Invertebrates
Isabella Tiger Moth
Pyrrharctia isabella · also called woolly bear, woolly worm
A fuzzy black-and-reddish-brown caterpillar hurries across a cold autumn sidewalk, and passersby stop to examine its band widths for a hint of the coming winter, unaware that whatever the woolly bear predicts, it has nothing to do with the weather and everything to do with simply surviving that winter itself.
Overview
The Isabella tiger moth, Pyrrharctia isabella, is a moth whose larval stage, the woolly bear caterpillar, is native to forest, grassland, and garden habitat across a broad range spanning most of the United States and southern Canada. Its caterpillar’s remarkable ability to survive freezing solid, enduring folk legend about winter prediction, and status as one of the most recognisable caterpillars in North America make it far more famous in its larval form than as an adult moth.
A caterpillar more famous than the moth it becomes
While the adult Isabella tiger moth is a plain, easily overlooked insect, its woolly bear caterpillar has become a genuine cultural icon, inspiring festivals and folklore built entirely around a creature most people never realise eventually grows wings.
Status
Least Concern — widespread and abundant across suitable forest and grassland habitat throughout most of the United States and southern Canada.
Isabella Tiger Moth: common questions
Can a woolly bear caterpillar really survive being frozen?
Yes, the woolly bear caterpillar produces natural cryoprotectant compounds that protect its cells from fatal ice damage, allowing a significant portion of its body to freeze nearly solid during winter without killing it, after which it thaws out again and resumes normal activity once temperatures warm up in spring, a remarkable feat of cold tolerance for an insect.
Does the woolly bear's band width really predict winter severity?
No, despite the popular folk legend, there is no established scientific evidence that the width of the woolly bear's reddish-brown middle band reliably predicts how harsh the coming winter will be, since band width actually varies with the caterpillar's age, diet, and how far along it is in its own development rather than with future weather conditions.
Why does the woolly bear caterpillar take so long to become a moth?
Because this species lives in a climate cold enough to require overwintering as a larva, and because that process may need to repeat across more than one winter depending on conditions, the woolly bear can spend several years as a caterpillar in total before it finally pupates and emerges as an adult Isabella tiger moth.
What does the adult Isabella tiger moth look like?
Unlike its famous, boldly banded, fuzzy caterpillar, the adult moth is comparatively plain, a fairly dull yellowish-orange colour with just a few small dark spots scattered across its wings and body, attracting considerably less attention and recognition than the woolly bear stage most people are familiar with.