A Blog by Carl Zimmer

Save the Zombie-Makers!

Parasites may seem too gross or too wicked to be worth saving from extinction. Or they may just seem so skilled in their sinister arts that we don’t have to worry about them, since they’ll always find a new victim.

In fact, parasites warrant our concern, right along with their hosts. That’s not to say that we’d better off if smallpox or rinderpest were still running wild. But letting parasites hurtle into oblivion due to our ecological recklessness is a bad idea.

Here’s a case in point: The World Wildlife Fund has just drawn attention to a parasitic wasp, Ampulex dementor, that makes cockroaches its zombified victims. The wasp was found in 2007 in Thailand, and in 2014 a German museum held a contest to give it a species name. Museum goers voted to name it after the soul-sucking dementors in the Harry Potter series.

WWF highlighted A. dementor in a new report on the 139 new species from the Greater Mekong Region that were described in 2014 alone. This region, which includes Cambodia, Laos, Myanmar, Thailand, and Vietnam, is stunningly rich with species. It’s also incredibly productive, yielding a quarter of the world’s catch of freshwater fish. But it’s also under intense pressure, ranking in the top five threatened biodiversity hotspots on Earth. Dams, roads, logging, and hunting are all taking their toll on the species there. Climate change will only add to the threats the Mekong’s species face.

A species like A. dementor is caught in a special bind. We didn’t even know it existed until recently, so it’s hard to know precisely how well the species is faring. No one has a detailed map of its range before human pressure ramped up in the past century, and no one has a corresponding map of its current range.

On top of that, the published scientific literature–pretty much just a single paper published last year–doesn’t even tell us about the particular cockroaches the wasp parasitizes. Does it zombify several species of cockroaches? Does it zombify just one? These questions matter a lot to the survival of A. dementor. If it parasitizes a single rare species, it could become extinct if its host disappears. (While a few species of cockroaches have become global champions by adapting to our homes, the vast majority can only survive in wild forests.)

While we know little about this parasite, the ecological threats to the Greater Mekong Region should make us concerned about it. And losing a species of parasite can be a bad thing. Parasites, for example, are important players in food webs. If they disappear from an ecosystem, their hosts–and the species that are affected by those hosts–may undergo wild swings. If you don’t like cockroaches, the last thing you want is for the parasites that devour them from the inside out to vanish.

Parasites are also worth saving for what they have to teach us. And that’s especially true for wasps like A. dementor. It belongs to a lineage known as Ampulicidae or the cockroach wasps, which contains 200 named species–and probably many more waiting to be discovered. The best known of these species is Ampulex compressa, sometimes called the emerald cockroach wasp. Phenomena readers may be quite familiar with the emerald cockroach wasp, because fellow blogger Ed Yong and I just won’t shut up about it. (I also added an epilogue to my book Parasite Rex pretty much just to write about it.)

The reason we know so much about the emerald cockroach wasp is that a team of researchers led by Frederic Libersat at Ben-Gurion University in Israel have figured out how to rear the wasps in their lab, and for years now they’ve been observing its remarkable skills.

The female emerald cockroach wasp searches for roaches, probably scanning the ground while sniffing the air. The wasp swoops down on the roach and stings it in its abdomen, temporarily paralyzing it. It then delivers a second shot to the head–literally snaking its stinger into the recesses of the cockroach brain. Now the cockroach loses all motivation to do much of anything. You can even shock its leg and it won’t budge on its own. But the wasp can grab onto an antenna and lead it into a burrow.

There, the wasp lays an egg on the roach’s underside and then leaves, sealing the burrow behind it. The egg hatches and the larva sucks on the roach in tick-like fashion for a while, before squirming inside the host’s body to finish off its growth. To keep its host from dying of infections, it smears an antibiotic cocktail on the roach’s inner body wall. The wasp larva forms a cocoon inside the roach, which then finally dies. Later, the fully-grown wasp pokes its head out of the roach, wriggles entirely free, and leaves the burrow.

These wasps may have many lessons for us. Most of their antibiotics are new to science, for example, and so they may be worth investigating further for medicine. The wasps have also evolved a remarkable skill at manipulating the cockroach brain. Figuring out how they do it might tell us more about how the nervous systems of insects work. And it might provide some inspirations for ways to manipulate our own brains–not to turn ourselves into zombies, but to treat psychological disorders.

But almost all the insights we’ve got about cockroach wasps come from a single species. Far from being degenerates, as they were traditionally viewed, parasites can evolve rapidly, hitting on new strategies for conquering their hosts. So it’s entirely possible that A. dementor uses a soul-sucking arsenal that’s significantly different than its cousin species A. compressa. The only way we can enjoy discovering that arsenal is to make sure this species doesn’t vanish first.


If you want to find out more about cockroach wasps, here are some pointers:

Me at TED-Ed, grossing out high schoolers with the details of the emerald cockroach wasp:

Ed at TED, talking about wasps and other parasitic mind-controllers:

Here’s a post I wrote in 2006 after a scientist first told me about Ampulex compressa, and after I confirmed he wasn’t just trying to fool me into thinking a crazy story was true.

Here’s Ed on how the wasp’s venom affects cockroach behavior.

Me on parasite antibiotics, and Ed’s write-up in Nature.

Me on how the wasps snake their stinger through the cockroach brain.

And on Radiolab, I talk to Jad Abumrad and Robert Krulwich about how these wasps put science fiction to shame.

A Blog by Carl Zimmer

Lemon-Scented Malaria

Parasites are life’s great success story, abundant in both species and sheer numbers. One secret to their success is the ability that many parasites have to manipulate their hosts. By pulling strings like a puppet master, they use their hosts to advance towards their own goal of planetary conquest. Creepy is the best word to describe most of their strategies. They turn some hosts suicidal. They castrate others. They turn still others into zombie bodyguards. But a new study published today suggests that the parasite that causes malaria may use a more pleasant strategy. It lures mosquitoes to infected hosts with a lemony scent.

Malaria is caused by single-celled parasites called Plasmodium. A female mosquito carries them in its gut as it flies around in search of a victim to bite. After the parasites mature, they push through the insect’s gut wall, eventually making their way into its salivary glands. When the mosquito lands on a person and drills into the skin, it pushes some of its Plasmodium-laden saliva into the wound.

The parasites now begin their long journey through the human body. They get pushed by the surges of the bloodstream to the liver, where they invade cells and multiply inside them. The infected liver cells erupt with the next stage of Plasmodium’s life cycle, called merozoites. The merozoites end up back into the bloodstream, where they now invade red blood cells. They multiply yet again, rupturing the blood cells and invading new ones. Eventually the parasites achieve the next stage in their life cycle, when they’re ready at last to get sucked up by a hungry mosquito in a meal of blood.

If Plasmodium  can’t get into a mosquito, all of this multiplication is for naught. So anything that the parasite can do to increase the odds of a successful exit can potentially be favored by natural selection. Last year, for example, a team of researchers found that mosquitoes were attracted to mice infected with Plasmodium parasites–but only when they were ready to leave their rodent host. The scientists found evidence that the parasites engineer this attraction by changing the odor of the mice. Infected mice give off odor molecules that draw mosquitoes to them.

Those scientists speculated that perhaps the parasite alters its hosts chemistry to make new odors. But recently Audrey Odom of Washington University and her colleagues raised another possibility: maybe the parasites themselves produce mosquito-attracting chemicals.

Other scientists had explored this possibility before without much to show for their efforts. But Odom and her colleagues suspected that previous researchers hadn’t looked hard enough. So the Washington University team added Plasmodium to much larger volumes of blood than before–400 milliliters–and then snagged odors rising off the blood with more sensitive traps. The efforts paid off: Odom and her colleagues found that when Plasmodium infected red blood cells, it produced chemicals called pinene and limonene.

You have probably smelled these chemicals before. Pinene is part of the blend of odors that make up the scent of pine trees. Limonene gets its name from lemons, which produce it in their rinds.

If you’re confused at this point about a single-celled blood parasite producing a fragrant odor, you have every right to be. To make sense Odom’s weird discovery, we have to take a sharp detour through more than a billion years of evolution.


It’s 1.3 billion years ago. The planet is ruled by bacteria and protozoans. Animals and plants won’t evolve for many hundreds of millions of years. On the surface of the ocean, some bacteria are capturing sunlight with photosynthesis, while protozoans are preying on them. Somehow, this story goes off-script, and some protozoans end up with photosynthetic bacteria trapped inside them. Instead of becoming food, the bacteria supply the food, powering the protozoans with photosynthesis. Over many generations, the bacteria become an inseparable part of their host. The combination of these two kinds of life become a new kind, which we call algae.

This primordial algae had many descendants. Some of them evolved into green algae, and eventually gave rise to plants on land. Another lineage of algae were swallowed up by yet another protozoan, and became another form of algae found on Earth today, known as red algae. Some red algae live now as free-floating photosynthesizers in the ocean. Others took up inside corals, providing coral animals with sustenance from the sun. And still other red algae became parasites of animals. Some of these parasites eventually became Plasmodium.

Plasmodium’s ancestors lost the ability to photosynthesize a long time ago. But they still hold onto some of the ancestral enzymes from the bacteria that their forebears swallowed 1.3 billion years ago. As a result, Plasmodium is weirdly similar to flowers and trees. Some scientists have even taken advantage of this evolutionary kinship by looking at weed-killers as potential drugs for malaria.

This ancient heritage also explains why Plasmodium can smell like lemons. Odom and her colleagues found that the parasite make pinene and limonene using enzymes that are related to the ones that plants use to make these chemicals.

There are reasons to think that the parasite are using these chemicals to lure mosquitoes. While we’re painfully aware of the appetite mosquitoes have for blood, the fact is that mosquitoes also feed on flower nectar. They depend on the nectar for sugar they need to fuel their flights. Many insects are keenly sensitive to certain colors and odors that flowers produce, which guide them reliably to their next meal of nectar. Odom and her colleagues found that the antenna of malaria-carrying species of mosquitoes are exquisitely sensitive to pinene and limonene. If you want to attract mosquitoes, it makes sense to make those chemicals.

While this research is tantalizing, it is only the first step in testing the hypothesis that Plasmodium makes a fragrant odor to lure its next host. So far, Odom and her colleagues have only demonstrated that the parasites are making these chemicals inside red blood cells. It’s certainly conceivable that in a living host, these odors could escape into the lungs and leave the body with exhaled air. It remains to be seen if they really do get out, and if they make a difference to the success of Plasmodium. Or perhaps this parasite perfume has some other function, and it remains bottled up inside sick hosts.

(For more examples of parasite manipulation, see my cover story in the November 2014 issue of National Geographic or my book Parasite Rex.)

Reference: Kelly M, Su C-Y, Schaber C, Crowley JR, Hsu F-F, Carlson JR, Odom AR. 2015. Malaria parasites produce volatile mosquito attractants. mBio 6(2):e00235-15. doi:10.1128/ mBio.00235-15.

[Update: Link to paper fixed]

A Blog by Carl Zimmer

Parasitic Wasps Infected with Mind-Controlling Viruses

Parasite-NG-cover-550In November, National Geographic put a ladybug and a wasp on its cover. They made for a sinister pair. The wasp, a species called Dinocampus coccinellae, lays an egg inside the ladybug Coleomegilla maculata. After the egg hatches, the wasp larva develops inside the ladybug, feeding on its internal juices. When the wasp ready to develop into an adult, it crawls out of its still-living host and weaves a cocoon around itself.

As I wrote in the article that accompanied that photograph, the ladybug then does something remarkable: it becomes a bodyguard. It hunches over the wasp and defends it against predators and other species of parasitic wasps that would try to lay their eggs inside the cocoon. Only after the adult wasp emerges from its cocoon does the bodyguard ladybug move again. It either recovers, or dies   from the damage of growing another creature inside of it.

How parasites turn their hosts into zombie slaves is a tough question for scientists to answer. In some cases, researchers have found evidence suggesting that the parasites release brain-controlling chemicals. But the wasp uses another strategy: there’s a parasite within this parasite.

In the Proceedings of the Royal Society, a team of French and Canadian researchers now lay out the evidence for this strange state of affairs. As they studied this manipulation, they reasoned that the best place to look for clues was inside the heads of parasitized ladybugs. They discovered that the brains of these hosts were loaded with viruses. When the scientists sequenced the genes of the virus, they found it was a new species, which they dubbed D. coccinellae Paralysis Virus, or DcPV for short.

The scientists found DcPV in the wasps as well–but not in their brains. In female adult wasps, the virus grows in the tissues around their eggs. Once a wasp egg hatches inside a ladybug, the virus starts replicating inside it, too. The larva then passes on the virus to its host, and the ladybug develops an infection as well.

DcPV causes no apparent harm to the wasps, but the ladybug is not so lucky. The virus makes its way into the ladybug’s head, where it attacks brain cells and produces new viruses in pockets inside the cells. Many brain cells die off during the infection.

The researchers hypothesize that the virus is responsible for the change in the ladybug’s behavior. To get the ladybug to guard the wasp, the virus may partially paralyze its host, so that it becomes frozen over the parasite. Because the paralysis isn’t complete, the ladybug can still lash out against predators. But these may just be wild spasms in response to any stimulus. The bodyguard effect may grow even stronger as the infection robs the ladybug of the signals from its eyes and antennae. Closed off the world, its sole purpose becomes protecting its parasite.

The fate of a parasitized ladybug–to die or to walk away–may depend on how it handles a DcPV infection. In some cases, the virus may be fatal–possibly by triggering a massive immune response that kills not just the virus but the ladybug itself. In other cases, the ladybug’s immune system may eventually be able to clear the virus out of its system, letting its nervous system heal.

In either case, the bodyguard paralysis lasts long enough to protect the wasp while it develops into an adult. Whether the ladybug lives or dies doesn’t matter to the wasp–or to the virus. The new wasp carries a fresh supply of DcPV. If it’s a female, it will be able to use the virus to infect both its own young, and its ladybug slave.

In recent years, scientists have developed a deepening appreciation for the importance of our microbiome–of the bacteria and viruses that make our bodies their home. While some microbes invade our bodies, others reside inside of us and help keep us healthy. Parasitic animals have microbiomes of their own, and this new study suggests that they can use them for suitably sinister ends.

(For more information on the sophisticated tricks of parasites, see my book Parasite Rex.)

A Blog by Carl Zimmer

For Your Halloween Viewing Pleasure: Two Mindsucker Movies

Last night at the National Geographic Society in Washington, I gave a talk with photographer Anand Varma about how parasites manipulate their hosts–the subject of my cover story in the November issue of National Geographic and Varma’s aesthetic obsession for the past couple years. Along with his gorgeous photos, Varma also showed off some lovely/creepy videos. I thought I’d share a couple of them with you. Pop them into full screen for full appreciation.

First off: Ophiocordyceps, a fungus that takes control of an ant. The fungus spores invade an ant’s body and then fill much its interior with tendrils. Under the fungus’s spell, the ant climbs up a plant and clamps down on the underside of a leaf. This movie, which Anand took in the Amazon working with Joao Araujo, a biology grad student at Penn State, shows the ant in its last hours, shot upside down for clarity. The fungus shoots a spike of spores out of the ant, which can then rain down on unfortunate ants below.

The second video shows how the white butterfly wasp takes over the cabbage butterfly caterpillar. A female wasp inserts her stinger into the host and injects dozens of eggs. They grow inside the caterpillar, which continues munching on leaves–leaves that now fuel the growth of the parasites inside. They chew their way out all at once, and yet they don’t kill the caterpillar. They prevent it from bleeding to death by thickening its bodily fluids and seal up their exit holes with bits of their own tissue. The caterpillar recovers from this strange birth and then spins a cocoon for the wasp larvae. It then sits atop its parasitic brood, fending off any animals that try to get at the wasps. At the end of the movie, you see the most dreaded of these enemies: another species of wasp that only lays its eggs in the larvae of the white butterfly wasp.

If you’re in Connecticut, you can come hear me talk about these puppet masters at the Westport Library on Saturday at 4 pm. And if you still crave more, check out my book Parasite Rex.

[Update: Added Araujo to post]


This Month In National Geographic: Parasites and Their Zombies

Parasite-NG-cover-550I’ve written the cover story for the new November issue of National Geographic about the biology of parasite manipulation. I’ve been obsessed by this subject for a long time. (In my book Parasite Rex I wrote a chapter on this bizarre slice of reality). So it’s a huge delight to help give these mind-controllers the Nat Geo treatment: gorgeous pictures. When I wrote Parasite Rex, I gathered up what photos I could find, but none of them did the parasites justice. Anand Varma has journeyed to a number of countries to find the creepiest examples of this surprisingly common (and medical useful) phenomenon.

Anand and I will be speaking at the National Geographic Society in Washington DC about the story and photos on October 29. Please join us. Details are here.

A Blog by Carl Zimmer

Ebola: New or Old?

As viruses go, Ebola has a grim star power. When a new outbreak hits, Ebola kills a high fraction of its victims, causing horrific bleeding along the way. The latest outbreak started in March in Guinea. As of today, the World Health Organization reported 231 cases and 155 deaths.

In order to better treat Ebola, Pardis Sabeti of Harvard and her colleagues have been analyzing the virus’s evolution. It turns out that Ebola is not some freakish new plague, but rather old. If that seems puzzling, a research scientist in Sabeti’s lab, Stephen Gire, has created this animation, which I’ve embedded below, to explain it.

Gire’s video is part of a contest being run by the National Institutes of Health. Check them all out and vote for your favorites. It’s great to see scientists being encouraged to explain what they do in a new medium like this. Wonderful things can evolve from this kind of experiment. A few years ago, for example, Brown University biologist Casey Dunn and his students started toying around with stop-action animation to tell stories of marine biology, and now their “Creature Cast” series is regularly featured on the New York Times web site.

Who knows–perhaps the next Carl Sagan will be a cartoonist?

A Blog by Carl Zimmer

The Information Parasites

Parasites can take many forms. Just this week, I’ve written about a giant virus that reproduces inside amoebae (and has survived being frozen 30,000 years in permafrost), along with a wasp that performs brain surgery to zombify hosts for its young. Viruses and wasps are radically different organisms–some would say that viruses don’t even deserve the label of organism. And they make use of their hosts in different ways. The virus sits inside a cell, manipulates its biochemistry to build virus proteins and DNA. The wasp, on the other hand, sips fluids inside a still-living roach, and builds its own proteins and DNA–and then becomes a free-living creature that can climb out of its host and fly away.

So why are they both parasites? The answer lies beyond the details of anatomy and molecules. It’s all about relationships.

Species can have all sorts of influences on each other. They can eat or be eaten, they can pollinate or steal pollen. But there’s one yardstick that scientists can use to measure all the variety in these interactions: the change that one species has on how many offspring the other can have. By that measurement, the differences between giant viruses and brain-surgeon wasps melt away. Each one is a disaster for its partner species. The viruses multiply inside amoebae until they burst. The roach lives until its wasp parasite is ready to depart. In each case, the relationship is good for the parasite (more offspring) and bad for the host (fewer).

When scientists look at life with this definition in mind, they can see a lot of parasites that might not look like parasites. We don’t think of birds as parasites–they’re too beautiful and not in the least bit creepy. But when a cuckoo pushes out the eggs of a reed warbler and puts her own in their place, and when the cuckoo chicks use all sorts of tricks to fool the reed warbler to feed them as if they were its own, we are seeing another parasite at work.

In the journal BMC Evolutionary Biology, a team of scientists in Finland describe another kind of parasite–one that doesn’t steal food or protein synthesis or even parental care. In the words of the scientists, these are “information parasites.”

Top: Great tit. Bottom: Pied flycatcher. Flickr: http://flic.kr/p/7o6KjK http://flic.kr/p/dGnDRc
Top: Great tit. Bottom: Pied flycatcher. Flickr: http://flic.kr/p/7o6KjK http://flic.kr/p/dGnDRc

These information parasites are, once again, birds. Lovely birds, in fact, known as pied flycatchers. And their victims are another species of bird, the great tit (twelve-year-olds at heart are allowed a few moments to get sniggers out of their system).

The pied flycatchers and great tits, both found across much of Europe, have evolved to the point where their existence is quite similar. They eat a lot of the same kinds of food, get killed by the same predators, and even choose the same sites for their nests. This similarity leads to a fair amount of competition, sometimes quite violent. If a bird from one species flies into a crevice to check out a potential nest spot, only to find the other species there, the two birds will fight–sometimes to the death.

The two species aren’t identical, though, and there a couple differences that are particularly intriguing.The great tits build their nests earlier in the year, and the pied flycatchers have a habit of paying visits to great tit nests before building their own.

In recent years, the Finnish researchers have found a likely reason for these visits. The pied flycatchers are gathering intel. They inspect the nests of great tits to help them decide where they will make their own nests. One piece of information they’re interested in is the number of eggs are in a great tit’s nest. If a nest is loaded with eggs, it’s probably a good place for a pied flycatcher to make its own nearby.

The great tit suffers for letting the pied flycatcher get this information. Now a rival bird sets up house on the same territory and starts to compete for the same food. The researchers have found that great tits that attract these neighbors end up with fewer nestlings as a result. The pied flycatchers, on the other hand, have more success in reproducing because they build their nests on good real estate. One species benefits, and one suffers. But the benefit doesn’t come from cockroach innards or cell proteins. The pied flycatcher is stealing information.

Once parasites evolve a strategy for taking advantage of a host, the host generally evolves defenses. Immune systems recognize pathogens and destroy them. The hosts of some wasps will fly away or fight off their attacker. If pied flycatchers really were information parasites, then great tits might evolve defenses to safeguard their information.

When great tits are laying eggs, they search for sheep hair and other materials to keep the eggs covered. It’s not clear why they bother. You could imagine that the covering is a blanket to keep the eggs warm. But the birds don’t bother to keep the eggs covered once they’re all laid and the embryos start to develop. So it’s possible that they’re doing something else with the hair.

One thing that the hair does is hide the eggs. The Finnish scientists wondered if the great tits use hair to hide information from flycatchers. To find out, they ran an experiment.

They put a decoy of a pied flycatcher five meters from great tit nests and played a recording of a pied flycatcher singing for five minutes. The next day, they collected the hair in the nests. The scientists then ran the same experiment, but with decoys of cedar waxwings–birds that live alongside great tits but don’t compete with them.

The great tits responded to pied flycatchers by adding over 40% more hair on top of their eggs than they would otherwise. The scientists concluded that the birds hide the eggs when pied flycatchers show up so that the pied flycatchers won’t see just how well the great tits are doing. Seeing what looks like a meager nest, the pied flycatchers will be more likely to move on.

When hosts evolve defenses against parasites, parasites sometimes evolve counter-defenses. When flu viruses infect a cell, for example, the cell can respond by making an anti-viral protein called interferon. The interferon guides the cell to chop up the invading virus genes. But flu viruses have proteins that block interferon.

Do information parasites have their own counter-defenses? The scientists don’t offer any solid scientific evidence in their new report, but they do mention that they’ve seen something odd. They’ve seen pied flycatchers sneak into great nests and pull hair from the eggs. That may seem like a pointless exercise, since pied flycatchers don’t use hair on their own nests. It’s possible that they’re just trying to steal some reliable information.

When I first started writing my book on the triumph of parasites, I burrowed into the science and was stunned at how many ways there were to be a parasite. Eventually, the bottom just fell out. This is the first time that I’ve become aware of the concept of “information parasites,” but I suspect it won’t be the last.

[Update: Correction–reed warblers are hosts of cuckoos, not cowbirds]

A Blog by Carl Zimmer

Crawling Through The Brain Without Getting Lost

Ampulex compressa. Photo by K. Seltmann, via Creative Commons. Link: http://www.morphbank.net/?id=102143
Ampulex compressa. Photo by K. Seltmann, via Creative Commons. Link: http://www.morphbank.net/?id=102143

If you’ve never met the emerald jewel wasp, let me introduce you to my little friend.

The wasp (Ampulex compressa) lives the first stage of its life as a parasite, growing inside the body of a living cockroach. That’s absorbingly horrific on its own, but how it gets into the cockroach in the first place is an especially gruesome delight. Its mother has to play neurosurgeon.

A female wasp seeks out a cockroach host and ambushes it. She inserts her stinger into its thorax and delivers a paralyzing shot of venom that immobilizes the insect for a few minutes. She pulls her stinger out and then delivers a second injection. This one goes into the cockroach’s head, delivering more chemicals to two sites in the brain of the host.

Image copyright Quade Paul
Image copyright Quade Paul

The result is a cockroach zombie. The neurosurgically altered victim recovers from its paralysis but now lacks the will to flee or fight. The wasp pulls on an antenna and leads the roach, like a dog on a leash, into a burrow. There she glues an egg to the underside of the roach. She leaves the burrow and seals it shut. In the darkness, the roach stands motionless as the wasp larva hatches from its egg and chews a hole into its side. The wasp feeds through the hole for a while, and then slithers inside. Later, it pops out as a full-grown adult.

ampulex emerging
Image by Ram Gal

At Ben-Gurion University, Frederic Libersat and his colleagues have been studying the emerald jewel wasp for over 20 years, and they continue to learn new things about it. In the journal PLOS One, they’ve now published previously unknown details about the creepiest part of the wasp’s attack: its injection of zombie drugs into the cockroach brain.

To appreciate just how tricky this can be, consider what it takes for doctors to deliver drugs to a human brain. They scan the patient’s brain to map its anatomy in three dimensions. Then they put their patient’s head in a cage, drill a hole in the skull, and then slowly push a tube into the brain. A wasp does much the same thing in about a minute, without ever glancing at a brain scan of its victim.

Some days I wish I was a  jewel wasp. Photo source: http://neurosurgerycns.wordpress.com
Some days I wish I was a jewel wasp. Photo source: http://neurosurgerycns.wordpress.com

They pull off this feat with their extraordinary stinger. It measures 2 millimeters long, enabling the wasp to insert it into the roach’s neck and snake it up to the brain. The tip of the stinger has two sets of valves. One set hold the equipment for laying eggs, and the other set hold the equipment for delivering venom. The valves interlock in a tongue-and-groove arrangement so that they can slide over each other, allowing the wasp to lay an egg or deliver a sting with the same organ.

Portion of a wasp stinger. Red arrows mark touch-senstive bell-shaped bumps. Black arrows mark touch- and chemical-sensitive dome-shaped bumps. Gal et al PLOS One. http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0089683
Portion of a wasp stinger. Red arrows mark touch-senstive bell-shaped bumps. Black arrows mark touch- and chemical-sensitive dome-shaped bumps. Gal et al PLOS One.

The stinger, the scientists also found, are studded with little bumps–some shaped like bells, others like domes. Each bell-shaped bump has a touch-sensitive nerve ending inside it, while the dome-shaped bumps have a touch-sensitive nerve ending along with four or five chemical-sensing ones.

To see what those bumps are doing, the scientists put electrodes inside the nervous system of wasps and then pushed the stinger against rubbery lumps meant to simulate a roach’s brain. The wasps’s nerves crackled with activity when the bumps on the stinger tip pushed along a lump. This response suggests that the wasp uses its stinger to feel its way through the roach’s brain.

To see if this was true, the scientists stripped the stinger bumps off of wasps and then let them attack a cockroach. The average time the wasps spent probing the cockroach brain shot up from just over a minute to nearly 20 minutes. That’s what you’d expect if the wasps suddenly were unable to find their way inside a cockroach brain.

The scientists then ran another type of test, presenting healthy wasps with roaches that they had altered in various ways. They took the brains out of some roaches and left them with hollow heads. In other cases, they swapped the brain with a rubbery lump (some lumps were hard and others soft). In still other cases, they injected a toxin into the brains of the cockroaches that silenced their neurons. And in still other cases, they insert scissors into the roaches’ heads and snipped up the brains into a homogenous mush.

The scientists found that some–but not all–of these altered roaches posed a challenge to the wasps. If a wasp stung a roach without a brain, she spent over ten minutes probing its head. A soft rubbery lump also stretched out the time the wasps stung their hosts. And after that long struggle, the wasps withdrew their stinger in defeat, without delivering their zombie venom.

But when the wasps encountered a hard rubbery lump–a lump with the same texture as a brain–they spent just a minute poking the cockroach, after which the scientists found venom in the victim’s head.

Nor did the scientists find any change when the wasps were presented with roaches whose brains had been silenced–suggesting that the wasps don’t sense electrical activity to guide their stinger. On the other hand, a shredded brain left the wasps groping. That result suggests that the wasps need to do more than just feel the roach brain–they need to feel different parts of the brain in order to get where they need to go.

Taken together, these results offer a picture of an exquisitely evolved sensory organ–one adapted not for some all-purpose perception, but solely to navigate the interior of a cockroach’s brain by sense of touch. The full magnificence of this sensory organ may yet to be revealed, however. In their new study, Libersat and his colleagues didn’t determine what the chemical-sensing dome-shaped bumps are for.

Do the wasps taste their way through a cockroach brain? It’s possible–but perhaps the dome bumps provide it with other kinds of information. The scientists speculate that the dome bumps may taste the wasp’s own venom as it’s released into the roach’s brain, so that the parasite can carefully control how much she delivers to her victim (this is neuropharmacology, after all). Or perhaps the wasp can taste the flavor of larva of other species of parasites–in which case she may abandon the already-infected cockroach for a fresh host.

If someone answers that question, I guarantee to let you know. In the meantime, here is a video of a talk I gave for TED-Ed in 2012 about the jewel wasp. And if that’s just not enough for you, check out my book Parasite Rex.