A parasitic wasp that injects its eggs into the leek moth - a serious agricultural pest - also adds a lethal virus that stops the moth’s immune system from harming the eggs, according to a new study.

Without this stealthy alliance the wasp’s eggs never hatch and the viruses find it much harder to infect the moth, says a report of the study in the magazine Microbiology Today, published by the British Society for General Microbiology.

"The overall picture is of a wasp and a series of viruses that combine to exploit a moth for their mutual benefit," says the report by a team of French researchers led by Dr Sylvaine Renault, of the François-Rabelais University in Tours.

The caterpillars of the leek moth – Acrolepiopsis assectella - bore into and damage the leaves and bulbs of leeks, garlic and other crop plants in the onion family. The moth is a serious pest in parts of Europe, It is also found in Africa and Asia and has recently spread to Canada.

The discovery by the French researchers not only reveals how the wasp – a solitary species, Diadromus pulchellus - boosts its parasitic success but may also open up new avenues for biological control of the moth and other insect pests.

Until recently, all wasps of this species tested in France were infected with one of two types of virus that can interfere with the moth’s immune responses. Scientists are uncovering increasing numbers of similar viruses that infect other agriculturally important pest insects, notably cotton bollworm, corn earworm and various cutworms and budworms.

Renault’s team found one wasp in a leek field near the university that was free of both the known leek-moth viruses but was still extremely effective at parasitising the moth: they ran tests and discovered that the female wasps carried in their genitalia a previously unknown species of cypovirus, which they named DpRV-2.

Many researchers internationally are investigating the viruses that infect commercially and medically significant insects, in the hope of finding safer and more specific substitutes for chemical insecticides, and cypoviruses in particular are thought to have potential as useful biological control agents.

The French team established that the wasps injected DpRV-2 into pupating moths with each of their eggs. The virus then developed in the moth’s gut cells, while the wasp larvae hatched and grew in the abdominal cavity. Within four days, the wasps had filled the cavity and killed the host, which also stopped the viral infection.

To discover whether the virus could interfere with the moth’s immune response, they carried out a series of experiments that involved inserting thin nylon monofilaments into moth pupae.

Pupae that had not been parasitized reacted to the filaments by encapsulating and isolating them with special tissue – the same response they had to wasp eggs when the virus was not present. But far fewer pupae responded this way when they were also infected by the cypovirus.

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