Yet Another Caterpillar Parasitoid Wasp

2010 September 25

If it doesn’t say Sunkist, you don’t know what you’re getting“[1]

Sam and I were out in the yard next to the herb garden, when Sam said, “Look, caterpillar poop!” and pointed at the sage patch. Sure enough, there were caterpillar droppings on the sage leaves, and looking a bit closer we found a good-sized, nearly featureless green inchworm eating the sage. I thought this was unusual, because nothing local seems to eat sage. So, to try to identify it we brought it in and put it into a jar with some sage leaves, to try to raise it to adulthood. And, like a fool, I didn’t photograph it right away. Next thing we knew, it spun a cocoon,

and then evidently died before it could become a proper pupa.

We kept it around for about two weeks just to be sure it was dead. I thought that maybe there was some chance of identifying it based on what it was eating, since sage is not a common food plant, but no luck.

Then, on September 10, hundreds of tiny, tiny little wasps suddenly appeared all over around the mummified caterpillar!

And by “Tiny”, I mean “about a millimeter long” and “perfectly capable of going right through the holes in a standard window screen”. S_ got these pictures of them for me, because I was at work at the time and they were busily dispersing.

By the time I got home, there was only one left anywhere near the caterpillar, so S_ caught it for me and froze it to photograph later.

That blue object is the tip of a toothpick, if you want a sense of scale

And then, it turned out that they had all flown over to the main picture window in the house, and hung out there, invisible and unnoticed, until they died and fell onto the windowsill.

So, how do I know that they are wasps, and not gnats? Two things: first, their antennae are fairly long and elbowed, while gnats have tiny antennae, like the other Diptera. Second, if we look closely, they have four wings, while gnats, being Diptera, only have two wings.

So: wasps.

As for what kind of wasps, well, that’s where you’ve stumped me. At first I was thinking that they were one of the Braconids, if only because braconids are generally smaller than Ichneumons, and I thought that parasitoid wasps were mostly either braconids or ichneumons. But now, I see that the “parasitic Apocrita” includes many, many other kinds of parasitoids , and now I don’t know what they might be. There are thousands of species of parasitic wasps in North America (and the number of species seems to increase as the size of the insects gets smaller) so I’m doomed as far as an ID, I think[2].

Edited to add: Hey, we might have an ID after all! Down in the comments, Foxfier suggested that maybe the caterpillar had been a cabbage looper, Trichoplusia ni (a green, nondescript inchworm that is very common in gardens), and when I checked it out on BugGuide, I found this series of pictures that shows one that was parasitized! The caterpillar mummy looks almost exactly like the mummy of my caterpillar here, and the hundreds of wasplets that came out look very like the ones that came out of mine! In that case, the wasps are almost certainly Copidosoma, and probably Copidosoma floridanum. Holy cow! These are the record-holders for the greatest number of parasitoids born from a single host – their official record is 2,750 individuals, and there are unpublished counts as high as 3,055!.

The fact that these wasps are so tiny, and yet so very, very many of them emerged from this one caterpillar, brings up an interesting question: how on earth could just one tiny wasp have room in her body to lay so many eggs? Either the eggs would have to be unimaginably tiny, or she would have to have a trick. And, it turns out, she was probably using a trick – it’s called “polyembryony“[3]. The way it works is this: The mother wasp finds a host, and injects into it a very small number of eggs (maybe even just one), and flies off. The small number of initial eggs then start dividing and separating into multiple embryos, essentially making scores of identical twins. They keep doing this until there are enough embryos to fully consume their host, and then stop dividing and actually mature. This gives the mother wasp several advantages:

– She still gets a lot of offspring without having to pack so many eggs in her abdomen, so she can be pretty minute;

– The number of parasitoid grubs gets automatically matched to the capacity of the host to feed them; and

– She can spread her eggs around into many hosts, rather than chancing them all on just one.

The downsides (which are why not all parasitic wasps do this) are probably (1) It only does any good if the parasitoid grubs are much, much smaller than the host. If the host is only big enough to support a few parasitoids, then there’s no point; and (2) The parasitoid grubs are all clones of each other, so they lack genetic diversity – which means, in practice, that anything that kills one of them is likely to kill all of them[4].

Incidentally, I suspect that the caterpillar wouldn’t normally have been eating sage. It has recently been discovered that woolly bear caterpillars will eat moderately toxic plants to self-medicate themselves, in an attempt to eliminate parasites. It is quite possible that such behavior is not limited to woolly bears, and that this caterpillar was trying to do the same thing. Sage is well-known as a spice and medicine, because the plant is full of moderately toxic (and therefore flavorful and biologically active!) defensive chemicals that normally save it from being eaten. So, it may very well be as much of a medicine for caterpillars as for humans. Unfortunately for the caterpillar, though, it evidently wasn’t quite good enough.

[1] I saw this Sunkist commercial when I was a kid, and for some reason it has stuck with me as symbolizing all the times that you get something completely unrelated to what you were expecting.

[2] This is a case where there is actually a non-negligible chance that I’ve found an undescribed species, but will never know for sure because I don’t know how to find out for certain. Never mind, it looks like we have an ID after all!

[3] I knew I’d heard of wasps getting multiple offspring out of a single egg before, but I couldn’t remember what it was called. Finding something online, when you don’t know its name, is quite frustrating, you know? I eventually found it defined in a Google Books scan of A Dictionary of Zoology. Once I knew the name of the behavior, finding information about it was a piece of cake.

[4] That’s an argument that comes up pretty often: why do so many organisms expend the time and effort on sexual reproduction, when simply dividing asexually or producing eggs parthenogenically can produce so many more offspring in a shorter time, and often with less risk? The argument I find most convincing is that sexual reproduction reduces the odds that all of your offspring will be killed off by some plague.

For example: Imagine we have a population of aphids, and two diseases (X and Y). Also, let’s say that for each disease, there is a gene mutation that makes the aphids resistant to that disease (but they are only resistant to both diseases if they have both resistance genes).

Now, let’s assume that there were two founder aphids, one of which was resistant to X, and the other was resistant to Y. If those two aphids reproduce parthenogenically, then they can churn out the offspring at an incredible clip, but none of those offspring will have both resistance genes. All of the descendants of the X-resistant mother will carry that gene, but with straight parthenogenic reproduction they will have no way to acquire the Y-resistance gene from the other mother. If the colony is infected with disease X, then the only survivors will be those resistant to that disease. But, this will mean that all of the aphids resistant to disease Y just died. And so, if the colony is then infected with disease Y, all the rest of them die. If, on the other hand, the two founder aphids have both male and female offspring, and they mate and recombine genes before laying eggs, then some fraction of the offspring will be resistant to both Disease X and Disease Y, so that after the colony is infected with both diseases, at least some of the offspring will survive.

So, basically, genetic diversity through sexual reproduction makes the difference between having a smaller number of offspring with a few that live, and a much larger number of offspring that all die.

5 Responses
  1. September 25, 2010

    And little wasps have lesser wasps, and so ad infinitum! Things that make my Saturdays special: Car Talk and The Backyard Athropod Project!

  2. September 26, 2010

    Thanks, Anne!
    Hey, maybe I *could* get my wisecracking brother to join me on this to inject weird non-sequitur comments! Except that he’s in Tennessee these days, so it would be hard to organize. Then again, my daughters could probably fill that role just as well. Hmm . . . .

  3. September 28, 2010

    I did have hopes of identifying it from “Caterpillars of Eastern North America”, because it has a food-plant index. But, neither of the caterpillars listed as eating “sage” looked at all close.

    The cabbage looper is a possibility, it’s certainly an inchworm, and is green enough – Oh, hey! I just checked on the cabbage looper (Trichoplusia ni) on BugGuide, and there’s a series of pictures showing one being parasitized by tiny little wasps that look just like these, and turned into a mummy that looks just like the one I have! That’s it! Thanks very much!

    I just added some more information to my main article based on this, with some links.

  4. September 28, 2010

    Welcome! ^.^ My mom will be delighted to hear that a pest website was handy!

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