In November, National Geographic put a ladybug and a wasp on its cover. They made for a sinister pair. The wasp, a species called Dinocampus coccinellae, lays an egg inside the ladybug Coleomegilla maculata. After the egg hatches, the wasp larva develops inside the ladybug, feeding on its internal juices. When the wasp ready to develop into an adult, it crawls out of its still-living host and weaves a cocoon around itself.
As I wrote in the article that accompanied that photograph, the ladybug then does something remarkable: it becomes a bodyguard. It hunches over the wasp and defends it against predators and other species of parasitic wasps that would try to lay their eggs inside the cocoon. Only after the adult wasp emerges from its cocoon does the bodyguard ladybug move again. It either recovers, or dies from the damage of growing another creature inside of it.
How parasites turn their hosts into zombie slaves is a tough question for scientists to answer. In some cases, researchers have found evidence suggesting that the parasites release brain-controlling chemicals. But the wasp uses another strategy: there’s a parasite within this parasite.
In the Proceedings of the Royal Society, a team of French and Canadian researchers now lay out the evidence for this strange state of affairs. As they studied this manipulation, they reasoned that the best place to look for clues was inside the heads of parasitized ladybugs. They discovered that the brains of these hosts were loaded with viruses. When the scientists sequenced the genes of the virus, they found it was a new species, which they dubbed D. coccinellae Paralysis Virus, or DcPV for short.
The scientists found DcPV in the wasps as well–but not in their brains. In female adult wasps, the virus grows in the tissues around their eggs. Once a wasp egg hatches inside a ladybug, the virus starts replicating inside it, too. The larva then passes on the virus to its host, and the ladybug develops an infection as well.
DcPV causes no apparent harm to the wasps, but the ladybug is not so lucky. The virus makes its way into the ladybug’s head, where it attacks brain cells and produces new viruses in pockets inside the cells. Many brain cells die off during the infection.
The researchers hypothesize that the virus is responsible for the change in the ladybug’s behavior. To get the ladybug to guard the wasp, the virus may partially paralyze its host, so that it becomes frozen over the parasite. Because the paralysis isn’t complete, the ladybug can still lash out against predators. But these may just be wild spasms in response to any stimulus. The bodyguard effect may grow even stronger as the infection robs the ladybug of the signals from its eyes and antennae. Closed off the world, its sole purpose becomes protecting its parasite.
The fate of a parasitized ladybug–to die or to walk away–may depend on how it handles a DcPV infection. In some cases, the virus may be fatal–possibly by triggering a massive immune response that kills not just the virus but the ladybug itself. In other cases, the ladybug’s immune system may eventually be able to clear the virus out of its system, letting its nervous system heal.
In either case, the bodyguard paralysis lasts long enough to protect the wasp while it develops into an adult. Whether the ladybug lives or dies doesn’t matter to the wasp–or to the virus. The new wasp carries a fresh supply of DcPV. If it’s a female, it will be able to use the virus to infect both its own young, and its ladybug slave.
In recent years, scientists have developed a deepening appreciation for the importance of our microbiome–of the bacteria and viruses that make our bodies their home. While some microbes invade our bodies, others reside inside of us and help keep us healthy. Parasitic animals have microbiomes of their own, and this new study suggests that they can use them for suitably sinister ends.
(For more information on the sophisticated tricks of parasites, see my book Parasite Rex.)
Originally published February 10, 2015. Copyright 2015 Carl Zimmer.