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.

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[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 soundboard.com. 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.