None dwell in their tents

tent caterpillar on black birch trunk

Let their habitation be desolate; and let none dwell in their tents.
Psalm 69:25 (KJV)

Eastern tent caterpillars (Malacosoma americanum) are a common sight this time of year, especially in their last instar, after they abandon their tents. They stray onto the porches and wander over the furniture, looking for protected places in which to pupate. On a visit this past Monday, my three-year-old niece Elanor decided that they were cute — not typically my own reaction — and began petting them, prompting the caterpillars to arch their heads back like housecats. Yesterday afternoon, I watched one crawling up the neck and across the face of a box turtle, which merely shut one eye while the caterpillar took its measure.

tent caterpillar tree (black cherry)

The tents began appearing at the end of April in unusual numbers, especially on black cherry trees, which are the favorite food source for tent caterpillars and occur in unnatural abundance across Pennsylvania due to 150 years of clearcut logging practices and the reversion of old fields and pastures. Black cherry is a common first-succession tree species in many forest types, and thanks in part to its relative unpalatability to white-tailed deer, it can form almost pure stands in many areas that would have formerly hosted oak-hickory, beech-hemlock, or mixed deciduous forests. Here in Plummer’s Hollow, many of our southeast-facing slopes are dominated by black cherry stands, and I figured they’d be completely defoliated by this time.

blossoming black cherry

Instead, Sapsucker Ridge is white with blossoms, filling the air with an ambrosial scent. One finds only a few black cherries as badly defoliated as the one in the second photo; the tree above is more typical. Though dotted with tents, only scattered branches have actually been stripped of their leaves.

dead tent

A closer look reveals that most of these tents are filled with dead caterpillars. The few still alive twitch spasmodically. What happened? I’d guess that the unusually cold, wet weather over the past few weeks is at fault. Nighttime temperature routinely dropped into the low 40s this month, and sometimes even into the high 30s; daytime temperatures rarely exceeded the mid-50s; and rain was almost constant for the first three weeks of the month. Not only would the cold have shut down their temperature-sensitive digestive systems for prolonged periods, but the rain would have kept them confined to their silken tents, and the two together would’ve made them much more susceptible to starvation and disease. According to the Wikipedia article on tent caterpillars,

The tent is constructed at a site that intercepts the early morning sun. The position of the tent is critical because the caterpillars must bask in the sun to elevate their temperatures above the cool ambient temperatures that occur in the early spring. Studies have shown that when the body temperature of a caterpillar is less than about 15 °C [59 °F], digestion cannot occur. The tent consists of discrete layers of silk separated by gaps and the temperature in these compartments varies markedly. Caterpillars can adjust their body temperatures by moving from one compartment to another. On cool mornings they typically rest in a tight aggregate just under a sunlit surface of the tent. … Later on in the spring, temperatures may become excessive at mid day and the caterpillars may retreat to the shaded outside surface of the tent to cool down.

Entymologist Vincent G. Dethier’s wonderful and evocative classic, The World of the Tent-Makers: A Natural History of the Eastern Tent Caterpillar (University of Massachusetts Press, 1980) describes in Chaper 13 (appropriately enough) the battery of predators and diseases that keep this native insect in check. He details the spread of a deadly virus from colony to colony, then adds:

As if that were not enough the unusually wet late spring had been kind to molds, mildews, smuts, blasts, and bacteria. A particularly virulent spore-forming species of bacterium struck many of the colonies. … The enormous population of tent caterpillars had been cropped by weather, starvation, ants, bugs, parasites, fungi, viruses, bacteria, and misadventure in general. It had been a particularly trying year. Summer had hardly begun and the die had already been cast for the year to come. There would be fewer moths, fewer egg masses, and fewer colonies of the next generation.

So a spring that spells bad news for many farmers is good news for the wood products industry, which relies heavily on black cherry in Pennsylvania. Unlike fall webworms, which come too late in the season to have much of an impact on the trees they defoliate, tent caterpillars can greatly stress the trees they defoliate during their periodic outbreaks.

As global climate change plays hob with our weather patterns, it will be interesting to watch the effects on insect outbreaks, and over the long term, on forest succession. For example, if black cherries continue to predominate as many foresters would like, more-frequent icestorms would have a much greater impact than they would if less brittle species such as red oaks and tulip poplars took their place. Warm winters are said to promote the spread of pest insects, but what about warm winters followed by cold springs? I’d heard that the state was due for some pretty large gypsy moth outbreaks this summer as well, but here in Plummer’s Hollow, at least, their caterpillars are few and far between.

Cross-posted to the Plummer’s Hollow blog.

14 Comments


  1. Dave, that’s so weird. When I first saw the top photo and the quote in my feed reader, I didn’t see the caterpillar at all but “read” the photo as an aerial view of the same rain forest site and thatched huts I was writing about yesterday. And the quote fit too.

    Your post, of course, is fascinating!

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  2. Nice piece and great photos, Dave.

    I was out checking the tents around here earlier today and found quite a lot of dead cats in them too. Must be a widespread thing. A year or two ago, I think I posted photos and maybe a movie clip of some type of fly laying its eggs on the tent caterpillars – even through the tent. I’ll bet the percentage of survivors of these cats that ever make it through to adulthood must be quite small.

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  3. Down here in VA, those guys went off to pupate back at the beginning of May. (That’s when you see them most — when they reach full size and scatter to find places to pupate.)

    Today I went down with family to the Ivy Creek nature preserve, where my niece and nephews spent a few hours trying to rearrange a fairly large stream. Various wildlife sightings, notably a couple of snakes which I’ve tentatively ID’ed as Eastern Milk Snakes, perhaps immature. (A bit skinnier and darker than the “mugshot” I turned up at What’s that Snake?.)

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  4. Excellent post, I love finding out that kind of detail about all the interconnections in a local ecology… It will be fascinating to see what happens in the future

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  5. beth – No matter how I squint at that photo, I can’t see what you saw! But I do like the photo and would’ve featured it at Visual Soma if I hadn’t planned on using it here. They’re attractive caterpillars, and the blue is produced by structural means, by absorbing other wavelengths of light, rather than through pigment, according to Dethier.

    bev – Thanks. Yeah, I imagine it is pretty widespread.

    Dethier mentions “five different species of coarse, hairy tachnid flies” as caterpillar parasites, in addition to several species of wasps. Direct predators include ants and shield bugs.

    David – Wow, you’re really ahead of us! Of course, we did get that hot spell at the end of April, which may have sped things up a little too much considering how the weather turned after that.

    Young black snakes are also patterend a bit like milk snakes.

    maria – Glad you enjoyed it.

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  6. Young black snakes are also patterned a bit like milk snakes.

    Hmm… I’d send you a photo if I had your E-mail address… (one of these days I will get onto Flickr, really!)

    Which is more likely to go swimming? We first spotted the smaller of the snakes actually swimming in the river. (I think it was the same — I found them basking, by heading in the same direction the swimmer had left by.) The swimming posture was interesting — mouth wide open at water level, with only the top jaw above water. In retrospect, I think he was hunting water striders, which were plentiful.

    Also, the two snakes were different sizes – the smaller one was darker, with less red in his markings.

    PS: Thank you again for setting up the comment-editing plugin!

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  7. CGP – Oops, I didn’t see your comment earlier – i was working on my own comment when it came in. I’m glad to hear this was still of interest to someone from a very different ecoregion.

    David – Please don’t mistake me for a snake expert. I do think most snakes swim, though. It’s always such a surprising sight to me.

    (Glad you’re finding the comment editing feature helpful. I think I’ve about reached the point of plugin saturation – if i add any more, I’ll have to take some out to avoid slowing the site down too much. But that’s one I’ll definitely leave alone.)

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  8. Interesting post, Dave! Here in Hannibal tent caterpillars are common, but we don’t have as many wild cherry trees as you seem to have. Wild cherries here are scrubby edge trees and rarely attain much stature.

    The tent caterpillars around here like the cherries, but they also seem to thrive on black locust and elm.

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  9. I haven’t kept track, but it seems as if we’ve had warm winters and quite cool springs for several years running here in the northern Virginia Piedmont.

    The Scouts and I hiked six miles on the Appalachian Trail yesterday and got wind, sunshine, and then thunderstorms with heavy rain. It was nice to be outdoors.

    I think the turtle was winking at you.

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  10. This is why I like visiting…so much information to satisfy a curiosity I didn’t know I had.

    We’ve got tents here, in the Pacific Northwest. I see their structures in the summer, though, in the Columbia Gorge. Now I’ll be looking for them and will have a better eye, and idea.

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  11. Larry – We have wild sweet cherries, too — I think of them as feral cherries. But the tree I’m talking about here, black cherry or Prunus serotina, is a full-sized forest tree with scaley rather than lenticilate bark, beautiful red-brown wood, and clusters of small, sour fruit that are mostly pit.

    Tent caterpillars are fond of most fruit trees, I think, and like gypsy moths with oaks, will spill over into less-preferred species during peak outbreaks.

    Peter – Our last couple springs have been cool but dry here. Two years ago we had a big tent caterpillar ourbreak in many parts of the northeast – you may remember Rachel Barenblat writing about it at Velveteen Rabbi.

    Glad to hear your hike went so well, and good on you for helping introduce kids to the outdoors.

    Sometimes, I wish I were a turtle.

    …deb – Glad you liked the post. I gather that there are a number of different species of tent caterpillars world-wide, but I’m only familiar with the eastern. We also have fall webworms, though. Several years ago – 2004, I think – they turned our western ridge completely white with their webs in August.

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  12. all that was said.

    the tent caterpillars up in Canada used to bite something fierce when they fell from a leaf and landed on your neck. So arched back like cats, and sharp little mandibles, too.

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  13. Spring in Charlottesville seems to mean lots of evening storms, with a few all-day storms for a change-up. I just had to dash out to get the garbage out to the dumpster, and I could easily see both sheets and columns of rain. The columns were mostly shaded against the sky, while the sheets were visible at ground level.

    Lee’s River: Caterpillars that bite just when they fall on you? Nasty!

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