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War report: fighting armyworms

War report: fighting armyworms - 28 January 2013. Masaiti Emmanuel

Video diary reveals the battle against country-wide insect plague.

Many important jobs require people to be on call, waiting for the phone to ring and to spring into action at a moment's notice. For BBSRC-funded Professor Kenneth Wilson at Lancaster University, the call doesn't take him to a hospital, an accident, or even an office. Between October and April in any year, when Wilson gets the call he embarks on a research trip to Africa where he combats the march of serious insect crop pests called armyworms that can devastate agricultural production and cause food security issues in many countries across eastern and southern Africa.

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  War report: fighting armyworms

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This call, however, turned out to be a little different. And just before Christmas, Wilson found himself meeting the Vice President of Zambia, and advising him on the impact of armyworms, how it might spread to neighbouring countries, and options for present and future control of the pest using a natural virus. "Usually I get about a week's notice," says Wilson of the calls that take him to Kenya, Tanzania or Zambia.

The Zambian government are interested in Wilson's expertise, which has been supported by BBSRC and other programmes such as the SARID (Sustainable Agriculture Research for International Development) initiative co-funded with DfID. That work helped establish a state of the art laboratory in Tanzania that can formulate the virus locally to counter the threat posed by the voracious armyworms. This means that the use of expensive imported chemical pesticides could be reduced or eliminated entirely which benefits people and their local environment. The use of local expertise also helps to develop indigenous skills and promote more equal knowledge exchange.

Control strategy

The African armyworm, Spodoptera exempta, is an insect pest of widely grown cereal crops such as maize, wheat and rice. It can reach plague-like densities of 200-1000 caterpillars per square metre (ref 1) – you wouldn't be able to take a step without squashing tens of them. Outbreaks can occur throughout sub-Saharan Africa but mostly originate in primary outbreak areas in Tanzania and Kenya (ref 2) before over several generations spreading to Zambia, Uganda and even as far as South Africa and Yemen. As is often the case in developing countries, the poorest farmers are worst affected, especially if they cannot afford pesticides to limit the damage.

Armyworm caterpillars devastate important food crops across entire countries. Image: Masaiti Emmanuel
Armyworm caterpillars devastate important food crops across entire countries.
Image: Masaiti Emmanuel

Wilson has been studying armyworms for nearly 25 years. During this time he's seen that most resource-poor farmers in Africa cannot afford imported pesticides, and those who can are not properly equipped to protect themselves from the noxious chemicals. Having witnessed the plight of these farmers first hand, he and his colleagues at the University of Greenwich and a small Tanzanian business, Crop Biosciences Ltd, have spent the last 15 years looking to see whether a viral disease of the larval stage of the armyworm, called SpexNPV (for Spodoptera exempta nucleopolyhedrovirus), can be harnessed as a cheaper and safer biological control agent (ref 2, ref 3, ref 4)

Wilson's expertise is collated on his Armyweb website that carries news and outbreak forecasts as well as summaries of his research. Armyweb was seen by Paul Desmarais, who runs the Kasisi Agricultural Training Centre in Lusaka, Zambia. He wanted advice on non-chemical means of controlling armyworms because they grow crops organically. "I told him about SpexNPV and he was keen to learn more. So I decided to go to Zambia to assess the situation for myself and to meet with him," says Wilson. "As soon as I had confirmed that I would be going, Paul phoned the Vice President, who he knows, to suggest that the two of us meet so that I could advise the government on mitigation options including biocontrol."

Wilson says they also discussed specific measures to tackle the looming food security crisis: replant the crops that had been destroyed (which they had already begun to do), and ensure they had a working pheromone trap network to monitor subsequent armyworm adult moth movement which would tell them where the next outbreaks would be.

Harvesting infected larvae to produce a biopesticide. Image: Wilfred Mushobozi
Harvesting infected larvae to produce a biopesticide. Image: Wilfred Mushobozi

"I also discussed the armyworm life cycle and advised that the next generation of moths would soon be migrating to initiate the next wave of outbreaks. This could be elsewhere in Zambia, but because the moths migrate hundreds of kilometres, it was just as likely to be one of the neighbouring countries in the south, such as Zimbabwe, Malawi and Botswana," Wilson explains. In fact, all of these countries have now reported outbreaks and much of Malawi is affected (ref 5).

Hopeful harvest

Analysing details of an organism's life cycle might seem an old-fashioned way to go about modern biology, but combined with genetic analysis of the virus, these fundamental insights into the behavioural ecology of the insect could turn a ubiquitous virus into a safe, environmentally friendly biopesticide.

And it could all be a matter of timing and applying the virus, which can be sprayed from a conventional machine or sprayed from an aeroplane (ref 6), early in the armyworm season. The armyworm season starts with the first rains and virus levels in the larvae also rise and fall and cycle with the armyworm population – in fact by the late season virus levels are so high than many armyworms are naturally killed by it. In the early armyworm season, however, viral loads are low because it follows the dry season when there hasn't been much vegetation to eat, and larval densities are too low for horizontal transmission of the lethal form of the virus. Hitting the armyworms with SpexNPV in early season is best because this will have the biggest impact of the growth of the armyworm population because infected larva will each produce more than 2Bn new infective particles to cascade control (ref 7). Applying the viral biopesticide early also allows plenty of time for new stocks of virus to be harvested for later use.

Using a cocktail of viral genotypes increases mortality compared to a single isolate, probably because armyworms vary in terms of which virus genotype they are most susceptible to. This favours local field production of the virus because a single, most efficacious strain does not have to be isolated which can be expensive, time consuming and technically challenging. Avoiding overuse of just one strain also greatly reduces the likelihood of resistance evolving.

Ken Wilson (second left) with Zambian Vice President Dr Guy Scott (third left) who chairs the Zambian Government's Disasters and Mitigation Committee.
Ken Wilson (second left) with Zambian Vice President Dr Guy Scott (third left) who chairs the Zambian Government's Disasters and Mitigation Committee.

And there's a recent, but significant, twist to the story. Working under the BBSRC/DfiD SARID programme, Wilson and colleagues have found that armyworms infected with a bacterium called Wolbachia are between 6 and 14 times more susceptible to SpexNPV than armyworms that had had their bacterial passengers removed (ref 8). It's an intriguing finding, because the presence of Wolbachia often confers resistance to viruses to their insect hosts, as seen in mosquitoes that carry the virus that causes dengue fever.

The finding also opens up an additional avenue for control: SpexNPV plus Wolbachia. And with six countries in Africa experiencing outbreaks in early 2013, the need is as great as ever. In Zambia alone Wilson says armyworm outbreaks were reported in 7 of the country's 10 provinces; more than 96,720ha of maize and pasture were infested, affecting close to 73,000 farmers.

It's not surprising that the Zambian Vice President was very interested in the work Wilson and colleagues are doing with field-produced SpexNPV, and expressed interest in conducting field trials in Zambia under a research license. However, Wilson says his team's priority this year is to produce a formulated product that could be registered for sale. "I explained that it would be simpler to import it from Tanzania, where the new production facility is, but first Crop Biosciences need to register the formulated product in Tanzania and then elsewhere in Africa."

References

  1. High levels of genetic diversity in Spodoptera exempta NPV from Tanzania
  2. Evaluation of Spodoptera exempta nucleopolyhedrovirus (SpexNPV) for the field control of African armyworm (Spodoptera exempta) in Tanzania
  3. Density-related variation in vertical transmission of a virus in the African armyworm
  4. Pathogen persistence in migratory insects: high levels of vertically-transmitted virus infection in field populations of the African armyworm
  5. African armyworm spreads across country
  6. Novel technologies for control of African armyworm on smallholder cereals in East Africa (PDF)
  7. NPV: a new biological control for Armyworm (PDF)
  8. Wolbachia in a major African crop pest increases susceptibility to viral disease rather than protects