Influenza, smallpox, dengue and, of course, Ebola, are all prime examples of viruses that can devastate human populations. But, with the advent of modern molecular technologies and the discovery of new microbes, it is now becoming clear that not all viruses are bad.

According to a new study¹, published this week in the journal PLoS Pathogens, researchers from the Chinese Academy of Agricultural Sciences (CAAS) and Lancaster University, UK, have discovered a virus which is beneficial to its host - the global insect crop pest the Old World cotton bollworm moth (Helicoverpa armigera). Caterpillars with this virus were both healthier and more resistant to biopesticides.

The cotton bollworm is one of the most damaging migratory pests throughout Africa, Asia, Europe and Australia, and it has recently arrived in South America.

To combat this voracious caterpillar, a number of biological control approaches have been developed, including a baculovirus known as HaNPV, and a number of genetically-modified crops that produce lethal toxins.  Since the 1990s, there has been widespread planting of Bt cotton to combat bollworm caterpillars and other pests in China, and this has proved to be successful in reducing pest outbreaks on this important cash crop.

But the new study, led by Chinese scientists Professor Kongming Wu and Dr Pengjun Xu from the Institute of Plant Protection at CAAS in Beijing, and co-authored by Professor Ken Wilson and Dr Rob Graham from the Environment Centre at Lancaster University, shows that these control approaches are now being undermined by another microbe that has only just been discovered using modern molecular methods - a new densovirus called HaDNV-1.

Professor Ken Wilson said: "Surprisingly, rather than being deadly, the new densovirus appeared to make the bollworms healthier. Caterpillars carrying HaDNV-1 were heavier, fatter and developed at a faster rate, and adult females produced more offspring and lived longer lives."

With such positive effects on the bollworms, the team predicted that the virus would be widespread throughout the population. When they collected lots of caterpillars from field populations in China they discovered that around three-quarters of all bollworms carried the densovirus.

Professor Wilson said: "The fact that not all bollworms were infected suggested that there must also be a down-side to carrying HaDNV-1. We knew from our previous work on armyworms in Africa that one possibility was that those caterpillars carrying the beneficial microbe might be more susceptible to another, more lethal, microbe so we conducted laboratory bioassays to see how they responded to two commonly used microbial biopesticides: Bt and HaNPV.

Once again, the scientists were surprised by what they found. Rather than being more susceptible to the biological pesticides, they found that densovirus-carrying caterpillars were more resistant to Bt at low doses (but not high), and were more resistant to the HaNPV baculovirus across a range of doses.

This latter finding is consistent with the observation that in the wild, fewer than expected bollworms carry both the densovirus and the baculovirus – most likely, those caterpillars that already carry the densovirus are more resistant to subsequent infection by the lethal baculovirus.

So far, the China-UK team cannot explain why there appears to be no cost to carrying the densovirus. One possibility is that we are witnessing natural selection in action and that eventually all bollworms will carry the beneficial densovirus – but research has shown there has been a steady decline in densovirus prevalence in China in recent years rather than the predicted increase.

“Another possibility”, explained Professor Wu, who led the study, “is that under field conditions densovirus-carrying moths or caterpillars are less competitive, less mobile, or are more susceptible to natural enemies or insecticides than those that are densovirus-free. At the moment we simply do not know and we need to do more research.”

¹ Xu P, Liu Y, Graham RI, Wilson K & Wu K (2014) Densovirus is a mutualistic symbiont of a global crop pest (Helicoverpa armigera) and protects against a baculovirus and Bt biopesticide. PLoS Pathogens.