Nature at its most gruesome can rival a horror movie. A perfect example is the disturbing way the parasitoid emerald jewel wasp turns the American cockroach into a zombie, kidnaps it, and precisely lays its eggs on the roach. The larval wasps then develop on and in this living nursery, slowly consuming it as they grow.
From the outset, this seems an unlikely interaction. The cockroach is much larger than the wasp. “This has led to the evolution of some really remarkable tricks of the trade for the wasp”, Ken Catania told Quirks & Quarks host Bob McDonald. Catania, a biologist at Vanderbilt University in Nashville, Tennessee, just published a new study in which he showed how the wasp uses a series of precisely targeted stings to take control of the cockroach..
Precision stings make for a good zombie
The first sting targets the first thoracic ganglion, part of the central nervous system. This paralyzes the front legs of the cockroach. This is followed shortly after by a second well directed sting into the brain of the cockroach. The stings aren’t meant to kill, but to give the wasp control.
“It can’t simply paralyze the cockroach and drag it to a hole” said Catania. “So it has instead made a zombie of the cockroach so it can lead it to a hole, lay an egg on the roach, barricade it in, and then unpleasant things happen from there.”
The first part of this interaction was already known to biologists. Catania’s new insight is that the wasp’s manipulation of its victim doesn’t stop there. He found that the wasp stings the cockroach three more times as it lays its eggs.
The wasp needs to place her egg in a precise position on the body of the cockroach, called the coxal plate, or the larvae will die. This target on the body can sometimes be blocked by the roach’s long, jointed legs.
“The wasp has this last challenge,” said Catania. “The legs of the cockroach might be in the wrong position, so the solution is to go back to the nervous system, and sting directly into a new place. And this new place is in the area that controls the second legs of the cockroach, and that causes the leg to essentially extend and that allows the last stage of egg laying.”
A blood meal, a cocoon and an alien-style birth
With the coxal plate now exposed, the wasp lays her egg, and the larvae begins to develop — and get hungry.
“This is where it gets even more grisly,” said Catania. “After it [the larvae] takes a blood meal for a couple of days on the outside of the cockroach, it burrows inside the cockroach, eats the living cockroach from the inside, forms a cocoon, and then eventually bursts out of the cockroach kind of alien style.”
Keeps on stinging
It turns out the wasp is something of a zombie itself, Catania found, in that it is locked into a biologically programmed behaviour that it cannot escape.
He was curious to learn what would happen if the set of three venomous stings did not work, and the legs of the cockroach remained unextended, blocking the critical location for egg placement.
He glued the legs in place on the cockroach and then observed how the wasp reacted. What he saw startled him. Like a frustrated robot, the wasp continued to sting the critical location, sometimes over 100 times, in an attempt to straighten the leg.
Catania said he can’t help but be impressed by the great precision of the wasp’s behaviour.
“This wasp has evolved to essentially find the central nervous system, to find the brain and precisely deliver venom to affect the cockroach behaviour in just the right way without killing it.
“It’s not quite like the lunch scene on the Nostromo from [the movie] Alien, but it’s as close as you can imagine for the biological world. So it’s straight out of science fiction.”
Written and produced by Mark Crawley