Woolly Bear Caterpillar update

Woolly Bear Caterpillars are frequently seen crossing trails in late autumn and early spring

Common Name: Woolly Bear Caterpillar, Woolly Worm, Fuzzy Bear, Hedgehog Caterpillar – The dense tufted bristles are black at both ends and brown in the middle with a texture that is similar to wool. The short, rounded and blunt shape suggests an ursine association ― ergo, woolly bear. The caterpillar is the larval stage of the Isabella moth.

Scientific NamePyrrharctia isabellaPyyrh is the Greek word meaning red or tawny. Arctia is derived from the Greek word arktos, which means bear. The Isabella moth has red markings on the wings and is a northern species ranging into the arctic regions. The association of bear with arctic is due to the astronomical importance of Ursa Major, the Great Bear also known as the Big Dipper. The pole star Polaris at one end of Ursa Minor and  pointed to by two of Ursa Major’s stars is used in celestial navigation to locate true north. Isabella is a color that ranges from yellowish brown to olive brown, the basic wing color. Former scientific name Isia isabella.

Potpourri: Caterpillars are the larval stage of insects of the order Lepidoptera comprised of butterflies and moths. The distinction between these two groupings is as arbitrary as the common names given to animals and plants that are mostly descriptive with occasional mythic etymologies. Butterfly, as a case in point, is thought to derive from the belief that witches in the shape of flying insects stole milk and butter. Butterflies are most notable for their  brightly colored wings and zigzagging drunkenly across meadows in daylight. Moths are everything else that flies like a butterfly (and doesn’t sting like a bee). They are united in one order due to the physiology for which they are named, as butterflies and moths all have scaly (lepis in Greek) wings (pteron in Greek). The other almost universal difference is that butterflies are diurnal and moths are nocturnal. Their larvae, for the most part, stay hidden in the foliage as a survival matter. The trundling woolly bears are an exception to this rule. [1]

The woolly bear caterpillar is best known for its headlong scramble across trails in late fall and early spring. They have been observed by wayfarers since the colonial era, gaining a measure of notoriety. The reason for their haste at the larval sprint pace of four feet per minute is unknown, but there is a continuity of direction that suggests a specific goal. Conjecture is based in part on the well-established fact that woolly bears overwinter as caterpillars, freezing nearly solid as temperatures plummet according to season, latitude, and elevation. In the fall months, this would imply that there was some necessary location favoring cryogenic hibernation. However, since shelter from cold cannot be a factor, seclusion could only be to prevent predation ― but any nook or cranny would do. In the spring, the path is reversed for pupation, also a matter of finding an out of the way place to wait in helpless suspended metamorphic animation. Regardless of the destination, physiological actions occur in preparation for migration. A peak in the level of ecdysteriods (hormones that promote molting) triggers a cessation of feeding and gut evacuation followed by the quixotic quest. Since they are and have been a successful species, relocation has promoted propagation. This is in spite of the fact that caterpillars squashed while crossing busy roads and trails cannot equally promote longevity. The evolved ability to endure winters as larvae is certainly a relevant factor. [2]

Woolly bear is a contradiction on two counts: Caterpillars are not bears and their “wool” is not a winter coat. The stiff hairs called setae that extend outward in all directions from the larval body are the most notable features of the caterpillar, blocking out all other detail. The function of the hair is to some extent protective, as it blocks wasps like yellow jackets from direct access to administer their all too lethal sting. [3] Woolly bears instinctively roll up into a protective ball of spines to augment this defensive measure when frightened; this is the origin of the alternative common name hedgehog caterpillar. The dense hair also allows for controlled whole-body freezing, a rather surprising capability that is shared by only a few other animals, notably wood frog.

Woolly Bears curl up to use the bristles as protection, giving them the name hedgehog caterpillars.

As cold temperatures set in, a natural antifreeze compound composed of lipids and alcohols called glycerol is produced and distributed to the body by hemolymph, the blood-like body fluid of insects. The change in cryoprotectants with temperature has been verified in laboratory conditions. [4] As the cold slowly seeps in, the entire body except the very centers of the cells freeze solid in anticipation of the spring thaw. This capability has extended the range of woolly (polar) bears to the Arctic, where they have survived winters with temperatures as low as 90 degrees F below zero. Their life sequence is slowed to match the metabolic reduction to the extent that the normal one month metamorphosis from larva to adult moth can take up to 14 years. [5] 

The distinctive black-brown-black banding of the setae of the Isabela moth larva is the basis for the mythology of woolly worm weather prediction. It is not altogether unreasonable to believe that animals might be able to sense the mood swings of climate. The traditional folk wisdom of caterpillar color bands for winter forecasting may have been a direct assimilation of Native American lore. In any agrarian society, crop cycles are crucial to survival. Intelligence  about the beginning, duration, or end of winter would be of abiding interest. For example, the  counterpoint to woolly bears and the harshness of winter is the groundhog’s shadow that allegedly determines its duration. In its most general form, the definitive metric of woolly bear winter is the width of the middle brown band between the two black end bands. A narrow brown band is indicative of a severe winter whereas a broad brown band indicates a mild weather.

The quaint custom gained national credence in 1948 when two entomologists from the American Museum of Natural History in New York City collected 15 woolly bears and predicted a mild winter based on band width averages. Rather than publish a scientific paper, the two insect experts provided their results to a reporter whose article made the front page of the New York Herald Tribune, a respected newspaper. When their prediction proved correct, woolly bear wisdom gained a national audience. For the next seven years, the paper’s readers demanded annual weather predictions. The custom eventually fell out of fashion  as its randomness inevitably became obvious. [6]

There is no correlation between woolly bear coloring and winter.  The relative size of the brown center band relative to the two black end bands is a matter of nutrition and age.  The more fruitful the summer feeding season, the larger the caterpillar will become, its growth narrowing of the central brown section. The age factor involves molting. Woolly bears grow and mature in six intermediate steps called instars, shedding their skin each time and becoming less black and more brownish sequentially. It might be feasible to correlate the bounty of a summer season with woolly bear ring sizes, but that is after the summer fact and has nothing to do with the winter future. The established biology of caterpillar growth and molting has had little effect on the public embrace of the original myth, even adding new variants. For example, the woollier the coat, the worse the winter. A more creative version concerns the direction of transit. A woolly bear moving south is escaping the coming harsh winter and vice versa, going north if milder.

In an attempt to coopt the success of Punxsutawney Phil, several small towns have inaugurated fall celebrations featuring nature’s herald as star attraction. The most well known is the Woolly Worm Festival held annually in October to promote tourism to the small town of Banner Elk, North Carolina. The event is promoted as a race between contestant caterpillars in heats to determine the champion woolly worm. The festival has drawn as many as 20,000 attendees witnessing racing heats involving 1,000 participant larvae. The winner is the official  weather prophet, the color of each of its 13 segments correlated to the 13 weeks of winter. Black indicates below average temperature, light brown above average temperature, dark brown average temperature, and something called fleck is low temperature with light snow. This not without financial remuneration as incentive. The winning worm receives a $1,000 prize. [7]

The adult moth to which the woolly bear larva pupates is one of the tiger moths of the family Arctiidae that are mostly moths in butterfly clothing … many are brightly colored with spots and stripes in contrast to the whites, browns, and grays of the majority of moths. The Isabella moth is an outlier with muted yellow-brown wings as its name implies. Tiger moths in general are also capable of producing audible sounds, a trait not normally associated with moths, mostly seen but not heard. Noisemaking is to deter bats, the nemeses of “mothdom” in their shared nocturnal air space. Bats are small-bodied and warm-blooded, consummately voracious to maintain the necessary energy input. Tiger moths, and the closely related owlet moths (Family Noctuidae – the ones most frequently seen fluttering around lights at night as their nocturnal name implies; their larvae are cutworms) have large eardrum-like structures called tympanic organs to detect the echolocation sounds made by bats. Located on either side of the head with a sensor for intensity that correlates to distance, tiger and owlet moths can determine the direction and the distance of an approaching bat and take appropriate evasive action. The tiger moths take the listening and evading strategy a step further, emitting a clicking noise that is thought to disrupt bat sonar altogether. [8] Evolution is powered by predators.

Bright colors are counterintuitive for defenseless insects that would likely have better survival chances by being neither seen nor heard. This is as true for the larval, caterpillar stage as it is for the adult moth stage. The black-brown banding of woolly bears is eye-catching, particularly when it is moving across open areas. Tiger moths, at all life stages, widely employ distastefulness as a defensive mechanism. The use of vivid colors by animals to indicate that they are not palatable is called aposematism. Predators learn to avoid them after the first encounter, recognizing the color and pattern that is intentionally obvious for that reason. Monarch butterflies and red efts are good examples.

It has been established in laboratory testing that woolly bear caterpillars consume plants containing pyrrolizidine alkaloids for chemical resistance to lethal parasitic tachinid flies. In fact, experiments showed that they do this as a matter of self-medication, one of the first instances of some form of cause-and-effect invertebrate cognition. [9] Pyrrolizidine alkaloids found in some asters and legumes are one of the most toxic substances to both domestic animals and humans, causing severe metabolic disruption. Wild animals learn to scrupulously avoid them. Woolly bear caterpillars, along with many other tiger moth caterpillars that live exposed lives, are able to live rashly and openly only because they are chemically protected. Tiger moth larvae are also among the most polyphagous of all the lepidopterans, eating as many as 88 different plant species. [10] It is probable that the chemistry of the foods they eat is inclusive of pyrrolizidine alkaloids.  

It may be concluded that woolly bears are exceptional caterpillars. Taking survival of the fittest seriously, they have evolved a formidable suite of adaptations. Extending poleward to the frozen northern latitudes to escape bug and bird infested trailways, they eschew the warmth of woolens for woolly-ness. Seeking nutrition across a broad range of foliage choice, they ensure that there will always be a dessert of protective poisons. But why the annual diaspora? Maybe they are the tramps of the lepidopterans with Bruce Springsteen’s mandate … they are born to run. But they don’t predict the weather.


1. Milne, L. and Milne, M. National Audubon Society Field Guide to North American Insects and Spiders, Alfred A. Knopf, New York, 1980, pp 697-698, 790.

2. Wagner, D. “The Immature Stages: Structure, Function, Behavior, and Ecology”. In Conner, William E. (ed.). Tiger Moths and Woolly Bears: Behavior, Ecology, and Evolution of the Arctiidae. 2009 Oxford University Press  pp. 31–53.

3. Rich, G. “How woolly bear uses clever tricks to survive” Washington Post, 23 November 2021.

4. Layne, J. and Kuharsky, D..  Triggering of cryoprotectant synthesis in the woolly bear caterpillar (Pyrrharctia isabella Lepidoptera: Arctiidae)”. Journal of Experimental Zoology. 1 March 2000 Volume 286 Number 4 pp 367–371.

5. https://www.weather.gov/arx/woollybear – The U. S. Weather Service website.

6. https://bygl.osu.edu/index.php/node/1713               

7. http://www.woollyworm.com/     

8. Marshall, S. Insects, Their Natural History and Diversity, Firefly Books Ltd., Buffalo, New York, 2006, pp 174-177, 212-213.

9. Singer, M. et al “Self-Medication as Adaptive Plasticity: Increased Ingestion of Plant Toxins by Parasitized Caterpillars”. PLOS ONE. 10 March 2009 Volume 4 Number 3.

10. Wagner, op cit.