A study led by biologist Nolwenn Dheilly of New York's Stony Brook University uncovers a new process of when a wasp lays its eggs inside a ladybird. The ‘Dinocampus coccinellae’ wasp uses a virus to paralyse and modify the behaviour of a ladybird, transforming it into a kind of zombie.
At 20 days, a young ladybird emerges from its home and wraps itself in a cocoon that its ‘victim guard’ is still standing on it. They then deposit their eggs in them and the larvae eat the insect from the inside.
When the wasp's egg is deposited inside the beetle, a wasp larva sprouts from its belly and weaves a cocoon between its legs. In the face of this unfortunate fate, the ladybird does not die but becomes paralyzed, twisting its shell involuntarily to ward off predators until the adult wasp emerges a week later.
How the wasp D. coccinellae enslaves its host has been a mystery but based on this groundbreaking study, researchers believe this insect has an accomplice: a newly identified virus that attacks the ladybugs' brains, turning them into a kind of zombie.
Life cycles of the parasitoid D. coccinellae and its endosymbiotic virus (DcPV) together with responses to parasitism and infection of the host C. maculata (drawing by Franz Vanoosthuyse)
What is this zombie transformation?
Many parasites modify their host behaviour to improve their own transmission and survival, but the mechanisms involved remain poorly understood.
As detailed in the publication, the wasp, in addition to depositing its eggs, inoculates the ladybird with the DcPV virus, which attacks its nervous system and causes paralysis. Finally, once developed, the wasps eat the bug from the inside.
The great milestone of the scientific team was to characterize and detail the new RNA virus that actually caused paralysis and behaviour modification. The virus, called DcPV is stored in the oviduct of female parasites, it replicates in the parasite larvae and is transmitted to the host during larval development.
Then, replication of this virus in the host's nervous tissue induces severe neuropathy and an antiviral immune response that correlates with the paralytic symptoms that characterize behavioural manipulation.
Surprisingly, after elimination of the virus, normal behaviour is also recovered.
These results provide evidence that changes in the behaviour of some insects are probably the result of DcPV replication in the brain nodes rather than manipulation by the parasitoid. In addition, the research offered exciting perspectives for research on parasitic manipulation by suggesting for the first time that behavioural manipulation may be mediated by symbionts.
Further experiments are now needed to characterize the nature of symbiosis between the D. coccinellae wasp and the DcPV virus. It is not yet known whether symbiosis is mandatory for the host.
Reference: Who is the puppet master? Replication of a parasitic wasp-associated virus correlates with host behaviour manipulation. Nolwenn M. Dheilly. (2015) Proceedings of the Royal Society B. doi:10.1098/rspb.2014.2773