New discovery could help in the battle against the armyworm

An international team of researchers believes that the intra-cellular bacterium Wolbachia, could enable farmers to overcome the challenge posed by African crop pests. Whilst investigating new affordable control measures to tackle the African armyworm, researchers from Lancaster University, the University of Greenwich and Tanzania-based Eco Agri Consultancy Services Ltd, discovered that Wolbachia rendered the creatures vulnerable to the biopesticide SpexNPV.

The African armyworm – a caterpillar that feeds on staple cereal crops such as maize, wheat, millet and rice – can be found in densities of up to 50,000 larvae per hectare, and can cause total crop failure. As many farmers cannot afford the chemical pesticides currently on the market, the armyworm poses a significant threat to food security.

Almost three quarters of insect species carry Wolbachia, and some African armyworms are amongst their number. Around 12 per cent of armyworms carry the bacterium, but this varies between populations from as low as zero to as high as 56 per cent. As a recent study demonstrated that Wolbachia-carrying mosquitoes are protected from viruses such as that which causes dengue in humans, the Lancaster-led team decided to investigate whether this bacterium offered armyworms any protection from biopesticides. To their surprise, however, they discovered that the bacterium made the caterpillars more vulnerable to SpexNPV than their Wolbachia-free counterparts.

I spoke to project leader Ken Wilson, Professor of Evolutionary Ecology at Lancaster’s Environment Centre, to find out more about this surprising discovery. I began by asking Professor Wilson whether he understood how Wolbachia makes the African armyworm more susceptible to SpexNPV.

"No, I don’t," he conceded. "In many ways, we are still in the early stages of our investigations. This wasn’t even the primary focus of our initial research. Our main focus was on the baculovirus, and it was only when we found this that we decided to examine the effects of Wolbachia on susceptibility.

"It could be that Wolbachia just triggers, or drains, the armyworm of resources that it needs for fighting the virus. Maybe, the host is so busy fighting or interacting with the Wolbachia that it has got fewer resources available for interacting with the virus, rendering it more vulnerable. We really don’t know at the moment. In mosquitoes, of course, a different strain of Wolbachia has the opposite effect. It makes them more resistant to a different type of virus. The reason our story is perhaps so newsworthy is because Wolbachia has completely the opposite effect on armyworms than on mosquitoes."

I went on to ask Professor Wilson whether this discovery came as a surprise.

"It was a surprise because there are quite a few studies, particularly on mosquitoes and fruit flies, that suggest particular strains of Wolbachia make insects more resistant to viruses," he replied. "One of our initial concerns was that the bacterium might have a negative effect in terms of using SpexNPV as a biological control agent. We expected Wolbachia to have either no effect at all, or to make the armyworm more resistant to the biopesticide.

"Our initial studies were based on field data that we had collected over the course of a year. Wolbachia appeared to be making the armyworm more susceptible to the virus, and we thought that this was quite unusual. We collected more data over the next couple of years and it became evident that the bacterium was making the creatures more susceptible to SpexNPV. It was at this point that we started to conduct laboratory experiments. We used antibiotics to ‘cure’ the armyworms of Wolbachia before infecting them with the virus. Our previous results were confirmed. The bacterium was making armyworms more susceptible to the virus."

Professor Wilson and his colleagues found that Wolbachia-carrying armyworms were between six and 14 times more susceptible to SpexNPV than caterpillars who had had their bacterial passengers removed. Of course, these results seem to point to a simple solution. Simply spread Wolbachia amongst the armyworm population and use SpexNPV to protect crops. However, as Professor Wilson explained, due to the mobile nature of adult armyworm moths, this is not an easy task.

"Wolbachia makes mosquitoes more resistant to the dengue virus, so in Australia, scientists have released mosquitoes carrying the bacterium into areas where dengue is prevalent. The hope is that as the levels of Wolbachia increase, the transmission of dengue from mosquitoes to humans will be interrupted.

"However, these mosquitoes are fairly sedentary, which means that you can concentrate the Wolbachia within a relatively isolated population. The adult moths of African armyworms, on the other hand, migrate across such large areas that we would probably need to release massive numbers of them to sufficiently increase levels of Wolbachia.

I concluded our interview by asking Professor Wilson about the further research that he would like to conduct in this area.

"The research grant that we were operating under has just ended, so we will be looking to apply for new funding in order to take things forward," he explained. "As I said, we obtained these results fairly recently, and unfortunately, the main project had already finished by this point in time. I think that we are still in the initial stages of our research. We haven’t even started to think about how we might apply our newly gained knowledge in the field. Whilst there are potential challenges in exploiting the effects of Wolbachia to control African armyworms, not all major crop pests are as mobile. With more research, it might become possible to apply our findings to other species."