Wednesday, October 13, 2021

Explore Outdoors ~ Woolly Bear Behavior

Broly0, Wikimedia Commons
Woolly bears ~ the fuzzy black and brown caterpillars of the Isabella Tiger moth ~ are on the move. As the days grow shorter and cooler, the fuzzy caterpillars are searching for a place to curl up and hibernate.

My friend, Colleen, is passionate about all things lepidoptera. She stops to help woolly bears across roads whenever she comes upon them. She picks them up and carries them to the other side, in the direction they were headed. And over the past few years, she’s noticed something interesting about the woolly bears in our part of upstate New York.

“Normally, when you pick up a woolly bear, it curls up in your hand,” says Colleen. This is its defense: hide the tasty soft parts of its body and look like a prickly hedgehog to potential predators. “But some of them thrash back and forth.”

Curious, she decided to raise some “thrashers”. She put them in a container with food – pilewort and dandelion leaves – and put a screen on top. Later, she noticed some bullet-shaped pupae. Tachinid flies, perhaps?

Tachinid flies parasitize other caterpillars. The female fly lays an egg on the unsuspecting insect and the larvae grow inside - eating their host from the inside out. Then they drop to the earth and pupate in the soil. Colleen wonders whether tachinid fly parasites might cause the thrashing behavior of the woolly bears.

photo by Colleen Wolpert

A Backyard Citizen Science Project:

You can help collect data. Just pick up any woolly  bear caterpillars you come across and make a few notes:

  • do they curl or thrash?
  • when did you observe this (date)?
  • where were you? 
  • Put your observations in the comments below.

Then, if they were crossing the road, put them on the side where they were headed. 

You don't have to raise any of the "thrashers", but if you want to, make sure to give them fresh food every day – they love nibbling on dandelions and pilewort (shown in the photo). Also put some grass and fall leaves in for them to hide under. Clean out the woolly bear home daily, so moisture doesn't build up and cause mold - and keep a tight cover on the bear cage. You don't want them to escape!


  1. There's so much to love about woolly bears, they even get their own festival in Vermilion, Ohio--I used to go when I lived in northern Ohio. You should write a book about them!

  2. I don't see many caterpillars out here in Northern California. I miss the woolly bears. I miss a lot about living in the midwest. Today this poem appeared in my email, and I thought you would enjoy it. I hope you do.

  3. Sue, You might want to mention that if captive caterpillars stop feeding at this time of year, it's not the capturers fault: they go into dormancy ("diapause") to overwinter as caterpillars.

  4. Woolly Bears will wake up, wander and feed. I have taken photos of them crossing snow which seems insane. As insects, I don't even know how they manage but some insects have life stages that only function well in cold temps. Amazing. If you ever get the chance to take Charley Eiseman's Bugs in Winter class, please do. Some cool stuff out there in the winter in the Northeast and beyond. BTW, I have had more non-curlers than curlers. About 4:1 so far.

  5. My count so far: 4 curlers; 0 non-curlers.