Parasitic wasp’s novel weapon

A parasitic wasp employs a “biological weapon” — a virus — to paralyse a beetle, turning the creature into a bodyguard for its larva, according to research just published.

Parasites are known to manipulate the behaviour of their hosts in a fashion that benefits themselves. Rats infected with the single-celled parasite, Toxoplasma gondii, for instance, stop fleeing from the smell of cats and thereby readily fall prey to them. That allows the parasite, which can sexually reproduce only in alimentary tract of cats, to complete its lifecycle.

In the case of the parasitic wasp, Dinocampus coccinellae, a female wasp will attack a ladybeetle, stinging it and then laying an egg inside its abdomen. The developing larva feeds off its unfortunate host’s tissues and, after nearly three weeks, proceeds to squeeze its way out of the latter’s body.

The beetle’s travails do not end there. The wasp larva then spins a cocoon between the beetle’s legs. While the larva undergoes further development inside the cocoon, the beetle becomes partially paralysed, unable to move about properly and with its legs twitching occasionally. By remaining perched on top of the cocoon, it acts as a bodyguard, its body protecting the larva from predators.

After about a week, an adult wasp emerges from the cocoon and flies away. Some beetles survive this traumatic experience and gradually recover.

Was it some sort of venom secreted by the larva that paralysed the beetle?

In the course of investigations to understand the phenomenon, Nolwenn M. Dheilly, then a postdoctoral researcher at Ecologie et Evolution des Interactions in Perpignan, France, found instead the partial genome sequences of a virus in the head of such beetles. He then put together the entire viral genome.

In a paper just published in the Proceedings of the Royal Society B, Dr. Dheilly, along with colleagues in France and Canada, named this virus ‘D. coccinellae paralysis virus’ (DcPV).

When the wasp injects its egg into the beetle, it also introduces the virus into the latter’s body. The beetle’s initial anti-viral immune response is suppressed, which, according to the paper, might be induced by the developing larva or perhaps by the virus itself.

By the time the larva is ready to leave the beetle’s body, virus levels in the creature’s head and abdomen have shot up. The virus was found in profusion in cells in the beetle’s cerebral ganglia, its brain.

“We believe that changes in ladybeetle behaviour result from DcPV replication in the nervous tissue,” said Dr. Dheilly, who is moving to Stony Brook University in New York, U.S., in an email. However, it was not yet known whether the virus was directly responsible for the paralysis and tremors that the beetles exhibited or whether that resulted from the damage produced by an antiviral immune response.

In beetles that recover, their immune system returns to normal and gets rid of the virus, allowing the nervous system to repair itself.