The green-eyed wasp Dinocampus coccinellae turns ladybugs into zombie babysitters. Three weeks after a wasp lays its egg inside the hapless beetle, a wasp larva bursts from her belly and weaves itself a cocoon between her legs. The ladybug doesn’t die, but becomes paralyzed, involuntarily twitching her spotted red carapace to ward off predators until the adult wasp emerges a week later. How D. coccinellae enslaves its host at just the right time had been a mystery, but now researchers believe the insect has an accomplice: a newly identified virus that attacks the beetle’s brain. The findings raise questions about whether other parasites also use viruses as neurological weapons.
“This is one of the more interesting papers I've read in a long time,” says insect virologist Kelli Hoover of Pennsylvania State University, University Park. “It was very elegant.”
Many parasites use mind control to manipulate their hosts. The Ophiocordyceps fungus turns tropical ants into zombie spore dispensers, for example, and the single-celled protozoan Toxoplasma gondii eliminates a rodent’s fear of cats so it can get back into the feline digestive system. In contrast, parasitoid wasps like D. coccinellae don’t always kill their hosts. The wasps need them for child support. A ladybug goes about her business for weeks as the D. coccinellae larva grows within her, feasting on her internal organs. Even after the larva emerges and turns her into a zombie bodyguard, a quarter of ladybugs eventually recover. Some even get parasitized again.
One of the more puzzling aspects of this bodyguard behavior is its timing, says parasitologist Nolwenn Dheilly, who led the new study as a postdoc at the University of Perpignan in France. The ladybug doesn’t turn into a zombie until weeks after the wasp has laid its egg, so what causes the paralysis? “Honestly, we had no idea how it worked,” she says.
Dheilly and colleagues wondered if wasp larvae or parasitized ladybugs make some toxic protein that builds up over time to eventually trigger paralysis. They began to search for suspicious gene activity that could produce such a molecule by sequencing RNA gene transcripts in beetles and their larval passengers. To their surprise, the researchers discovered that the brains of the beetles were full of unfamiliar viral RNA not present in healthy beetles. “I ran around the lab saying, ‘I've got a virus! There's a virus in the head of the lady beetle!’ ” Dheilly recalls. On further inspection, the researchers classified the stowaway as a new species of Iflavirus, a type of RNA virus related to the polio virus. They dubbed it D. coccinellae paralysis virus (DCPV).
DCPV seems to be exquisitely tuned to help D. coccinellae control its victims, the team reports online today in the Proceedings of the Royal Society B. The adult wasp carries the virus and injects it into the ladybug along with its egg. The virus replicates rapidly but for unknown reasons doesn’t spread to the beetle’s brain until just before the larva emerges from her belly. At first the virus seems to build up harmlessly in brain cells, but as soon as the larva breaks out, these cells burst open, unleashing destruction all around them.
The researchers suggest that the beetle’s own immune system may be responsible for this damage. They discovered that crucial immune genes are suppressed while the larva lives within the ladybug, but then reactivate after the larva emerges. The researchers speculate that the beetle’s revived immune system discovers and attacks DCPV-infected cells. The self-inflicted brain damage could temporarily paralyze the beetle just when the newly vulnerable wasp larva needs protection.
The new findings are exciting, but the team has not yet proven that DCPV is responsible for turning ladybugs into bodyguards, cautions Shelley Adamo, an insect neurobiologist at Dalhousie University, Halifax, in Canada. “It's many different lines of circumstantial evidence, which makes it compelling, but it does miss that final causal link,” she says.
Dheilly, who is now at Stony Brook University in New York, acknowledges that there’s more work to be done. But if the research team’s hypothesis holds, she says, it illustrates the importance of studying parasites, hosts, and all of their microbes together. “Whatever species you look at, there might be another, smaller organism that is within it and participating in its evolution and ecology.”