Parasite tales: The jewel wasp's zombie slave - Carl Zimmer

Transcriber: Andrea McDonough
Reviewer: Bedirhan Cinar

I would like to introduce you to my favorite parasite. There are millions that I could choose from, and this is it: it's called the jewel wasp. You can find it in parts of Africa and Asia. It's a little under an inch long, and it is a beautiful looking parasite.

Now, you may be saying to yourself, "This is not a parasite. It's not a tapeworm, it's not a virus, how could a wasp be a parasite?" You are probably thinking about regular wasps, you know, the ones that build paper nests as their house. Well, the thing is that the jewel wasp makes its house inside a living cockroach.

Here's how it happens. A jewel wasp is flying around, looking for a cockroach. When it sees one, it lands and bites on its wing. So, I'll be the cockroach. Be-wha! Bewha! And the cockroach starts shaking it off, "Get away from me!" The wasp very quickly starts stinging the cockroach. All of a sudden, the cockroach can't move for about a minute. And then it recovers and stands up. It could run away now, but it doesn't. It just doesn't want to. It just stays there.

It's become a zombie slave. Again, I'm not making this up. The wasp goes off, it walks away and finds a hole and digs it out, makes it into a burrow. It walks back. This can take up to half an hour. The cockroach is still there. What do we do now? The wasp grabs onto one of the antennae, bites down on it, of the cockroach, and pulls the cockroach. And the cockroach says, "Alright," and walks like a dog on a leash.

The wasp takes it all the way down into the burrow. The cockroach says, "Nice place." The wasp takes care of some business and then goes and leaves the burrow and seals it shut, leaving the cockroach entombed in darkness, still alive. The cockroach says, "Alright, I'll stay here if you want."

Now, I mentioned that the cockroach took care, ah, the wasp took care of a little business before it left the burrow. The business was laying an egg on the underside of the cockroach. The egg hatches. Out comes a wasp larva. It looks kind of like a maggot with big, nasty jaws. It chews a hole into the cockroach and starts to feed from the outside.

It gets bigger, like you can see over here. And then when it gets big enough, it decides to crawl into the hole, into the cockroach. So now it's inside the still-living cockroach and the cockroach doesn't mind much. This goes on for about a month. The larva grows and grows and grows, then makes a pupa, kind of like a cocoon. Inside there it grows eyes, it grows wings, it grows legs, the cockroach is still alive, still waiting.

Finally the wasp is ready to leave, and that's when the cockroach finally dies because the fully-formed adult wasp crawls out of the cockroach's dying body. The wasp shakes itself off, climbs out of the burrow, goes and finds another wasp to mate with to start this whole, crazy cycle again. So, this is not science fiction; this happens every day, all over the world.

And scientists are totally fascinated by this. They're just starting to figure out how all this happens. And when you really start to look at the science of it, you start to kind of respect this very creepy wasp. You see, the thing is that when it attacks the cockroach, it's not just stinging wildly; it delivers two precise stings.

It knows this cockroach's nervous system like you know the back of your hand. The first sting goes to that spot there, called the "walking rhythm generators," and, as you can guess, those are the neurons that send signals to the legs to move. It blocks the channels that the neurons use to send these signals. So the cockroach wants to go, it wants to run away, but it can't because it can't move its legs. And that lasted for about a minute.

This is really sophisticated pharmacology. We actually use the same method, a drug called Ivermectin, to cure river blindness, which is caused by a parasitic worm that gets into your eye. If you take Ivermectin, you paralyze the worm using the same strategy. Now, we discovered this in the 1970s; the wasp has been doing this for millions of years.

Then comes the second sting. Now the second sting actually hits two places along the way. And to try to imagine how this can happen, I want you to picture yourself with a friend who's got a very long, very, very scary looking needle. And your friend, or at least you thought he was your friend, sticks it in your neck, goes into your skull, stops off at one part of your brain and injects some drugs, then keeps going in your brain and injects some more.

These are two particular spots, marked here, "SEG," and you can see the tip of it in the brain, marked "Br." Now, we can do this, but it's really hard for us. It's called stereotactic drug delivery. You have to put a patient in a big metal frame to hold them still, you need CAT Scans to know where you're going, so you look at the picture and say, "Are we going the right way?"

The jewel wasp has sensors on its stinger and scientists think that it can actually feel its way through the cockroach's brain until it gets to the exact, right place, and then penetrates an individual neuron and then delivers the goods. So, this is quite amazing stuff, and what seems to happen then is that the wasp is taking away the control that the cockroach has over its own body.

It's taking away the cockroach's free will. We didn't really appreciate that cockroaches have free will until this wasp showed us. And we have no idea how it's doing this; we don't know yet what the venom has in it and we don't know which circuits it's hitting in the cockroach's brain, and I think that's why this is, most of all, my favorite parasite because we have so much left to learn from it. Thank you very much.