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Ask Sam: What's Up With Those Tent Caterpillars?

Marie-Eve Jacques / UNH Cooperative Extension
Webs of Eastern Tent Caterpillars

Every other Friday on Morning Edition NHPR’s Sam Evans-Brown tracks down answers to questions about the environment and outdoors for our listeners in a segment we call “Ask Sam.”

Donna from Campton (aka: JUST DOWN THE HALL … she works here) asks: A thought I had this morning while driving down. The trees have the tent caterpillar nests in them right now. What are they and are they harmful? And whatever happened to the gypsy moth caterpillars that devastated the trees and leaves years ago. They seem to have gone away… or are the tent caterpillars and the gypsy moths the same thing?

Donna! Am I detecting here is a certain amount of discomfort — perhaps even a bit of distaste, for tent caterpillars?

In that case, we’ve got to flip the script: how can we get you to come at tent caterpillars from a place of curiosity rather than disgust? The key to appreciation is knowledge. Think baseball. It’s a super boring game, until you learn the rules, who all the players are, what’s going on strategically, and the same is true for kinda gross tent caterpillars.

To answer this question I spoke to Terrance Fitzgerald at SUNY Cortland in upstate New York, who has been watching them since he was a kid in the 70s.

For starters, here in the East, there are two common tent-weaving caterpillars that you could be referring to. One is the Eastern Tent Caterpillar, which builds tents almost exclusively on apple and cherry trees, and the second is called the fall webworm and those are on all kinds of trees. But in the question Donna said tent caterpillars, so that’s where we’re going. (Also, frankly speaking, they are cooler than webworms so I want to talk about them.)

So Why Do Tent Caterpillars Make a Tent?

If you grew up in New Hampshire, you’ve almost certainly seen tent caterpillars outside the tent. Because of my bug-hunting days as a small person, I think of them as the quintessential caterpillar, the one that spits out a little bubble of foul-colored-something-or-other when you poke at them.

So why the tent? One reasonable guess to make would be that being in a tent might keep predators away, but they have other adaptations for predators. That foul bubble is composed of cyanide laden juices (coming from the leaves of the apple and cherry trees they eat) that birds and insectsaren’t fans of. Also, they thrash when threatened to confuse and intimidate insect predators.

The tent is really about keeping warm, and it’s what sets them apart from nearly every other caterpillar. Tent caterpillars have a niche, which is that they hatch from their eggs very early in the spring, when it’s still quite cold. And because they’re ectotherms (sciency-speak for cold blooded) and can’t regulate their own body temperature, when it gets very cold they can’t move or digest any food they’ve eaten.

The tent is typically built on the south side of trees, or at least in a sunny spot, and the caterpillars take refuge inside or bask in the sun on the outside, depending on the weather. The web tent acts a bit like a greenhouse, and the temperature inside can soar to well above100 degrees Fahrenheit.


The building of a tent means that they can hatch out first thing in the spring, and eat the very first leaves that bud out on these fruit trees. By June, they metamorphose into adults, lay eggs and die after just one day as a moth. The eggs lay dormant for the whole rest of the year until the next spring.

So How Do I Get Rid of Them

Again, I want to flip the script on you. Why do you want to get rid of them? They’re a native species, so their population is kept in check by natural predators. We have ant farms that we buy for our kids so they can watch the social behavior of ants, and yet, tent caterpillars may munch some of the leaves on your backyard apple tree, but you’ll have the opportunity to observe one of America’s more interesting social arthropods.

Social? Yes, social! Because tent caterpillars are gregarious! They hang out together!

And just like ants, they leave little pheromone trails that lead to food sources, in fact, they’re the only caterpillar that we know of that does so. Others leave “exploratory” chemical trails that lead them back to their nest, but it’s just tent caterpillars that reinforce the trails that lead to food, turning them into “recruitment” trials.

So if we think that ants are cool enough to buy for kids to watch, then I think tent caterpillars are cool enough that we should cut them some slack, and just observe them a little bit. And now that you understand them a little better — just like watching a game of baseball — you can watch the caterpillars come and go for hours worth of entertainment.

Now, About Those Gypsy Moths

Gypsy moths are not a native species. And this part of the question makes it abundantly clear that Donna (my own co-worker, *gasp*) has not been keeping up on her Outside/In podcast episodes. We covered the story of how the gypsy moth invasion was brought under control in one of our earliest episodes, and I encourage you to give it a listen. It’s a fascinating tale, involving Indiana Jones-esque trips to Asia to search for the gypsy moth’s natural predators. The short version is that the control of the gypsy moth is an excellent example of how biocontrol(the introduction of a new species to control an out-of-control invasive species) can be done incredibly effective when done right.

That’s Sam Evans-Brown, host of NHPR’s Outside/In which you can subscribe to wherever you get your podcasts, If you’d like to submit a question you can record it as a voice memo on your smartphone and send it to oustidein@nhpr.org, OR call the hotline, 1-844-GO-OTTER.

Sam Evans-Brown has been working for New Hampshire Public Radio since 2010, when he began as a freelancer. He shifted gears in 2016 and began producing Outside/In, a podcast and radio show about “the natural world and how we use it.” His work has won him several awards, including two regional Edward R. Murrow awards, one national Murrow, and the Overseas Press Club of America's award for best environmental reporting in any medium. He studied Politics and Spanish at Bates College, and before reporting was variously employed as a Spanish teacher, farmer, bicycle mechanic, ski coach, research assistant, a wilderness trip leader and a technical supporter.
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