By tradition, the woolly bear caterpillar’s stripes forecast the winter ahead: lots of black means a rough season, and a wide brown middle means a gentler one. The bands actually record the caterpillar’s own past, not the weather to come.1mothman.org. ADULT AND LARVA OF MOTHS OF PENNSYLVANIA The more interesting winter story belongs to the caterpillar itself, which gets through the cold by freezing nearly solid.
How to Read a Woolly Bear the Folklore Way
A woolly bear has a black band at each end and a rusty-brown band in the middle. Folklore says the amount of black you see in autumn matches how severe the coming winter will be where the caterpillar is found.2National Weather Service. Woolly Bear Caterpillar – Winter Predictor Or Not? Long black bands point to a longer, colder, snowier winter, while a wide brown middle band is said to signal a milder one.3National Weather Service. Woolly Bear Caterpillar – Winter Predictor Or Not?
To try a reading on a caterpillar you find, look at it from above while it is stretched out and walking, since a curled-up one hides its bands. Compare the length of the brown middle with the two black ends combined. The lore applies to your own area, meaning wherever you found the caterpillar. Some readers go further and split the caterpillar’s 13 body segments into the weeks of winter, with each black segment standing for a harsh week.4NPR. The Myth of the Woolly Bear
Make sure you have the right caterpillar first. The classic woolly bear has short, stiff, even bristles in a black-brown-black pattern. All-black or all-yellow fuzzy caterpillars are different species and carry no folklore reading. Seeing woolly bears crossing roads in fall is normal, because they are looking for a winter hiding spot.
Even when you play along, it stays a game. A caterpillar that has crossed a road is not a clean sample of your area, so a reading made on a driveway is only a bit of fun. Fans often say to check several and go with the majority, which is a friendly way to play but doesn’t turn the result into a forecast.
Why the Stripes Can’t Forecast Winter
As a woolly bear grows, it sheds its skin several times. With each molt it shows less black and more reddish-brown.5mothman.org. ADULT AND LARVA OF MOTHS OF PENNSYLVANIA By late fall, caterpillars of different ages naturally carry different band widths, so the color differences reflect age, not the severity of the winter ahead.6mothman.org. ADULT AND LARVA OF MOTHS OF PENNSYLVANIA So a caterpillar with a wide brown middle has usually shed its skin more times and is further along in growth than a blacker one, which is often younger with more growing left to do.
Growing conditions matter too. Temperatures while the caterpillar is young are reported to shape its coloring, with warmth linked to more brown and cold to more black.7Waterborne Environmental. Insects as Climate Change Indicators: Meet the Woolly Caterpillar – Waterborne Environmental How much it ate and how fast it grew affects the banding as well.8Sanford Springvale News. Out In the Woods – Feeding and Age Determine Wooly Bear ‘Forecast’ Finding one mostly black and one mostly brown in the same yard is expected, since they can differ in age and growing conditions.
Weather is driven by large-scale atmospheric conditions, not by animal behavior or appearance.9City of Oak Grove Kentucky. Groundhog Day: Separating Fact from Fiction Checks of Groundhog Day “forecasts” found no link to the weather that followed.10City of Oak Grove Kentucky. Groundhog Day: Separating Fact from Fiction
For planning, use a real seasonal outlook from your national weather service.
The myth still feels right sometimes, and there are reasons. Winter can only come out roughly milder or harsher than usual, so a random guess lands on the right side a good share of the time. People remember the years the caterpillar seemed right and quietly forget the misses. And what counts as a mild or harsh winter is loose, depending on place and personal impression, so it is easy to fit the outcome to the caterpillar after the fact.
How a Caterpillar Became a Weather Celebrity
The idea has been part of American folklore since colonial times.11Farmers’ Almanac. How Did A Woolly Bear Caterpillar Become A Weather Forecaster? Then in 1948, Dr. Howard Curran, insect curator at the American Museum of Natural History, went to Bear Mountain, New York, to test it.12Farmers’ Almanac. How Did A Woolly Bear Caterpillar Become A Weather Forecaster? He counted the bands on about 15 caterpillars.13NPR. The Myth of the Woolly Bear Finding that over half had wide brown bands, he predicted a milder winter, and it was.14Farmers’ Almanac. How Did A Woolly Bear Caterpillar Become A Weather Forecaster? A reporter’s story in the New York Herald Tribune then spread nationwide.15Sanford Springvale News. Out In the Woods – Feeding and Age Determine Wooly Bear ‘Forecast’
One season’s match could easily be luck. About 15 caterpillars from one hillside can’t stand in for a whole region either, because they may have been of similar ages or grown in similar conditions. A real test would need many caterpillars across many places over many winters, compared with the weather that actually followed. Curran is generally described as treating the outing as light-hearted, yet the newspaper coverage turned it into an annual tradition. Today, towns across the country hold woolly bear festivals with costumes, parades, and caterpillar races, all built on the story that began in the 1940s.
How Woolly Bears Survive Winter by Freezing
The caterpillars you see in fall are nearly full grown. They spend winter as caterpillars and become moths in spring.16Journal of Experimental Zoology. Triggering of cryoprotectant synthesis in the woolly bear caterpillar (Pyrrharctia isabella Lepidoptera: Arctiidae) In colder northern areas they enter a built-in winter pause that they must go through.17Elsevier / Journal of Insect Physiology. Effects of photoperiod and temperature on the development of a univoltine population of the banded woollybear, Pyrrharctia (Isia) isabella So they crawl into leaf litter or under logs to wait out the cold.
Woolly bears can survive freezing at moderate below-zero temperatures.18Journal of Experimental Zoology. Triggering of cryoprotectant synthesis in the woolly bear caterpillar (Pyrrharctia isabella Lepidoptera: Arctiidae) As fall cools, they build up a natural antifreeze in their body fluid.19PubMed. The influence of freeze duration on postfreeze recovery by caterpillars of Pyrrharctia isabella (Lepidoptera: Arctiidae): when is survival enough to qualify as recovery? Drying out triggers its production quickly as well.20Journal of Experimental Zoology. Triggering of cryoprotectant synthesis in the woolly bear caterpillar (Pyrrharctia isabella Lepidoptera: Arctiidae) With that antifreeze, less of their body water turns to ice, about a quarter at -3°C and about half at -10°C.21Elsevier / PubMed Central. Effect of freeze temperature on ice formation and long-term survival of the woolly bear caterpillar (Pyrrharctia isabella) Repeated freezing and thawing pushes them to make even more.22PubMed. The sub-lethal effects of repeated freezing in the woolly bear caterpillar Pyrrharctia isabella
Freezing has costs. Thawed caterpillars start crawling within minutes,23Elsevier / PubMed Central. Effect of freeze temperature on ice formation and long-term survival of the woolly bear caterpillar (Pyrrharctia isabella) but deep freezes still take a toll, with about half dying in the following weeks after -10°C and a fast thaw.24Elsevier / PubMed Central. Effect of freeze temperature on ice formation and long-term survival of the woolly bear caterpillar (Pyrrharctia isabella)
If you find one in late fall or winter, the best choice is to leave it outdoors. If it is on a path or driveway, nudge it onto leaf litter or under a log nearby so it isn’t stepped on. Bringing it into a warm house can disrupt its winter pause and timing, so it may wake and wander at the wrong season with nothing to eat. A caterpillar that is stiff and motionless in cold weather may be frozen rather than dead, since it is built to survive partial freezing, so leave it where it is. Leaving fallen leaves in garden beds gives woolly bears and other small creatures a place to shelter.
When spring arrives and it warms up, the caterpillar wakes and feeds for a short time before it changes form. It spins a cocoon, using its own bristly hairs as part of the material. Inside, it becomes the Isabella tiger moth, a fairly plain yellowish-orange moth that looks nothing like the caterpillar. The moth lays eggs, and the caterpillars that hatch from them are the next fall’s woolly bears.
Is It Safe to Pick Up a Woolly Bear
Some people get a rash after touching woolly bears.25Pointe Pest Control. Woolly Bear Caterpillars Aren’t Warm & Fuzzy The rash is temporary and not dangerous.26Pointe Pest Control. Woolly Bear Caterpillars Aren’t Warm & Fuzzy Wash your hands after handling one and avoid touching your eyes.
When disturbed, a woolly bear curls into a tight bristly ball. With kids, let the caterpillar crawl onto a leaf or stick rather than grabbing it, since a gentle approach keeps it from curling up and keeps skin contact down. Keep the handling short and put it back where you found it, so it can continue toward its winter hiding place. If a child gets itchy, rinse the skin with soap and water, and the mild rash should fade on its own.