Woolly Bear Predictions?

Fall is here.  My resident hummingbirds have vanished, preferring warmer parts where they can find nectar to sustain their high energy needs.  The leaves are dropping, carpeting my lawn, and providing a needed change to my exercise routine.  And, when I do still walk, the Woolly Bear caterpillar dots the roadways and trails, searching in earnest for places to hibernate for the winter.

Or so I thought.  The Woolly Bear caterpillar doesn’t actually hibernate.  It freezes solid!  Having been born in the fall, it’s not yet ready to pupate into an adult and complete its life cycle.  Somehow in its evolutionary journey, the process of cryoprotection was deemed more successful than, say, the monarch’s habit of flying hundreds of miles south to overwinter in a warmer climate as an adult butterfly.

All moth and butterfly species have evolved to survive the winter in one of the stages – egg, larva, pupa, or adult – of its life cycle.  The Eastern Tent caterpillar overwinters in the egg form.  The Monarch is a well-known example of a species that overwinters in the adult form.  The Woolly Bear has found success overwintering in its larval form.

It sounds like science fiction!  The Woolly Bear larva produces a cryoprotectant, which allows it to survive temperatures down to -90°F.  The little caterpillar’s heart stops beating, its guts and blood freeze solid, and then the rest of its body freezes. In the spring it warms up and continues its life cycle first eating some more, then pupating, emerging as an adult, mating, laying eggs and dying.  The busy adult stage only lasts a few days.

The Isabella Tiger Moth (Pyrrharctia isabella) lives all over North America.  In the Arctic, short summers mean not enough feeding time to pass through each instar – the stages between each molt while a larva.  In this case a Woolly Bear larva might freeze and thaw up to 14 times –that means 14 winters – until it is big enough to pupate into an adult!  This is incredible!  I don’t like to be cold, and I can’t imagine freezing and thawing (if I could even survive the process) without pain.

So the truth really is stranger than fiction!  As a child I loved the folklore stating that one could predict the severity of the coming winter based on the amount of red-brown or black hairs on the caterpillar in late fall.  The lore asserts that a bigger red-brown section forecasts a mild winter.  Conversely, more dark hairs mean a more severe winter.  It’s cute.  I love winter, even though I hate to be cold, and as a child, I delighted in sighting a caterpillar with more dark hairs on its ends.  There are even Woolly Bear festivals that are the fall cousin to the groundhog that, for fun at least, predict spring in Punxsutawney, Pennsylvania.  But this ability to predict the weather, which I don’t believe has any scientific basis, pales in comparison to being able to survive freezing!

Woolly Bear larva
Woolly Bear larva

No one has been able to prove the folklore about predicting winter weather, and people have tried.  That doesn’t stop some people from still believing it.  I guess they conveniently forget about their predictions when the results don’t pan out.  Or perhaps, they rationalize one severe storm as proof of their prediction for a harsh winter.  I have never found a fall season where all the Woolly Bears I see have the same or similar amount of red-brown and black hairs, a feature that would have to exist if this little larva was capable of predicting the weather.

It turns out that the hair coloration of the larvae is actually evaluative of the summer just ending rather than predictive of the coming weather.  This is because the red-brown middle section grows larger as the caterpillar ages, measured as number of instars.  More red-brown hairs indicate that a caterpillar has had longer to feed before the fall weather arrives.  In other words, it means it is older.  After the first frost, the caterpillar seeks a place to ‘freeze up’ for the winter.

I can’t imagine heading south as a human or a butterfly to avoid winter, but I really can’t imagine curling up under some bark and freezing solid for months . . . waiting, yet not conscious of when or what was next.  I have a new admiration for what the Woolly Bear tells me:  not that it can predict the winter, but that there are so many amazing, stranger than fiction, feats to discover in the animal world.  Humans seem so boring sometimes.

Leopard moth larva - also overwinters in larval stage.  I'm not sure if it freezes also.
Leopard moth larva – overwinters in larval stage. I’m not sure if it also freezes.
Leopard moth adult
Leopard moth adult



4 thoughts on “Woolly Bear Predictions?”

  1. Thanks Cathy! I was actually wondering if the folklore was still passed around. It seems so from places like Farmer’s Almanac. I wonder if it was more common to talk about in the north.


  2. I’ve wondered about that! I’m glad you posted this. We’ve found them under leaves in the yard in the winter and I wondered if they just perished or what? And now I know! 😀


    1. Glad you liked it; I’m enjoying learning this stuff and sharing it. I am seeing so many woolly bear caterpillars right now. They must know we are about due for our first freeze of the season.


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