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Killer Wasps And Zombie Cockroaches

The emerald jewel wasp is a deadly and venomous insect, like all wasps are. Their sting can be excruciatingly painful for humans. Nevertheless, this brilliant metallic blue-green tropical wasp found in South Asia, Africa and the Pacific Islands is of little concern to humans unless disturbed. But for cockroaches, this wasp is the stuff of nightmares.

The emerald jewel wasp is a parasitic wasp that enslaves cockroaches by injecting mind-controlling venom into their brains. After being stung by the wasp, the cockroach loses control of its behavior and becomes a slave to the wasp. The wasp then lays eggs on the cockroach, and as the larva grows it consumes the cockroach alive from within eventually killing it. The young offspring then emerges out of the corpse like an alien bursting out of the victim’s abdomen.

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An emerald jewel wasp and a cockroach battles for dominance. Photo credit: Glass and Nature/Shutterstock.com

The female wasp, which is often just a fraction of the size of her victim, begins her attack from above, swooping down and grabbing the roach by its neck. She then stings it—twice. The first sting goes at the middle of the body, the thorax, in between the first pair of legs temporarily paralyzing the legs with a dose of venom. The loss of mobility allows the wasp to deliver the second sting with remarkable accuracy. The second sting delivers a neurotoxin at a precise spot in the victim's brain, the section that controls the escape reflex. After the second sting, the cockroach is unable to run away even after it had regained control of its legs. At first it becomes sluggish as if drunk, and then it begins a curious grooming routine.

For half an hour, the cockroach meticulously cleans itself rubbing its forelegs and wiping its antenna, while the wasp busies itself getting a burrow ready. There is dopamine in the venom, a chemical that controls the brain's reward and pleasure centers. Scientists believe that it’s the dopamine that triggers this grooming response in the cockroach, although whether the behavior is a necessary part of the process or an unexpected side effect, they are not sure. Some believe that grooming ensures a clean, microbe-free host for the vulnerable baby wasp. Others think it’s simply a way to keep the cockroach busy and distracted while the wasp prepares the cockroach's tomb.

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The emerald jewel wasp grabs a cockroach by the plate-like pronotum on its neck as it prepares for the first sting. Photo credit: Ken Catania

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After having paralyzed the front legs with the first sting, the wasp delivers the second sting into the brain of the cockroach. Photo credit: Ram Gal/Ben-Gurion University

By the time the cockroach is done cleaning, the venom completely takes over its free will. It becomes unable to flee or do anything else for that matter. The cockroach doesn’t lose its motor abilities, but the insect simply doesn't seem inclined to use them, explains the Scientific American. “So the venom doesn't numb the animal's senses—it alters how its brain responds to them.”

The cockroach, now completely docile, allows itself to be led to the wasp’s nest. The wasp does this by grabbing one of the cockroaches' antenna and walks it around like a dog on a leash (see video below).

Now the wasp breaks the antennas at a precise location and uses the antenna stumps as straws to drink hemolymph, a blood-like fluid in insects, to replenish fluids and energy lost during the fight. Some researchers believe that this action is done to regulate the amount of venom in the cockroach as too much could kill the victim before the larvae could grow. The wasp then lays a single egg on to the cockroach’s leg and entombs it by sealing the burrow entrance with miscellaneous debris to keep predators out.

About two days the egg hatches, and the larva grows latched onto the cockroach. At first the larva chews on the cockroach’s leg drinking the nutritious hemolymph that oozes out. Later, it bores a hole into the cockroach’s abdomen and climbs inside to feast on the internal organs. Once the insides are gone, the cockroach dies, at which point the larva enters the pupal stage and forms a cocoon still inside the now-empty insect. After several weeks, the pupa matures and an adult wasp emerges from the desiccated shell of the cockroach to complete the gruesome life cycle.

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A fully grown adult emerging out of the shell of a dead cockroach. Photo credit: Ram Gal/Ben-Gurion University

But cockroaches too have learned to defend themselves from the attack of the killer wasp.

Ken Catania, a neurobiologist at Vanderbilt University, was recording high speed videos of wasp and cockroaches battling between themselves when he saw one cockroach deliver an effective karate kick on the wasp’s head throwing it several inches away.

“Many cockroaches deter wasps with a vigorous defense,” wrote Catania on a recent paper published in the journal Brain, Behavior and Evolution. “Successful cockroaches elevated their bodies, bringing their neck out of reach, and kicked at the wasp with their spiny hind legs, often striking the wasp’s head multiple times.”

Other researchers had noted these behaviors anecdotally, but no one had made a detailed study of it until now. Catania found that the karate kick is very effective—it worked in 63 percent of adult cockroaches who tried.

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