BRAIN SHOT A wasp downs a cockroach with a double sting, one temporarily paralyzing it and the second delivering a venom to the brain that weakens the roach's drive to walk.Gal and Libersat

It’s a dog-walker’s dream, a dose that quashes the urge to dash around without guidance. Too bad it works just for cockroaches.

A new analysis in the June 24 Current Biology reveals how venom of the jewel wasp (Ampulex compressa) wields such power over roaches. Once stung, cockroaches become so biddable that the small wasp can grab an antenna and lead the roach to a wasp nesting site. A series of tests shows, though, that the venom isn’t just making the wasps generally dopy, decreasing their overall arousal, report Frédéric Libersat of the Mediterranean Institute of Neurobiology in Marseille, France, and Ram Gal of Ben-Gurion University of the Negev in Be’er Sheva, Israel. The venom targets the parts of the nervous system that prompt an animal to start walking and keep it moving.

The study “highlights the fact that insects have cognition,” Libersat says. He adds that studying the wasp venom might someday lead to new compounds for manipulating nerve cells.

When it’s time to lay eggs, female wasps bring down cockroaches with a one-two sting. The first strike hits the roach body, and the victim’s legs buckle as if temporarily paralyzed. The wasp then delivers a precision sting that injects venom into the roach brain.

The doomed roach briskly grooms itself for half an hour but never recovers the initiative to walk around on its own or to escape danger. The female wasp leads it to the site of her future nursery, lays eggs on the roach and then leaves it to become baby food. Eventually the eggs hatch and the youngsters burrow into the roach body. They feed on fresh roach and grow to maturity, ultimately bursting out as adults.

To clarify what the venom does, Libersat and Gal encouraged wasps to sting roaches and then presented the victims with a series of physical challenges. When released in a new enclosure, the stung roaches just stood there, twitching an antenna from time to time. In contrast, healthy roaches scurried around as if exploring the new space.

When researchers put stung roaches into an open chamber with an electrified floor, the insects stood in the same place as shock intensity doubled or even tripled before stepping away from the chamber.

The passive roaches retained some ability to move. When researchers flipped stung roaches onto their backs, the animals righted themselves within three seconds, and monitoring showed nerves firing normally. In a flying test, tethered roaches showed normal neural activity.

Other parasitic hunters may affect the central nervous system of their hosts, says David Biron of the French National Institute for Agricultural Research at Clermont-Ferrand. He and his colleagues suggest that proteins produced by parasitic hairworms might mimic cricket proteins that affect the central nervous system. The parasitic hairworms feed for a while inside living crickets. When the cricket plunges into water, possibly under hairworm influence, the worm wriggles free and the cricket dies.

Watch a wasp emerging from the roach where it spent its childhood:

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