Green Caterpillar Devoured by Parasitoid Wasps

2010 July 3

Warning: This is quite probably the creepiest thing I have posted to date.

We were looking over our cherry tree in late May, and S_ found this fairly nondescript green caterpillar eating the leaves. There are a lot of kinds of green caterpillars, and it is hard to identify them until they turn to the adult form.

So, she gave it to me to raise up, so that we could find out what it would become. Little did we realize what that would turn out to be . . . (Cue Ominous Thunder [1]).

It looked healthy enough at first, it mostly crawled around like any other caterpillar (except that about half the time it crawled backwards, which I thought at the time was a little odd)[2]. I have learned from experience that caterpillars need to be photographed right away because they have a tendency to pupate abruptly, so I took that first picture immediately and put the caterpillar in a jar with a cherry leaf. Then I took a few more pictures a couple of hours later[3], and went to bed.

The next morning, I got up and looked into the jar to see whether it had pupated, or eaten its cherry leaf, or anything like that. What I saw was . . . THIS! (Cue Scream [4])

It was a squirming mass of grubs! The caterpillar was gone! The grubs had eaten everything, except the hard parts of the head! Even the skin was gone![5]

So, even though that caterpillar looked OK when we found it, it was actually DOOMED, DOOMED, DOOMED. It had been infested by parasitoids[6] at some point; something had laid eggs in it that had grown inside, carefully consuming its non-vital parts as it grew and ate and grew and ate, until finally the dinner bell rang and they FINISHED IT OFF! The mass of grubs was initially loosely wrapped in silk, so it looks like the trigger to End It All was the caterpillar starting to spin a cocoon in preparation for pupating[7].

Over the next day or so, the mass of grubs dispersed and dropped to the bottom of the jar, where I had put some soil for them to crawl into. At this point, the big question was what these things were, so we wanted to raise them up to see what we got. They crawled around for a while, and most of them eventually made cocoons like this one:

It took a couple of weeks for the cocoons to hatch out, and we finally got to see that they were some type of Braconid wasp. When I posted it on BugGuide, Bob Carlson thought that it looked like something in the genus Macrocentrus (and he also thought that the caterpillar was some sort of tortricid, maybe in the genus Archips, which are known to have green caterpillars that sometimes eat cherry leaves).

They were pretty small, so I needed the high-magnification lens, and they didn’t want to sit still. It is hard to quickly find subjects and focus at high magnification, so I was kind of scrambling to get pictures before they flew off. One of them gave me kind of a dirty sideways look that I managed to catch:

They had really, really long antennae, longer than their bodies, which implies that they probably find new hosts by scent.

Finally I got exasperated and froze one of them so I could get at least one clear picture while it was sitting still, so here we can see the leg configuration, the wings, and the long, long antennae:

Afterwards, I let the survivors go so that they could go out and parasitize more caterpillars. I don’t feel too bad about it, because the caterpillar they were in was eating our brand-new cherry tree, after all.

So, once again, we see that the life of a caterpillar is a hard one. Death can strike suddenly, without warning, both from the outside, and from inside.

[1] That’s the “Castle Thunder” sound effect, originally recorded for the 1931 “Frankenstein” movie. It has been used in dozens of movies and probably hundreds of cartoons since then.

[2] It is possible that the caterpillar was not fully in control of its own body at that point. It may have been crawling backwards because the things inside were forcing it to do so.

[3] In retrospect, I don’t think it was looking so good in that second picture. It was starting to get kind of bloated and yellow.

[4] And that’s Fay Wray screaming, in “King Kong”. I downloaded it from I thought about using the Wilhelm Scream, but that one is more of a “Being shot/Falling off of a high place/Being eaten by alligators” scream. Wray’s is a more appropriate “Recoiling in Horror” scream.

[5] An interesting question: Since the only food source for the parasitoid grubs was the caterpillar they were eating, and they ate every scrap with no leftovers, how did they manage such a close match between what they needed and what was available? Did the mother wasp simply lay exactly the right number of eggs so that the grubs would precisely consume the whole caterpillar with no waste? Or did the grubs eat the caterpillar until it was gone, and then start in on each other if there wasn’t quite enough to go around? Or did they force the caterpillar to remain a caterpillar until it had supplied them all the food that they needed, even if that was longer than it would normally have existed as a caterpillar? Or did they just make do with whatever was available, and grew up to be larger or smaller wasps depending on how well they were fed as grubs?

[6] The distinction between “parasitoids” and “parasites”, is that parasites don’t necessarily kill their hosts. Parasitiods, on the other hand, ultimately do consume their hosts until they die. And then there are “kleptoparasites”, which mainly steal the food stores that a parent insect laid up for her offspring (things like pollen, or paralyzed spiders, etc.), although they usually also eat the original egg or larva more or less as an afterthought.

[7] Sometimes, I think that if I’d known what was going to happen, we could have watched and photographed the whole sequence as they erupted through the skin and tore their host apart. Other times, I think that would have been just too much, and maybe it’s better for one’s peace of mind not to see such things.

12 Responses
  1. July 3, 2010

    This is awesome, Tim. Thank you for posting this fascinating sequence!

    >It is possible that the caterpillar was not fully in control of its own body at that point.
    > It may have been crawling backwards because the things inside were forcing it to do so.

    …And that is extremely creepy.

  2. Carole permalink
    July 3, 2010

    Fantastic. I always learn so much from you.

  3. Anne Bingham permalink
    July 3, 2010

    Was the caterpillar’s name John Hurt?

  4. July 4, 2010

    Awesome, if squirmy, post. Truly horrible.

    You can embed audio players in your posts fairly easily. The browsers will supply the controls your readers need. Take a look at this.

  5. July 5, 2010

    Well, I suppose his name is John Hurt *now*. Although, Hurt only had one alien bursting out of him, not thirty or so . . .

    KT: Thanks for the pointer. For some reason, I can’t get it to work, though. Possibly because I’m using the Opera browser, or possibly because I’m missing some important point. Well, I’ll keep trying, and maybe it will work eventually. I do have a few more entries that are going to require sound files, so it will be worthwhile knowing how to do this properly.

    Hah! Got it! It turned out that what I was missing, was that I had to specify height and width parameters before the control bar would appear on screen. Ok, now I can use this in the upcoming Cicadas entry!

  6. July 7, 2010

    Cool beans! I do wish you’d caught the whole gruesome emergence and consumption though…blech/neato! (And yeah, it DOES look bloated in the second pic…I wonder if it had any inkling what was going on in there…)

  7. July 8, 2010

    Tim, if you’ve got access to Adobe Flash and a server to host your js and swf files, I’m sure you could come up with some really cool audio players with insect-themed skins. In the absence of that (and the work it implies), I’m really grateful that our browsers can provide us simple sound players.

  8. kellitapita permalink
    July 9, 2010

    Thought you might find this interesting!

  9. Ellen permalink
    July 10, 2010

    Wow. Like you, I think that on the one hand, it would’ve been cool to watch the whole emergence, while on the other hand, that might have just been too much to take. The stuff of nightmares. >shudder<

    Great fodder for a post, though!

  10. Della3 permalink
    August 31, 2010

    I love the photo with the sideways look. This story is amazing. Photographing or filming the entire process would have been fascinating. As for the gore – hey, wildlife filmakers film this type of stuff all the time. Just warn us what’s coming so that if we’re not in the mood for such things, we can skip that part. If I’m not eating at the moment, and you don’t dwell on it too much, I might find it quite interesting.

  11. wiktoria permalink
    July 25, 2016

    ohhb thats creepy!! but fascinating… nevertheless i caught one in my room (one before that too, it was green and i squished it with my shoe since it was midnight and it decided to fly dangerously close to me while sounding like a mosquito) and was interested what it was. my question is, though, would it lay eggs inside me if it got the chance

  12. July 26, 2016

    Thankfully, the parasitic wasps have never developed the ability to parasitize mammals. While one could in principle lay eggs under a human’s skin, our immune system is so different from that of the insects they normally parasitize that the eggs get eliminated immediately.

    So, if you ever start feeling down, just make yourself feel better by remembering that at least we aren’t in danger of being eaten alive from the inside by parasitic wasp larvae.

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