Tent Caterpillars – Eastern and Forest

2009 June 20

We have two species of tent caterpillars around here, the Eastern Tent Caterpillar, Malacosoma americanum,


which makes “tents” on tree branches and prefers to eat leaves from apple and cherry trees,


and the Forest Tent Caterpillar, Malacosoma disstria, which doesn’t make tents and prefers forest trees like aspen.


You can tell them apart pretty easily: the eastern tent caterpillar has a solid line all the way down its back, while the forest tent caterpillar has a dotted line running down its back.

Back in the summer of 2001, we had the peak of a massive infestation of both species at once, and it was pretty appalling – they completely defoliated practically every tree for miles around. Walking in the woods, there were hordes of the forest tent caterpillars rappelling down on silk threads, dropping all over your head and tangling their shroud lines around you. The normally pale-colored trunks of the aspen trees were absolutely black – because they were coated with caterpillars. If you stood still, you could hear them munching, munching, while their droppings rained down, patter-patter-patter.

Meanwhile, on our fruit trees, the eastern tent caterpillars were swathing the trees with silk, and denuding vast areas of the trees. We had a super-soaker filled with soapy water that we would shoot into the tents from the ground, which killed a lot of them and kept them reasonably under control but didn’t eliminate them.

But, our efforts to protect the trees were ultimately fruitless. When the forest trees were stripped by the forest tent caterpillars, they washed across the landscape in a seething wave, stripping every tree that they came across, and eating any vegetation they could find, digestible or not – grass, lilacs, rhubarb, pine needles, scraps of paper and dead leaves – until, twitching, millions of them finally died, with their hairy, festering corpses spurned by even the birds.

Even with millions dead, there were still millions left. The survivors built cocoons on any available surface, coating the siding of houses and outbuildings, filling the eaves, and lining crevices in tree bark. But then, hordes of earwigs abruptly appeared, ripping into the cocoons and devouring the contents[1]. There were no survivors, and then the earwigs, too, died.

The carnage [2] finally ended around the middle of July. The surviving trees leafed back out, and all returned to normal[3]. From the mind-boggling peak of the caterpillar population, the crash was almost total. In the eight years since, this is the first year that I have seen the tents of the eastern tent caterpillars reappearing on the trees. And this forest tent caterpillar that our neighbor found is the only one that I have seen recently.

The Department of Natural Resources has said that we can expect a plague like that every 15 to 17 years, with the severity depending on how dry it is in the spring (dry conditions make it worse). So we probably have another five years or so before they start getting bad again.

The caterpillars pupate in late June or early July, and emerge as kind of nondescript brown moths in mid-to-late summer. They mate, and the female lays an egg mass on a tree branch that overwinters. In the spring, the egg mass hatches out and the caterpillars all hang together in a large colony until they finish working their devastation. So if you see these on your fruit trees in the fall or winter:


then it would probably be a good idea to eliminate them before they hatch out in the spring.


[1] A less visible, but probably more important, contributor to the collapse of tent caterpillar populations was the “Friendly Fly”, Sarcophaga aldrichi, which parasitizes the caterpillar cocoons. After they finish with the caterpillar cocoon, they drop to the ground to pupate and overwinter, emerging as adults right about the time the tent caterpillars are ready to pupate the next year. In the years that there are no caterpillars, they keep going at a low level by laying eggs on carrion, but there is a lot of competition for dead bodies and so this doesn’t let them get very numerous. They are called “friendly flies” because they like to sit on people and don’t easily shoo away – they have to be brushed off. When they are numerous, they are very annoying and drive a lot of people crazy. A number of people around here call them “Government Flies” because they are under the mistaken belief that the DNR releases them to keep the caterpillars down, which isn’t actually true. It’s just that their populations rise and crash with the tent caterpillars, so they go from practically-nonexistent to very numerous usually in the second or third year of the infestation. Which, if I want to be cynical, would be another reason to call them “Government Flies”: they show up at the disaster site after the damage is already done, and they cause almost as much trouble themselves as the problem they are trying to fix.

[2] Hey, what would be the abuse-to-plants equivalent to “carnage”? “Foliage”?

[3] Except that all the defoliated apple trees (and all the other fruit trees, of course) lost all of their fruit[4]. They evidently lost so much of their resources that they didn’t even blossom the next year, so there was no fruit then, either. But then, two years after the infestation, every apple tree that we had produced a massive bumper crop of perfect, bug-free apples. What had happened was, the lack of fruit for two years had pretty much eliminated all of the fruit pests like codling moths, because they had nothing to eat that whole time. So the tent caterpillar plague wasn’t a completely bad thing.

[4] So our efforts to protect the trees were “fruitless”, see? Ha ha! I slay myself.

6 Responses
  1. June 22, 2009

    Impressive story, Tom. I have never seen anything close to this in forest tent caterpillars in my northeast Illinois county, though we do have them in small numbers. Our last significant outbreak of eastern tent caterpillars was in the ’80’s, and lasted 2-3 years. It was brought to an end mainly by a hymenopterous pupal parasite.

  2. Jenny permalink
    June 23, 2009

    Pretty cool post. I just found your blog and wanted to say
    that I’ve really enjoyed browsing your posts. Any way
    I’ll be subscribing to your feed and I hope you write again soon!

  3. July 5, 2009

    Awesome post. A link is on the way.

  4. Stacie permalink
    April 19, 2011

    heyy nice i have learned alot wit this thax 🙂

  5. zoey mccarthy permalink
    May 21, 2014

    i need to know where in the backyard i can find one…… like today.

  6. May 21, 2014

    Well, the best way to find them is to go somewhere where there are a lot of small fruit trees. They like apple, cherry, pear, plum, and similar trees. Look for forked branches that have white webbing on them, and you’ll find the caterpillars.

    They haven’t hatched out yet here in Houghton, but further south they are likely to be out (although probably still pretty small)

Comments are closed.