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Jewel wasp emerging from cockroach host. Photo by Ram Gal
ScienceThe Loom

If You’re Going To Live Inside A Zombie, Keep It Clean

If you haven’t already met Ampulex compressa, otherwise known as the jewel wasp, now is as good a time as any. Someday you may be very grateful that you did.

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Ampulex compressa. Photo by K. Seltmann, via Creative Commons. Link:

This gorgeous animal, which measures just under an inch from mandibles to tail, lives across much of Africa and Asia, as well as a few Pacific Islands. Don’t be fooled by its lovely glittering appearance, though. This is a deeply sinister creature. Jewel wasps don’t rear their young in a familiar paper nest. For them, home is the inside of a cockroach.

When the female wasps are ready to lay their eggs, they take to the air and search for roaches. They find them on trees, on the ground, and even in people’s apartments. Since cockroaches don’t want to play host to their young, the wasps have to sneak up on their victims and subdue them–without killing them. So a wasp will sneak up and clamps her mandibles on the roach. As the roach tries to shake her off, the wasp hooks her tail underneath and stings her victim just below the head, temporarily paralyzing the roach’s front legs. Now the roach is easier to handle. The wasp then conducts brain surgery.

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Ampulex compressa injects drugs in the brain of a roach. Photo by Ram Gal

As I’ve written about in greater detail here and here, the jewel wasp snakes its stinger up into the cockroach’s brain, using sensors at its tip to feel its way to specific regions where it then releases cocktails of neurotransmitters. The wasp removes her stinger and walks away to find a crevice that will serve as a suitable burrow. Her first sting wears off, and the roach is now free to run away. Except it doesn’t. It becomes the insect equivalent of a zombie, having lost all will.

The wasp returns–sometimes as long as half an hour later–and bites off one of the roach’s antenna. She slurps some roach juice from the wound, like a kid drinking a milkshake through a straw. Then she bites down on the antenna stump and guides the roach to her burrow, leading it as if it were a dog on a leash. The roach goes without a fight. The wasp leads the roach into the burrow and then lays an egg, shaped like a grain of rice, on its underside. Then she leaves the burrow, sealing it up to leave the roach in darkness.

The egg hatches, and out comes a larva. It chews a hole into the roach’s underside, from which it feeds. It grows larger and larger over the course of a week. And then it crawls inside.

Remember, the roach is still healthy. It could push away the larva, bolt for the exit, push aside the pebbles blocking the burrow, and scramble away. But it can’t, or it won’t–I’ll leave it to philosophers to find the right language to describe the free will of cockroaches. It just stands where it was led, while the wasp larva feeds on its insides and grows to fill the roach’s entire body cavity. Eventually the roach dies, and inside the cadaver, the wasp spins a coccoon around itself, inside of which it grows eyes and legs and wings. And then it pops out, ready to meet the world. The wasp shakes itself off, climbs out of the burrow, dries out its wings, and flies away. It leaves behind the dead husk that was the roach.

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There’s a world of adaptations that the jewel wasp deploys in order to turn this science fiction story into natural history. Many of those adaptations have to do with brains: how the wasp brain manages to carry out its complex stinging maneuver, and what exactly it does to its victim’s brain to make it a zombie. But scientists are also discovering adaptations of an entirely different sort. In order for the jewel wasp to make roaches its home, it has to be more than a brain surgeon. It also has to be a pharmacologist.

Gudrun Herzner, a biologist at the University of Regensburg in Germany, was curious about how the larvae manage to stay alive inside a cockroach. To a jewel wasp, the inside of a roach may be an all-night diner but it’s also a hot zone. Cockroaches carry plenty of microbes–including antibiotic-resistant pathogens that are the bane of hospitals and nursing homes. The pathogens aren’t just dangerous to humans, though. When Herzner looked inside roaches, she found them to be rife with a microbe called Serratia that’s both a medical menace and a threat to insect larvae. If the inside of a cockroach is an all-night diner, it’s a diner where the short-order cook douses all the orders with deadly bacteria before the waitress delivers the dishes.

Obviously the wasps are not dying inside their roach hosts, even after their hosts die and presumably start to rot. So they likely have some kind of defense against infection. To see what that defense might be, Herzner created a hole on the side of infected roaches, which she covered with a little window. She then spied on the wasp larvae inside.

Herzner saw something no one had seen before. The larvae produced clear droplets in their mouths, which they carefully applied to the inner walls of the cockroach’s body cavity. The wasps then crawled around the cavity, smearing the droplets smoothly across all the inner surfaces of their host.

Fortunately for Herzner, the wasps also applied some of the drops to the window. She was able to extract chemicals from the drops, which she then analyzed. Once she had identified those compounds, she gathered more of them by grinding up infected roaches.

She found a number of molecules in the droplets, two of which were especially interesting. They’re called mellein and micromolide. And Herzner found that they stopped the growth of Serratia she isolated from the cockroaches, along with other pathogens. When the wasp enters its cockroach diner, in other words, it sponges down the walls, the tables, the dishes, and the food with an antibiotic soap.

In my book Parasite Rex, I wrote about how parasites long had a reputation as degenerates. They supposedly evolved from free-living species by losing the adaptations necessary for a rugged life of self-reliance, trading their autonomy for a life as a moocher. This is merely a case of our imposing our own social anxieties on nature. In fact, parasites have to gain many adaptations in order to enter the alternate universe that exists inside living hosts. It’s true that for a wasp larvae, it’s great to grow up inside a roach where your next meal is one slurp away. But it also means that you’re cheek-by-jowl with bacteria that could kill you.

The antibiotics that the jewel wasp has evolved are not entirely new. Scientists have found other forms of mellein in fungi, for example. Micromolide was first discovered in a citrus plant called Micromelum hirsutum. The wasps, in other words, have unwittingly hit on the same solution for fighting off bacteria that radically different organisms did on their own.

The jewel wasp is worth getting to know just because it exists. But now it’s possible that we might someday benefit from that knowledge in the most practical way imaginable. What is good for jewel wasps might prove good for us.

The mellein found in fungi is effective against MRSA, the deadly strain of skin bacteria that is resistant to most antibiotics. When scientists tested out the micromolide from plants on the bacteria that causes tuberculosis, it proved to be among the most powerful anti-TB drugs ever found. Now the jewel wasp turns out to be a factory for similar antibiotics, which might turn out to be even better than what’s been found in fungi or plants. To Herzner, that possibility cries out for exploration, because right now the antibiotics we use to cure our own infections are failing at a distressing rate. “I personally believe that we have no other choice but to look for alternatives to the commonly used antibiotics,” says Herzner.

Of course, just finding a natural antibiotic does not mean that tomorrow we will be able to buy it in pill form, as Ed Yong recently reminded us. But the first step on the long road to replenishing the antibiotic cabinet is to start looking in unexpected places. Places like zombie cockroaches.

Reference: Herzner et al, 2013. Larvae of the parasitoid wasp Ampulex compressa sanitize their host, the American cockroach, with a blend of antimicrobials. PNAS, to be published the week of 1/7/13. When the paper is published, you’ll find it here.

[Update 1/7 7 pm: Herzner’s quote fixed]