Few of us has much sympathy for the cockroach, but pity the luckless roach that’s caught by a jewel wasp.
Dr Simon Pollard tells Megan Whelan about the parasitoid jewel wasp (ampulex compressa) that immobilises and performs brain surgery on cockroaches that renders them zombies and living hosts for the wasp’s young.
The jewel wasp is about one fifth the size of a cockroach, yet this tiny wasp is more than a match for the quicksilver roach.
Simon explains that understandably roaches are reluctant to be a living nursery for alien young, so the wasp has to use guile.
“Cockroaches have incredibly fast escape reflexes, they can run 80 centimetres in a second.
“So the wasp needs to firstly immobilise the cockroach. It finds it by sight and smell, it grabs hold of the cockroach and then stings it in the thorax and temporarily and paralyses the first pair of legs.”
Now the cockroach can’t run away and the wasp can execute the next part of its fiendish plan.
It uses its stinger, about the width of a human hair, to perform brain surgery on the roach – very precise brain surgery.
“It has sensors on it [the stinger] that allow the wasp to tell the difference between brain tissue and other types of tissue inside the cockroach’s head.
“The wasp inserts its stinger in the back of the cockroach’s head and moves the stinger around to find two parts of the cockroach’s brain.”
Once found, the wasp then injects venom into those two sites.
"What effect that venom has is extraordinary - the cockroach loses the will to walk. It’s totally capable of walking, but can’t be bothered,” says Simon.
The venom also lowers the cockroach’s metabolism so it uses less oxygen and it lives longer. Another effect of the venom is the cockroach starts grooming obsessively for about 30 minutes. Its increased longevity and cleanliness will come in handy later in its final role as a living wasp nursery.
While the roach is busy grooming, the wasp goes in search of a nest site.
“When the wasp comes back it bites off one of the roach’s antennae to check venom levels. It then leads it like a dog on a leash into the nest and lays an egg on it and seals up the nest,” says Simon.
The female wasp then leaves her egg and the roach entombed.
Life is about to get even less comfortable for the roach because baby wasp has hatched, and baby wasp is hungry – as Simon explains.
“The egg hatches and then drinks blood from the cockroach for a few days, and then burrows into the cockroach’s body and when it’s inside it eats all the non-essential organs; it wants to keep this roach alive as long as possible.
“If the roach dies too soon the larvae will die too soon, it won’t pupate and become a wasp.”
After a month the wasp emerges from the nest having now devoured the host roach. If it’s a female it will immediately go in search of another victim, just like its mum did, and the whole process starts again.
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