov/servlet/SingleRpt/SingleRpt?search_topic=TSN&search_value=117310)
2. [Woolly Bear Caterpillar – Winter Predictor Or Not? National Weather Service](https://www.weather.gov/arx/woollybear)
3. [Banded Woollybear / Isabella Moth, NC State Extension](https://content.ces.ncsu.edu/banded-woollybear)
4. [*Pyrrharctia isabella*, NatureServe Explorer](https://explorer.natureserve.org/Taxon/ELEMENT_GLOBAL.2.115923/Pyrrharctia_isabella)
5. [Nature's Notebook: Woolly Bear (*Pyrrharctia isabella*)](https://mynpn.usanpn.org/npnapps/species/Pyrrharctia/isabella)
6. [Davis, A. et al. Can woolly bear caterpillars really predict the weather? SSRN](https://papers.ssrn.com/sol3/papers.cfm?abstract_id=6140918)
7. [Can the woolly bear caterpillar predict the weather? MU Extension (2018)](https://extension.missouri.edu/news/can-the-woolly-bear-caterpillar-predict-the-weather)
8. [Do Woollybears Predict the Winter Weather? Iowa State Yard and Garden (1994)](https://yardandgarden.extension.iastate.edu/article/1994/11-11-1994/wooly.html)
9. [Layne, J. R. & Blakeley, D. L. (2002). Effect of freeze temperature on ice formation and long-term survival of the woolly bear caterpillar. *Journal of Insect Physiology* 48(12): 1133–1137.](https://pubmed.ncbi.nlm.nih.gov/12770036/)
10. [Marshall, K. E. & Sinclair, B. J. (2011). Transmembrane ion distribution during recovery from freezing in the woolly bear caterpillar. *Journal of Insect Physiology*.](https://www.sciencedirect.com/science/article/pii/S0022191011001181)
11. [Warmund, M. (2014). Wintery Weather Folklore. Missouri Environment and Garden, November 2014.](https://ipm.missouri.edu/MEG/archive/2014/v20n11.pdf)
12. [Coppinger, J. P. (2014). How wise is the woolly bear caterpillar's wintry weather prediction? The Conversation via Phys.org](https://phys.org/news/2014-12-wise-woolly-caterpillar-wintry-weather.pdf)
13. [Isabella Tiger Moth or Banded Woolybear, Butterflies and Moths of North America](https://www.butterfliesandmoths.org/species/Pyrrharctia-isabella)
14. [Layne, J. R. (2006). The influence of freeze duration on postfreeze recovery by caterpillars of *Pyrrharctia isabella*. *Journal of Experimental Zoology Part A*.](https://onlinelibrary.wiley.com/doi/pdf/10.1002/jez.a.300)
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*Topic: Encyclopedia › Life and health › Animals › Invertebrates › Arthropods › Insects › Butterflies and moths › Erebidae and tiger moths*
*Initially written Sep 17, 2026 · Reviewed: Sep 19, 2026; Oct 21, 2026 · Edited: Sep 19, 2026; Oct 21, 2026 · Last review: Oct 21, 2026*
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License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license