Cockroaches are famously indestructible. Tough, fast, and agile, they are renowned for their hardiness in the face of poisons and heavy boots. But unlike us hapless humans, the emerald cockroach wasp can kill roaches—and in the worst way imaginable.
The wasp is a beautiful creature decked out in Hollywood colors, with a metallic teal body and bright-orange thighs. It is also a parasite, and cockroaches are its victims. When a female wasp finds a roach, she uses her jaws to grab it by the pronotum, the plate that covers what is roughly its “neck.” Then, she stings it—twice.
The first sting goes into the nerves that control the roach’s front legs, and temporarily paralyzes them with a dose of venom. The second sting delivers venom directly into the brain, and pacifies the roach. At this point, it could run away if it chose to, but it never chooses to: The venom effectively removes its free will. The wasp can then lead the docile roach back to her underground lair by its antennae, as if walking a dog on a leash. Once there, she lays an egg on it, which eventually hatches into a grub that chews its way into the still-living, still-docile roach and devours it from the inside out (but not before disinfecting it with antibacterial saliva; even to a body-snatching parasite, roaches are a little gross).
The wasp’s strategy makes sense, because she’s not strong enough to carry a fully grown cockroach to her lair. She needs it mobile but pliable, so she has evolved a disturbingly precise venom that doesn’t kill or immobilize, but instead turns its victim into a mindless zombie. This ability, which was revealed through work by Frederic Libersat and others, has been written about extensively. But such writings, including this article, usually begin when the wasp has the upper hand. And as Ken Catania realized, that’s not always the case.
A neurobiologist at Vanderbilt University, Catania has a track record of investigating unusual animals, many of which he studies single-handedly. He has worked on star-nosed moles, tentacled snakes, naked mole rats, electric eels, crocodiles, and, er, humans. “I like getting firsthand experience about the animals I find are most interesting,” he says. “I teach about the emerald cockroach wasp, so I went ahead and got some. I thought I would just start filming them to show students and flesh out my lectures, especially the Halloween one.”
His videos, filmed at high speed, showed that the wasp’s attacks are all about the pronotum. If it gets its jaws on this plate, and delivers the first sting, “it’s game over,” says Catania. That happens roughly half the time without incident, usually when the wasp takes the cockroach unawares. But often, the cockroach defends itself.
First, it turns its flank to the wasp, stretches its legs as if standing on stilts, and tilts its head away. This not only keeps the vulnerable pronotum out of reach, but also turns the roach’s sharp leg spines toward the wasp. Those spines can inflict heavy damage, and they’re also riddled with sensitive nerves. These, combined with the cockroach’s sweeping antennae, track the wasp’s movements and warn of potential lunges. In effect, the roach becomes a living barbed-wire fence, complete with roving searchlights.
This posture has another benefit: It allows the cockroach to kick. “Crickets and grasshoppers kick defensively, but they are pre-built for it; the cockroach is not,” Catania says. “And it has a strikingly different form.” A cricket kicks outward by rapidly extending its bent legs, but a cockroach will fully extend a leg, wind it back, and then swing the whole outstretched limb like a baseball bat. “The kicks almost always landed squarely on the wasp’s head,” says Catania. “After it gets about five of these blows, it just gives up.”
Even if all of that fails, and the wasp grabs the pronotum, the cockroach still has a chance. It can now use the spines on its legs to rake the parasite off, parry its stinger, or even stab it. If it’s successful, the results can be fatal for the wasp. When Catania first acquired his wasps, he made the mistake of leaving one alone with a big cockroach. A few hours later, he returned and the wasp was dead, most likely from stab wounds.
But at this point, the odds are against the roach. Once the wasp bites down, it is very hard to dislodge, as the movie below shows. The roach bucks around like a rodeo bull, but its odds of unseating the wasp are just one in seven. That’s small, but “better than nothing if you don’t want to be a zombie,” says Catania.
Other scientists have noted these behaviors anecdotally, but Catania is the first to study them thoroughly, says Gudrun Herzner from the University of Regensburg, who also studies the wasp. The cockroach’s defenses “have presumably evolved in response to a wide range of different predators,” she says. “The wasps, on the other hand, experience extremely strong [evolutionary] pressure to overcome these defenses and are obviously well adapted.”
Catania agrees. When human doctors deliver drugs into a patient’s brain, they use medical scanners to guide their way. The wasp has no such help; it must perform neurosurgery in the dark, guiding its stinger with the sensory cells at the tip. That takes time, during which the roach could conceivably dislodge it. And that, Catania thinks, is why the wasp has come to deliver two stings, using the first to paralyze the legs that are closest to it.
“I like to tell people that this is the only thing that will make you feel sorry for a cockroach,” he says. “It still might not, but if anything would, it’s being turned into a zombie and being slowly eaten alive.”