Woolly Bears

Ever seen one of these caterpillars crossing the road in the fall and wondered where it is going with such a sense of purpose?

Woolly Bears (also known as”Fuzzy Wuzzies”) won’t sting or bite, and if you pick one up, they’ll usually curl right up into a tiny ball to protect itself.

They are woolly bear caterpillars (Pyrrharctia isabella), and they are the juvenile form of the Isabella tiger moth:

Photo by Dr. Everett Cashatt, courtesy of the Illinois State Museum.

You may wonder why wooly bear caterpillars put themselves at such risk by crossing roads. They are  are not fussy eaters, and will eat just about anything (including dandelions, grasses, plantains, nettles and many other common wild plants), so probably they’re not searching for some kind of rare or specialized food source.

More likely is that they’re scouting around for a safe place to spend the winter. When the weather turns cold in fall, almost- fully-grown woolly bear caterpillars curl up in the leaf litter under plants and trees, or in wood/brush piles, and hibernate til spring.

Amazingly, they have adapted to survive the frozen northern winters by producing a chemical substance that protects them from freezing! A natural antifreeze…

Unfortunately for woolly bear caterpillars, hungry winter birds consider them a great protein source, so many caterpillars don’t make it through to spring.  Those that survive wake up in March or April when grasses and plants start forming leaves, eat for a short time, then begin spinning a dark silky cocoon around themselves to continue the transformation into a tiger moth.

So what do you think? Why do you think the woolly bear is crossing the road? Is it looking for fallen leaves or the brushpile in your beautiful wildlife garden where it can sleep until spring?

Ellen Sousa is a garden coach and writer from Spencer, MA, where she maintains a small horse farm as NWF Certified Backyard Habitat 71074. Visit her habitat farm website and blog at THBFarm.com

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  1. Pam Resor says

    They might also be heat-seeking. Lots of critters are on the asphalt in early spring and late fall because the black asphalt surface is nice and warm in the sun, even when the air is quite cold.

    • says

      hi Pam (nice to see you here, btw!)
      You could be right, the warm pavement must be nice to tiny creatures when the weather gets cold, even ones with warm furry coats like the woolly bear ;-)

    • says

      That journey would even surpass the monarch butterfly’s migration! haha. Let us know whether your woolly bears also turn into tiger moths or similar. Do you have silkworm moths in South Africa?

      • Justin Johnson says

        Thanks Ellen I found one and I was so scared that it wouldn’t live but now that I know it eats grass and other plants I’m reliefed!

    • says

      You know, I’ve heard so many times that it’s a myth that the size of the woolly bear’s stripe is a portent of the winter’s severity, but I have heard a theory which might finally explain the history of it…a woolly bear sheds its skin several times through the autumn, and each time, its middle orange band becomes wider and black bands more narrow. If winter comes early, the caterpillars are more likely to dive down into the leaves and hibernate, so we are less likely to see cats with the wider orange band. Makes sense to me!

  2. Rick says

    We’re in Southern California and we’ve had some woolly bears in captivity for at least 4 weeks and they haven’t cocooned yet. Do they burrow or hang on branches? Perhaps we’re not feeding them enough? They are growing and seem healthy.

    • says

      Rick – they burrow down in leaf litter for the winter here in New England, not sure about California. In spring they wake up, feed for a while and then turn into a cocoon. Do they have some host plants to feed on where you are? They feed on a variety of plants, plantain, some grasses and forbs, also certain types of trees. I’d check up on what plants they feed upon in your area and transfer your cat to one of those plants…
      Ellen Sousa recently posted..Norcross Sanctuary – Hidden Jewel of Monson, MA


  1. […] furry moth species out there, including tussock and tiger moths (which include the familiar woolly bear caterpillars), but be very careful about touching them. Some people can get a rash by touching the hairs… […]

  2. […] Woolly Bears “Ever seen one of these caterpillars crossing the road in the fall and wondered where it is going with such a sense of purpose? They are woolly bear caterpillars (Pyrrharctia isabella), and they are the juvenile form of the Isabella tiger moth” by Ellen Sousa […]

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