Banded Woolly Bear

2009 October 10

On Wednesday, we were all out for a walk and I happened to mention that even though the Woolly Bear caterpillar was very common, and October is prime woolly bear season, I somehow hadn’t gotten a picture of one yet, and I’d really ought to do something about it. So, on Thursday, Sam went out and caught one, and S_ took a series of pictures[1]:

I’d say that this is probably the single most widely known caterpillar in North America, because they are (a) distinctive, (b) common, (c) widespread, (d) pretty big, and (e) often found crawling across roads and sidewalks in fall, just in time to appear in school playgrounds where as many kids as possible are going to see them.


People generally like woolly bears, probably because they are so furry. As Sam put it, “He’s a cutie!” The fur is actually kind of prickly and brush-like, so they aren’t as nice to touch as one might think from looking at them, but the hairs are only somewhat irritating to the skin if you touch them, not actively toxic or anything like that.


They are the caterpillar of the Isabella Tiger Moth, Pyrrharctia isabella, a fairly large, light-tan moth with black specks on the wings[2]. They are one of a large group of moths that generally tend to have large, fuzzy caterpillars, most of which also get called “woolly bears”, but this is the only one where they are basically black caterpillars with a band of brown around the waist[3]. So, “banded woolly bear” is the best common name if you really want to be precise.

Unlike a lot of other moths and butterflies that overwinter either as eggs, or as pupae, woolly bears actually overwinter as caterpillars. The eggs are laid in the late summer or early fall, and the caterpillars eat pretty much anything they come across (grasses, low herbs, you name it) until they get to a good size. Then, when it gets cold, they hike off looking for a good place to curl up and hibernate over the winter (which is when we mostly see them). They are loaded with “cryoprotectants”, chemicals to keep their body fluids from freezing, so they can survive the winter with only moderate shelter. It’s adequate for them to hang out in the “subnivean environment”, the little gap between the ground and the snow where the temperature hovers at just about freezing. Then, in the spring, they come out, gorge for a while on all the tender young plant shoots that come up early, and pupate. They come out as moths over the summer, and then they do whatever moths do until it gets to be time to mate and lay eggs again.


The hairs are pretty good defense against large predators like birds, swallowing a woolly bear would be a lot like choking down a bottle brush. I gather that the major predators of all types of woolly caterpillars are actually things like parasitic wasps and flies, which is a pretty gruesome way to die. These parasites are mainly what keeps woolly bears from turning into some all-devouring plague upon the land.

It turns out that the woolly bears do have some ability to counteract the parasites; they are the first insects that have been shown to use medicines. When they are infected with parasites, they will intentionally seek out toxic, alkaloid-laden plants to eat, which will help to knock off the parasites. They generally only eat small amounts of these plants when they are not infected. This is very unusual, up until this was discovered it was thought that only animals capable of some basic level of reasoning, like primates, were capable of seeking out medications when they were sick.

There is a persistent belief that the width of the color bands is an indicator of just how severe the coming winter is going to be, but there are some problems with this: (1) I’ve heard conflicting statements of which way it’s supposed to go, is a wide brown band a sign of a cold winter, or a warm winter? (2) The brown band generally gets wider as the caterpillar gets older, so your “prediction” is going to change depending on how old the caterpillar is at the time; and (3) People who have reared these caterpillars[4] say that the width of the brown band is all over the place, you will have ones with narrow bands, wide bands, and intermediate bands even from a single batch of eggs. So, I’d say that overall the woolly bears are not helpful for weather prediction. Not that other very-long-term weather forecasts are much better, mind you, and at least looking at woolly bears is slightly cheaper than buying an “Old Farmer’s Almanac”.

So goodbye, little woolly bear. Sleep well, and maybe we’ll see you next spring.


[1] I love my family. They happily go along with my weird hobbies.

[2] As it happens, some years after writing this we reared another wooly bear, which turned into this rather large brown moth:

[3] Or brown caterpillars, with black bands on the head and tail. One description is probably as accurate as another. Are Zebras black with white stripes, or white with black stripes?

[4] There are some good instructions for rearing various sorts of tiger moth caterpillars here, if anyone wants to give it a try.

9 Responses
  1. October 12, 2009

    The catepillars seem to have more appendages than the moths. If they lose a leg, is there a corresponding limb loss in the moth that emerges in the Spring?

  2. October 12, 2009

    Well, if I understand the function of imaginal discs correctly, the appendages of moths and butterflies don’t actually have any connection to the caterpillar’s original appendages. As long as the imaginal disc in the caterpillar that was supposed to grow into an adult leg is left undisturbed, the new leg will grow in just fine even if the caterpillar somehow loses some of its caterpillar legs.

  3. October 16, 2009

    its so cute. i caught 10 this year. i waited until they turned into moths

  4. October 28, 2015

    Man, have we been corresponding since 2009? Crazy! In any case, Fiesta Island, where the Catican Guards go for unleashed walks, is crawling with these guys right now.

  5. October 28, 2015

    KT: Yes, it has been a while. If I remember right, you found my blog from a link off of Friday Ark, sometime in early 2008, shortly after I migrated off of Livejournal.

  6. October 28, 2015

    That was definitely a watershed moment in my life. I’m very blessed to have discovered you!

  7. Maggie permalink
    October 30, 2016

    I’ve been looking for these guys for the past couple of years and I have yet to find even one, which in my area is really weird. Usually we find tons of them and see them crossing the highways.
    We live in Hermansville, MI… Directly in the middle of a swamp.
    I want to try to rear some of these but I honestly have no clue where to look since I generally end up having little problems finding these cute little guys. Typically we find 10 or 20 around the foundation of our house and garage but all I’ve found this year are these strange, some are hideous, nocturnal caterpillar grub things. Fat things too.

  8. November 1, 2016

    Maggie: The population does seem to fluctuate quite a lot. They are subject to being attacked by parasitic flies (in particular, there is a tachinid fly that was introduced to control gypsy moths that also attacks a lot of other caterpillar species, and that has come into Michigan relatively recently), and it is possible that the wooly bears are just at a low ebb for a few years.

  9. Maggie permalink
    November 5, 2016

    If that’s what’s happening to them, the flies, that’s just sad. 🙁
    I’ve seen quiiiite a few gypsy moths around here too so that just might be the case…
    I hope they recover from that…

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