Adult wasps are not to be messed with – but when they are mere larvae and pupae, they need to be protected from passing predators. One parasitic wasp has apparently evolved the perfect solution. Using biological weaponry, they enslave ladybirds to take care of their vulnerable young.
Nolwenn Dheilly at the University of Perpignan in France and colleagues discovered that the Dinocampus coccinellae wasp deploys a virus to hijack a ladybird’s nervous system and force it into babysitter duty. Even more remarkably, the virus gains as much from this arrangement as the wasp. The study, which appears to be the first known example of a pair of parasites teaming up to manipulate the behaviour of their host, has been published in Proceedings of the Royal Society B.
“The article looks very strong and convincing,” says Nancy Moran, a biologist who studies symbiotic relationships between insects and microbes at the University of Texas at Austin. “And it’s quite a weird story!” she adds.
It begins when an adult female D. coccinellae wasp spots a ladybird, swoops down and injects her egg into the ladybird’s soft underside. Once hatched, the wasp larva grows plump on the ladybird’s fat stores. After about 20 days it burrows out of the ladybird and weaves a cocoon between the ladybird’s legs.
At this point precisely the ladybird – which is still alive – loses its mind. It stops moving around and simply stands guard, zombie-like, over the cocoon until the adult wasp emerges a week later.
“My colleague Jacques Brodeur saw this in the field and thought, ‘what is that, a ladybeetle taking care of a parasitic wasp cocoon?’ ” says Dheilly. “At the time of the behavioural manipulation, the parasitic larvae is not even in direct contact with the ladybeetle anymore, so that was very weird.” So the team tried to figure out the wasp’s trick.
Genetic sampling of the zombie ladybird solved the mystery when a third protagonist, the D. coccinellae paralysis virus, was identified. Dheilly showed that the virus lies dormant inside the wasp and is injected into the ladybird alongside the wasp egg. From there, the virus finds its way into the ladybird’s nervous system, where it reproduces before reinfecting the growing wasp larva and hitching a ride with it as it tunnels out of the ladybird to spin its cocoon.
At this point, there’s so much virus in the ladybird’s nervous tissue it induces paralysis. “A very intriguing point is the very precise timing between the egression of the larva and the induction of bodyguard behaviour,” says Dheilly. “This timing is amazing and we do not understand how it works. Our leading hypothesis is the larva can somehow detect the beginning of bodyguard behaviour and gets out at that point.”
Each young female adult wasp that emerges from its cocoon carries the virus in its oviduct (fallopian tube). There it lays dormant until the female breeds and lays the next generation of eggs into ladybirds – and so the lifecycle of virus and wasp continues. “We found the virus in all tested individuals,” Dheilly says.
So the virus clearly benefits from the relationship. And the wasp seems to benefit as well – something the team plans to confirm by trying to create virus-free individuals to study how they fare in comparison to their infected brethren.
But how does the story end for the zombie ladybird? While clearly the loser of the threesome, the enslavement is not always fatal. After the adult wasp has left the cocoon, some ladybirds do manage to clear the virus from their body, recover from the paralysis and resume their daily business.