Sunday, October 26, 2008

Apple #348: Woolly Bears

Woolly Bear caterpillars, that is.

Woolly bear or woolly worm caterpillar
(Photo from Dr. Jeff Masters' WunderBlog)

A couple weeks ago, I saw one scurrying on its multiple, stubby little legs across the sidewalk. I haven't seen a woolly bear in years. I got a stick and put it in the caterpillar's path. After some hesitation, the caterpillar climbed on, and I walked for a while carrying the woolly bear on the stick.

More of its spiky fur -- if it can be called fur -- was brown than black. My mom's cousin who has a farm used to say that if a woolly bear has more brown, that means the upcoming winter will be mild. More black than brown, the winter will be harsh. Lots of people think that about woolly bears -- that they somehow know what the upcoming winter will be like and color their fur accordingly, kindly letting us in on their prognostication.

  • Woolly bears are the larvae of the Isabella tiger moth (Phyrrharctia isabella). It is very common in the US, Mexico, and southern Canada.
  • There are actually several different types of woolly bear caterpillars. They vary in color -- yellow, green, white, tan, etc. The one we're talking about here is the banded woolly bear.
  • The hair on a woolly bear is called bristles or setae. They are grouped with several other "bristled" species.
  • Only the legs on the first three segments of the woolly bear's body actually work. The other sets of legs behind those are "false" legs. The last leg on the back segment helps prop up the woolly bear as it reaches for leaves and other food.
  • They can't see very well, so they'll often rear up and kind of feel around for where to go next.
  • The woolly bear has two "defensive postures," as the scientists call it. When disturbed, they will either curl up into a ball or run away as fast as they can. For some reason, this cracks me up.

The woolly bear in its defensive posture. This is serious business as far as the woolly bear is concerned, never mind how cute and round and fluffy and funny-looking it is to us.
(Photo from WiseAcre Gardens)

  • Woolly bears, unlike some other kinds of caterpillars, don't congregate together. They like to go off by themselves. So if you see more than one woolly bear at a time, you've encountered something rare.
  • Woolly bears like to eat
  1. dandelions
  2. clovers
  3. weeds
  4. leaves of maples, asters, and birch trees
  5. if you're feeding them, they will eat spinach or cabbage
  • Usually, their quest for food takes them alongside roadsides or sidewalks, so that's why you often find them galumphing along in those sorts of places.
  • Actually, two generations of woolly bears are born in a year. The first round appears in May and the second in August. I'm not sure why, but we tend not to see the spring crop of woolly bears, only the ones in the fall.
  • Once they've eaten their fill, they'll go off looking for a place to sleep through the winter and work on becoming moths.
  • They like to nestle under tree bark or in cavities in rocks or in the nooks of fallen logs and spin their cocoons there.

Woolly bear in a finished cocoon, which is lying inside a milkweed pod.
(Photo by Ann on flickr)

  • If you find a woolly bear in hibernation state in late fall and bring it inside, it will warm up quickly and need food right away. If you want it to stay alive and become a moth, put it in a container with no food and no air holes -- counterintuitive, I know -- and put it in your refrigerator right away. It will go back to sleep until spring. Once spring comes around and there's vegetation available again, you'll have to give it food in its little container in the refrigerator for it to spin a cocoon and turn into a moth.

What the woolly bear caterpillar turns into: the Isabella Tiger Moth.
(Photo from EastTennesseeWildflowers)

The same moth with its wings spread a bit.
(Photo from Wing Watchers)

  • In 1948, one entomologist, Dr. C. H. Curran, decided that he wanted to find out if there was any truth to this folklore about the relationship between the color of a woolly bear's bristles and the forthcoming winter.
  • He studied the woolly bears for 8 years, and he did find that on average, around 5 of the 13 segments of the woolly bears during those years were brown. In other words, they were mostly brown. As it happened, the winters during those years were milder than average.
  • Before you get too excited, although Dr. Curran watched those woolly bears during those 8 years, he didn't keep an eye on very many of them. So he knew his data sample was small and therefore not very reliable.
  • More recent research has shown that the coloring of woolly bears may be related to the levels of food and moisture in the areas where the woolly bear was born and raised. The better fed the caterpillar, the less brown it will be.

Several bands of brown.
(Photo from Nathan Cook's site of naturalist photos of Grand Island, New York)

Fewer bands of brown. This one has had more to eat than the one above.
(Photo from Ecobirder)

  • Lots of people still hold onto this idea about the woolly bears and weather forecasting abilities. Elementary school classes often conduct scientific experiments involving woolly bears and weather. Newspaper columnists write about the woolly bear's predictions.
  • The good folks in Banner Elk, North Carolina, hold an annual Woolly Worm Festival (they're worms, not bears in Banner Elk) each October. Participants race their hand-picked woolly bears. The champion caterpillar is inspected by the judge, who then pronounces the forecast for the coming winter based on the winning woolly worm's coloring. 

This is 2011's winner, YoYoMa, on the nose of owner Matt Buckland. YoYoMa's winter forecast is very detailed, week by week. In general, he says it will be cold and snowy.
(Photo from the official Woolly Worm Festival in Banner Elk)

  • The owner of the winning woolly worm also wins a $1,000 prize.
  • Festival-goers say the woolly worms' predictions have been 70% accurate.

Old Farmer's Almanac, The Truth About Woolly Bears
Iowa State University Department of Entomology, Woollybear
Woolly, A Closer Look at the Woolly Worm
Bill Oehlke's tiger moth pages, Isabella Tiger Moth


  1. I found one outside my house remarkably I did find another one close by(it was frozen),but I did put it into a jar and I think it should like it in its new environment.

  2. This past Sunday, we found a Wooly worm under a log in our back yard. Not knowing the basics, we did put him in a container with small airholes, food, water, etc. Today the Wooly worm is gone, and there is a tiger moth, just like the one pictured, in the container. This seems impossible. There is no evidence of a cocoon, and he certainly wouldn't have had time to morph. Does this even seem logical?

  3. I grant you that that does seem like a rapid transformation. But is there another explanation that seems more logical?

  4. we just found one while camping! My children are very excited about watching it go through its transformation. Any suggestions for food or materials we should use to keep it in so it feels at home?

  5. 38th Annual Woollybear Festival in Vermilion, Ohio. Hosted by Fox8 Meterologist Dick Goddard. If you would like to join in on the fun contact the Vermilion Chamber of Commerce 440-967-4477 or email

  6. I found a Wooly Bear yesterday, hew was so cute and climbing on a house. I had to rescue him. I went on the internet to find what food he eats ( spinach, grass, etc.) and I found that they are supposed to hibernate frozen. I just let him outside and put him on a smooth leaf under a bush. Should I go get him and put him in a container, or will he be fine outside?


If you're a spammer, there's no point posting a comment. It will automatically get filtered out or deleted. Comments from real people, however, are always very welcome!