raise the alarm!

I believe the average person’s reaction to seeing an Eastern Tent Caterpillar nest may be to become grossed out, or perhaps angry and determined to rip it down. I’ll admit that I experience some of that urge to rip down nests when I discover them in our apple trees. In that case, I generally cut away the branch with the tent and toss it across the drainage creek where the caterpillars can do no real harm. However, when the caterpillars are in just about any other kind of tree, I leave them alone to do their thing. The truth is, they don’t really do much harm, and are actually an important part of the food chain. Some birds such as the cuckoo, feed on tent caterpillars. Also, a number of parasitic flies and wasps lay their eggs on the caterpillars. As is the case with many insects, and particularly caterpillars, it’s a bug-eat-bug world out there and few survive to adulthood.

Normally, I don’t pay tent caterpillars a great deal of attention, but the above nest caught my eye when Sabrina and I were out for a walk on Thursday afternoon. On the southwest side that was receiving the full afternoon sun, there were few caterpillars (see above – click on image for larger view). However, the shaded northeast side was crowded with caterpillars. These communities of caterpillars use their nests as a shelter from predators (birds and insect parasitoids), but also for temperature regulation. They spend much of their time inside the tents between forays for food, but also move about on the surface of the nest to take advantage of sunlight and shade. There’s quite a lot of interesting information on the natural history of these caterpillars and particularly on their thermoregulation on this wikipedia page.

Okay, so why was I attracted to this particular nest of tent caterpillars?

Curiosity! As I was passing by, I happened to notice that the few caterpillars on the sunny side of the nest, were thrashing frantically in a typical caterpillar “alarm” mode. From a distance, I couldn’t see what was making them so worked up, so I decided to approach and see what was going on.

Now, I’m not alone in my curious streak. It definitely runs in my family. One of my favourite childhood stories about my youngest brother is of finding him standing in the middle of the living room rapidly twirling a dayglo type yellow or pink eraser on the end of a very long string. I asked what he was doing, and he said he’d discovered something weird and that I should watch the eraser. In seconds, a fly of some type blasted through the air and gripped the eraser and rode along as it spun in huge circles. My brother would stop spinning the eraser and brush the fly off, and then repeat the process once more. The fly would immediately race from wherever it was perched and grip the eraser again. I’ve often wondered how my brother, who was probably about ten at the time, discovered that flies were attracted to dayglo erasers. Go figure.

Anyhow, back to the tent caterpillars. Yes, I had to investigate the reason for the violent thrashing. I approached the nest with my camera switched on movie mode to see if I could capture some of the action. At first, I couldn’t find the source of the obvious agitation, but then I noticed a fly dancing back and forth across the nest (see still frame capture below). It would repeatedly rush at the caterpillars, but immediately back off when one of them thrashed its head wildly in the direction of the fly. I managed to capture a few sequences of this and have put one up online for those who are as curious as me. The short movie is in the usual .mp4 format, with no sound (approx. 900b). The fly is, in all likelihood, some species of tachinid. They are the bane of of many insects as they deposit eggs or almost-ready-to-hatch larvae on or very near to the intended host (see Stephen Marshall’s “Insects: Their Natural History and Diversity”, pg. 409, for a good description of the tachinid’s modus operandi). If you watch the movie, take note of the actions of the fly. It dodges in and out, trying to move near to the caterpillars on the exterior of the nest, but from what I can see, it also seems to be injecting eggs through the webbing of the nest when it is over a caterpillar that is concealed within.

So, you see — the usual suburbanite preoccupation with tearing down or lighting fire to caterpillar nests is probably unnecessary when you’ve got deadly tachinid flies taking care of business.

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16 Responses to “raise the alarm!”

  1. Wayne Says:

    Excellent investigation and catch, Bev. I love tachinid and other predatory flies.

    I ran across what the writer, whose name I cannot remember, might have thought was a throwaway but struck me as *much* better than burning the nests. She, and I do recall it was she, noted that the growing caterpillar nests start out small and then expand as the caterpillars go through enlargment. They spin more web for more protection. So she pokes holes in the nests with a long stick. And then she watches the predators descend in the form of birds (and probably tachinids).

  2. bev Says:

    Wayne – Good point about just damaging the nests. I sometimes do the same with the nests in the apple trees as I figure that, once a section of nest is removed, the caterpillars are suddenly very vulnerable, although as mentioned, if it’s convenient, I do just trim off a branch and toss it across the creek. My rationale is that the caterpillars live to die another day, so to speak. As mentioned, I just leave them in the wild cherry and other trees around the farm as I figure they make good food for the cuckoos and anyone else who might find them appealing.

  3. robin andrea Says:

    What a great video, bev. Incredibly good catch on the fly. I have not done any research on tent caterpillars, so I did not know that they return to the nest. When we first moved to Washington, our orchard was full of tent caterpillars. We removed and killed them. I hated doing it, but there nests in every tree. We haven’t had them since. I think if they came back, I’d poke a hole in the nest and watch the birds have their way with them.

  4. Robert Ballantyne Says:

    Bev, I know that whenever Burning Silo lights up on my aggregator I am in for a treat. I have to admit that I react badly to the sight of the tent caterpiller nest. I grew up near Montreal, and I remember a year when there seemed to be a plague of those tents. All of the bushes around the property had so many tents that cutting them out didn’t seem to be an option. I remember the family involved in hand-to-hand combat with torches. Speaking of insect plagues, we do have them from time to time. Here in BC, the most dramatic consequence of global warming seems to be the current activity of mountain pine beetle. On a more amusing note, I remember the day (it must have been around 1960) Montreal was attacked by a horde of grasshoppers. I had a summer job in a downtown office on the 8th floor, and they were flying in the window. It was hard to walk down the street without crunching them underfoot.

    Thanks for posting the movie!

  5. Ontario Wanderer Says:

    Very interesting and neat movie. I prefer trees to caterpillars I guess. I tend to destroy the nests but our cuckoos are not starving by any stretch of the imagination.

  6. Cathy Says:

    I wonder if anyone has recorded a movie of this process before. That is just extraordinary, Bev. I watched an oriole picking over one of these nests this Spring. I tried to get a picture from a great distance and through the windshield. Nope.

    You make an interesting case for a familial tendency for curiosity. Do you have any sense as to how much was innate in you and your brother and how much was modeling from your parents? Pretty difficult to tease it apart, I suppose.

  7. Duncan Says:

    We’ve got caterpillars over here Bev, called Processional Caterpillars, they build similar nests, and as the name suggests follow each other in long lines. It’s an old trick to divert them and get them going round and round in a circle.

  8. bev Says:

    robin – thanks! I’m quite sure that tearing away some of the outside of the nest is enough to attract birds and other predators. I’ve done that to a few of the nest that were too high to cut down out of some apple trees here at the farm and the caterpillars seem to disappear soon after — perhaps simply dispersing as they become sitting ducks (?).

    Robert – thanks very much! I’m glad you find the posts here at Burning Silo of interest. You’re quite right about there being some years when there are huge outbreaks of insects. About 15 years ago, some kind of caterpillar or sawfly appeared in massive numbers and ate the needles off of many of the pines here at the farm. At the time, I didn’t know enouh about insects to know what they were, but if I ever see them again, I’ll take a better look. That’s funny about the grasshoppers — especially to have a population explosion within a city. Sometimes strange things do happen. I can tell you one thing — if there were any of the large garden spiders (Argiope) around that year, they would have been in seventh heaven as, based on my observations, grasshoppers are their favourite food!

    OW – I suspect there are so many tent caterpillar nests in wild cherry and other trees around the countryside, that the cuckoos will never go hungry!

    Cathy – I suspect that entomologists probably record this kind of thing on video quite a bit these days, so the footage probably isn’t too rare, but it’s still very interesting. I’m trying to do this kind of shooting a bit more often as I realize that it’s not possible to see many kinds of insect behaviour with the naked eye. If I have the camera clips in my computer, I can review them a few times and even slow them down to study frame by frame, and I learn so much more that way. For me, the camera has become a real tool for studying nature.
    I’m not sure about our familial tendency for curiosity but I’d say that it ran in our family (both of my parents were very observant). I sometimes wonder if, collectively in our time, we’ve just lost some of our curiosity about nature because we tend not to see it around us as much, or pay as much attention to it anymore (I’m just musing over this very subject at the moment). When I was studying the Victorian period in some English Lit and Art History classes at university about 15 years ago, I made a point of reading a lot of nature field journals. People used to spend a good deal of time watching insects, birds and other creatures. Also, when I think of someone like Jean-Henri Fabre (1823-1915) and his writings in his “Book of Insects” I realize that I haven’t even begun to scrape the surface of nature observation, but it’s never too late to start. (-:

    Duncan – Interesting! Okay, how do you divert them? Do you trick them into going onto a branch they haven’t been on before?

  9. Paul Decelles Says:

    Great video! It’s interesting that the response of the caterpillars to the fly seems pretty variable and a great story about the eraser. Have you tried replicating that?

  10. bev Says:

    Paul – Thanks! I noticed that some of the caterpillars seemed more “aware” than others. In general, I’d say the largest caterpillars seemed to react about the most — perhaps were more cognizant of the peril. I also experimented a little with poking the caterpillars with a stalk of grass and the caterpillars on the shady side seemed to react less than the ones on the sunny side of the nest. Also interesting – the fly didn’t seem to be bothering with the caterpillars in the shade as much as the ones in the bright sun. Perhaps it had already been on that side of the nest before I came along, or perhaps it just didn’t want to move about in the shade(?). And no, I never tried to replicate the dayglo eraser thing. I’ve been curious about it though, and have wondered if it only works with a certain species of fly. The eraser was fairly large in size, but I can’t remember if it was pink or yellow — I kind of think it was yellow. I must ask my brother if he can remember that. Btw, I have no idea how he even discovered that the eraser would attract flies — I think he was probably just spinning the eraser and noticed the fly reaction as something odd.

  11. Clare Says:

    Once again you continue to amaze me with your posts. I’ve long felt that the curious mind is the best mind set. If only we could all see the world through the eyes of a child. Everything for them is filled with wonder.

    Your post got me thinking about our (general) revulsion of tent caterpillars. They seem to invoke a similar response in many people – to destroy them. I wonder if part of it is our desire for nature to be neat and pretty, that the “messiness” of their nests is what inspires us to want to rid trees of them?

  12. Marcia Bonta Says:

    I’m a little late with this (too many holiday visitors and visits), but the best book about tent caterpillars is THE WORLD OF THE TENT-MAKERS: A NATURAL HISTORY OF THE EASTERN TENT CATERPILLAR by the late Vincent G. Dethier (University of Massachusetts Press, 1980). All of Dethier’s popularly-written books are excellent and this one is no exception.

  13. Duncan Says:

    Bev, the processional caterpillars can sometimes be found on the ground going in search of a new tree to defoliate, that’s when you can play the dirty trick on them ;-)

  14. bev Says:

    Clare – Yes, I agree about the curious mind. That’s really how many discoveries are made — from someone investigating something that seems just a little odd or unusual. That’s certainly been my own experience when exploring off the beaten track. I think you’re onto something about the reason for the revulsion many people feel towards caterpillars. They stick out like a bit of a sore thumb, especially once they defoliate a few branches around the nest. The damage isn’t usually serious enough to warrant doing much, if anything.

    Marcia – I shall check into the Dethier book. I borrowed the Stoke’s insect book from our library on the weekend and it’s quite interesting.

    Duncan – Aha! So that’s how it’s done! (-:

  15. Frederick W. Schueler Says:

    It has occurred to me that the best thing to do with the small nests of Malacosoma is to clip the stem they’re on, and place them in a nearby Buckthorn, to give only those caterpillars with a taste for the invader to mature. If many People did this, we might force an evolutionary saltation, and add one or both Buckthorns to the diet of the Tent Caterpillars, helping to level the playing field between the native and alien oldfield shrubs.

  16. bev Says:

    Fred – That’s a great idea and I’ll definitely implement it and try to check back on the caterpillars to see what they’re doing. Unfortunately, it looks like the peak of the caterpillar activity may have already occurred, but I may find a few nests yet. I’ve been checking on several nests and have found that there are a lot of dead caterpillars in them, or on nearby branches. Perhaps this was a good year for tachinids.