"Zombie" Roaches Lose Free Will Due to Wasp Venom
Mati Milstein in Be'ér Sheva, Israel
for National Geographic News
|December 6, 2007|
The parasitic jewel wasp uses a venom injected directly into a cockroach's brain to inhibit its victim's free will, scientists have discovered.
The venom blocks a chemical substance called octopamine in the cockroach's brain that controls its motivation to walk, the study found.
Unable to fight back, the "zombie" cockroach can be pulled into the wasp's underground lair, where an egg is laid in its abdomen. The larva later hatches and eats the still living but incapacitated cockroach from the inside out.
"The whole thing takes about seven to eight days, during which the meat has to be fresh," said study co-author and neurobiologist Frederic Libersat of Ben-Gurion University of the Negev in Be'ér Sheva, Israel.
"If you kill a cockroach, it rots within a day."
The mature wasp emerges from the bug victim's body after about a month.
The study recently appeared in the Journal of Experimental Biology.
The team of researchers at Ben-Gurion University believe that the octopamine discovery is an important piece of the puzzle of how the tropical wasp's venom turns its victims into the living dead.
Octopamine is a brain substance that places insects in an alert state, inspires them to move, and allows them to perform demanding physical tasks.
"It serves the same functions as noradrenaline, which is involved in the fight-or-flight reaction ... in the vertebrate brain," Libersat said.
The team determined that the wasp injects its venom into a specific area of the cockroach's brain, the protocerebrum.
This region, which contains octopamine-secreting nerve cells, controls the ability to start walking. The venom interferes with the release of octopamine, they found.
The researchers then reversed the process: they injected an octopamine-like substance directly into the protocerebrum of cockroaches that had already been turned into zombies by wasp stings.
The result was significant recovery and restoration of the cockroach's free will.
(Related news: "Robo-Roaches Can Control Insect Groups" [November 15, 2007].)
"This helps us understand how movement is initiated in animals," Libersat said. "We know how movement itself is generated, but to understand what makes an animal decide to move or not to move is a different issue."
The jewel wasp is the only parasite known to inject its venom directly into its host's brain.
But other parasites also control the behavior of their hosts, said David Richman, curator of the Arthropod Museum at New Mexico State University in Las Cruces, who was not involved in the new study.
"This is not uncommon. There are a tremendous number of parasites, and they all have different strategies for survival and for propagation of their species," Richman said.
The behaviors of land snails, grasshoppers, and types of ants, for example, can all be affected by parasites.
(Read about suicide grasshoppers brainwashed by parasitic worms.)
"Not only that," Richman added, "[some parasites] can take over certain aspects of the host's biology, particularly as you get into microorganisms."
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