The Hungry, Hairy Heroes

Kelly’s Field Notes

Common Name: Woolly bear caterpillar, Woolly worm, Isabella tiger moth

Family: Erebidae

Genus and Species: Pyrrharctia isabella (there are at least 8 woolly bear species in the United States!)

Life Cycle:

Female Isabella tiger moths lay 100+ eggs per batch, which hatch in about 5 days. The longest portion of their lives begins when those eggs hatch. There are two generations per year

Woolly bear caterpillars are the larval version of the Isabella tiger moth. When they hatch woolly bear caterpillars are only a few millimeters when they hatch but will reach a full size of nearly 3.8 centimeters (1.5 inches) in length. They get this large by spending all of their time munching away on their favorite plants. You can find woolly bears on many different types of leaves from tree leaves, to stinging nettles, to plantain, to dandelions. They are generalists when it comes to food. Woolly bears living in the Arctic (Gynaephora groenlandic) can spend up to 14 years in this stage, munching and hibernating (diapause) for seven winters until it’s time to become a moth (Kukal and Dawson, 1989; Saft, 2020). Woolly bear species living south of the Arctic can live to around 7 years.

Once the woolly bear has feasted enough, it’s time to pupate! The woolly bear uses its fur to create a cozy cocoon, attached to a twig or leaf. After about 10-15 days the Isabella tiger moth emerges in all of her splendor (Kanuckel, 2021). The adult moth only lives for five to ten days, where its only purpose is to find a mate then lay eggs (The Nature Conservancy, 2022). The adults cannot eat, surviving off of their fat stores when they were caterpillars.

Super Powers:

  • Woolly bears freeze in the winter and thaw in the Spring! Through dehydration and antifreeze-like chemicals their vital organs are protected until it’s time to thaw out at around 50oF (Payne, 1927; Layne and Blakeley, 2002). This doesn’t mean they are not harmed and even woolly bears have their limits as far as temperature and duration of freezing goes (Layne et al., 2006)!

  • Woolly bears are voracious eaters! They eat a wide variety of plants and a lot of them.

Woolly Bears in Culture:

  • Woolly bears can (maybe) predict the weather! Folklore from the colonial era of North America suggests that the wider the red band, the harsher the winter months will be in that region. According to the Farmer’s Almanac (2022), in 1948, Dr. Howard C. Curran, curator of insects at the American Museum of Natural History tested this long running theory. Unfortunately, his results were inconclusive.

  • There are festivals in the U.S. dedicated to the Woolly bear! The festival in Vermillion, Ohio is the largest single day festival the state puts on all year. Find their webpage here:


Kanuckel, A. How Did A Fuzzy Caterpillar Become A Weather Forecaster? The Farmers Almanac. (2021). Accessed July, 2022.

Layne Jr, Jack R., and Deborah L. Blakeley. "Effect of freeze temperature on ice formation and long-term survival of the woolly bear caterpillar (Pyrrharctia isabella)." Journal of insect physiology 48.12 (2002): 1133-1137.

Layne Jr, Jack Randall, and Benjamin James Peffer. "The influence of freeze duration on postfreeze recovery by caterpillars of Pyrrharctia isabella (Lepidoptera: Arctiidae): when is survival enough to qualify as recovery?." Journal of Experimental Zoology Part A: Comparative Experimental Biology 305.7 (2006): 570-575.

Payne, Nellie M. "Measures of insect cold hardiness." The Biological Bulletin 52.6 (1927): 449-457.

Saft, Carolyn. Can I touch a Wooly Bear Caterpillar? University of Florida. Accessed July, 2022.

The Farmer’s Almanac. “Woolly Bear Caterpillars and Weather Prediction.” Accessed July, 2022.

The Nature Conservancy. “Banded Woollybear (Isabella Tiger Moth larva).” Accessed July, 2022.