Interview with a Western Tent Caterpillar

Photo by Andrew Reding. Western tent caterpillars. 2014. CC.

Photo by Andrew Reding. Western tent caterpillars. 2014. CC.

(This post was inspired by my father, who saw tent caterpillars in July and told me I should write a blog post about them. Merry Christmas Dad!)

Me: What a lovely June day. The bees are buzzing, the flowers are blooming, the caterpillars are swarming. Wait, why are caterpillars swarming my Aspen tree? Get off of there!

Western Tent Caterpillars: Make me. I’m a native species, and I’m not doing anything wrong.

Me: You’re a gross creepy crawly mass of caterpillars, that’s what’s wrong.

W: Well we can’t all be hairless apes like you.

Me: And look at what you’re doing to this poor tree! You’ve eaten half of its leaves. How is it supposed to survive?

W: I wouldn’t worry about it. Trees are pretty good at bouncing back. Our population explosion will be over soon anyways. Maybe another four years or so.

Me: Four years? You mean I’ll have to stare at hundreds of wriggling caterpillars for another four years?

W: That’s what it looks like, yes. These explosions happen every decade or so.

Me: That is not very comforting. Why does your population explode anyway?

W: What can I say, sometimes the world needs more caterpillars. The cycles depend on weather, predators, pests and other factors. You guys haven’t cared to study us very much, so I’m not giving anything away.

Me: Will my poor aspen tree end up dead as a result of your munching?

W: It’s a pretty healthy specimen, so probably not. Healthy trees don’t usually die from infestation, it just make them weaker and more vulnerable to pests and drought.

Me: That’s not very comforting.

W: If it’s any consolation, the tree’s fighting back. It can make its leaves less nutritious next year, so fewer of us will survive. Anyways, getting rid of some leaves in the canopy actually helps the environment.

Me: Oh really? How’s that?

W: Fewer leaves means more light and rain reach the forest floor, giving saplings down there a chance to grow. I’ll also have you know that caterpillar poop is an awesome fertilizer.

Me: Eww, I didn’t really need to know that. How did you all get on my tree in the first place?

W: Mom put us here. Last August she laid a huge cluster of 300 eggs around a twig and covered us in this foam that hardened to look like grey Styrofoam. The finished clutch was as big as she was!

Me: Sounds pretty impressive.

W: It was! We started developing in our eggs, then took a long nap over the winter.

Me: What happened when you hatched? Why did you stick together?

W: We hatched just as the leaves started appearing in April. All of the larvae from that eggs sack decided to hang out because we are social butterflies. Okay, social moths.

Me: Why do they call you tent caterpillars?

W: Life as a soft and squishy caterpillar is not easy. Birds and rodents want to eat you, and parasitic wasps want to lay eggs in your insides. It’s very unpleasant. To avoid this, the colony builds a giant silken tent to hide under. If we aren’t feeding we’re in the tent. It’s shelter from the weather, protection from predators and a place to molt and grow.

Me: I’ve seen you stick your bodies out of the tent and twitch them. Is that a caterpillar dance party?

W: Nope, it’s a defense mechanism. We do it whenever a possible predator passes by. Hopefully they’re so confused they just keep walking and don’t try to eat us.

M: You said you turn into a moth, right? How long before that happens?

W: About 4-6 weeks after we hatch, everyone in the colony separates and finds their own spot to make a silken cocoon. We’ll pupate on anything from aspen leaves to house siding to lawn mowers.

M: I can’t wait.

W: Don’t worry, 10 days later we’ll emerge as beautiful light brown moths. Those moths flitting around your porch light are probably us. We usually mate within 24 hours of emerging.

M: Whoa, you don’t waste any time, do you?

W: Nope. Every female lays one egg clutch, and our whole life cycle takes about a year.

M: Yeah, but for most of it you’re stuck in an egg.

W: Touché.

M: So does this mean that my aspen tree will be infested by your children next year?

W: Probably. We are Canada’s national champion of eating leaves off aspen trees, after all.

M: I guess I’d better get used to you then.

W: Yep. You could spray pesticides on the tree, but that would kill the beneficial bugs too. It’s best just to let our predators or the weather finish us off.

M: Okay, I’ll leave you be… this time.

W: We appreciate it!


Click to access Tent-CaterpillarsDRAFT041113.pdf

Click to access stelprdb5303047.pdf

Worried Woolly Bears

 Photo by Tony Fischer. Wooly Bear Caterpiallar to Tiger Moth. CC.

Photo by Tony Fischer. Wooly Bear Caterpiallar to Tiger Moth. CC.

While biking to school these days I’ve had to dodge the fuzzy black and orange caterpillars scurrying across the bike path. What are these critters, and why are they running so fast?

Child stars

Apparently woolly bears are the most recognizable caterpillar in North America, which is funny because I had never heard of them until this year. They’re much more famous than their adult form, the Isabella tiger moth, or Pyrrharctic isabella for you Latin fans. That’s probably because the moth is nocturnal and a dull yellow-tan. The fuzzy orange-and-black caterpillars are more noticeable, especially with their habit of crossing roads on sunny fall days.


The real reason for the woolly bear’s fame isn’t its cute black head or its orange and black fuzz. It’s because early North American settlers thought the native caterpillars could predict the winter weather. The wider the orange stripe, the milder the winter would be. They were basically a fall version of Groundhog Day.

This seems silly to us now, but in an era where your survival depended on how much food you stored up the fall, any information these settlers had about the winter would have been comforting.

Scientists aren’t really sure why some caterpillars have wider stripes than others. It could be climate, natural variety in the population, or something that changes as a caterpillar grows. Woolly bears may not predict the weather, but they are awesome for a lot of other reasons.

Autumn angst

Photo by D.Fletcher. Wooly Bear. CC.

Photo by D.Fletcher. Wooly Bear. CC.

People usually see woolly bears in the fall when they’re in a hurry. They’re looking for somewhere to wait out the winter, and if they don’t rush, they’ll freeze. Well, they freeze anyway, but more on that later.

In the spring and summer woolly bears are solitary beasts. Native to southern Canada, they spend their days in meadows and fields hidden in patches of wildflowers. They’ll eat just about anything, from grass to maple leaves to dandelions.

But when winter rolls around they need to find logs, rocks, or tree bark to hide under, none of which are plentiful in wildflower meadows. So in the fall they wander out of their meadows and into more forested areas, which is why we see them crossing roads and bike paths. They’re just looking for somewhere cozy to wait out the winter.

Fuzzy popsicles

Over the winter woolly bears freeze solid. Starting with their heart. Think about how that would feel for a moment. Freezing is usually a supremely bad idea for a living thing. Water expands as it freezes, and prickly ice crystals damage cells and tissues. But woolly bears make a natural antifreeze called cryoprotectant, which keeps their bodies largely undamaged from freeze and thaw cycles.

If it warms up enough during the winter, the caterpillars might thaw and walk around a bit before refreezing again when it gets cold. However, just because they’re full of antifreeze doesn’t make them superheroes. Freezing and thawing multiple times over the winter increases their chances of death and organ damage. When it warms up in the spring, the caterpillars wake up and continue to eat before making a cocoon and transforming into a less-famous adult moth.

Why so hairy?

Woolly Bear fuzz close up. Photo by LadyDragonflyCC->;<. Woolly Bear close up. CC.

Soft and squishy caterpillars are the ideal snack for birds, rodents, frogs and snakes. The woolly bear has a few ways to keep from becoming their next meal. First, they can run away pretty quickly. They will also curl up into a ball and play dead, keeping all their hairy bristles called setae on the outside. These setae do discourage some predators, but skunks and a few other animals have been known to roll them off before chowing down. It’s also thought that the hairs act as insulation during the caterpillar’s long winter’s nap.

Heal thyself

Besides being surrounded by predators, some woolly bear caterpillars get eaten alive from the inside out. Parasitic flies lay their eggs inside the caterpillar’s body. After they hatch, the larvae munches on the woolly bear’s insides before exploding out of its side. Ick. A 2009 study at the University of Arizona found that woolly bears self-medicate on certain plants which cure the parasite. Caterpillars infected with parasites ate way more alkaloids than their non-infected peers. However, too many alkaloids are poisonous, so woolly bears have to balance poisoning themselves with getting eaten alive by fly babies. Not a fun choice.

So watch out for these speed demons on a road near you this fall, and please try not to squish them.


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