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Halting the armyworm march using biological control

SARID initiative advances knowledge of pest-killing virus

22 September 2011

Researchers studying how to control the rampage of armyworms that devastate crops have discovered key knowledge about the natural ecology of a specific virus that naturally infects the armyworms – fundamental insights that can be used to enhance food security without using chemical pesticides.

farmer in Korogwe, northern Tanzania, looks on in despair as his maize crop is lost to African armyworms. Image: Wilfred Mushobozi

A farmer in Korogwe, northern Tanzania, looks on in despair as his maize crop is lost to African armyworms.
Image: Wilfred Mushobozi

The secret may lie in the changing seasons. Applying the virus as a biopesticide early enough in the crop growing season so it not only protects vulnerable food supplies in a region where famine is a constant threat, but also allows plenty of time for new stocks of virus to be harvested for later use.

The work, funded by BBSRC and DfID, also builds local research capacity because local expertise is being developed to mass produce the pest's natural enemy right where it is needed.

March of the armyworm

African armyworm, Spodoptera exempta, is a voracious caterpillar pest of cereal crops, including the agricultural staples such as maize, wheat, sorghum, millet and rice. Armyworm outbreaks occur throughout sub-Saharan Africa but mostly originate in primary outbreak areas in Tanzania and Kenya; resource-poor farmers are hit hardest.

Armyworms can be managed using chemical insecticides. In June 2011, over 3,000 litres of the organophosphate Malathion was sprayed in an attempt to limit widespread outbreaks of armyworms in drought-stricken Ethiopia (ref 1). But most poor farmers in outbreak areas – estimated at 70% – cannot afford to buy them for their own farms (ref 2). And because insecticides can be damaging to the environment and agricultural workers' health if not applied correctly there is a clear case to look for natural control alternatives.

Armyworms can occur at densities in excess of 50,000 per hectare. Image: Ken Wilson

Armyworms can occur at densities in excess of 50,000 per hectare.
Image: Ken Wilson

Wilfred Mushobozi, a Tanzanian project partner, says farmers from the Korogwe and Same districts were hit by widespread armyworm outbreaks in April 2011. "In two districts, they caused up to 70% crop losses and if the armyworm were not controlled in time, there was total crop failure necessitating replanting. A major challenge for these farmers is their inability to purchase expensive chemical insecticide during the outbreaks," he says.

Counterstrike

Fortunately, armyworms are vulnerable to a lethal viral disease that occurs naturally in Africa (Spodoptera exempta nucleopolyhedrovirus, or SpexNPV). SpexNPV is a baculovirus that infects only armyworms and is entirely safe to humans, livestock and beneficial insects. Research has shown that this virus can be locally mass-produced in the field – DfID's Research Into Use programme has funded the construction of a local facility for the production of SpexNPV biopesticide. Harvesting the virus in the field, rather than in expensive insect-rearing facilities, massively reduces the cost and it could be produced for as little as five US dollars per hectare.

Using funding from the SARID programme (see 'Sustainable agriculture overseas') scientists in the UK and Tanzania are looking at why SpexNPV naturally controls some armyworm outbreaks and not others. The first finding was that all adult moths (and probably all caterpillars) carry the virus. "This is a new and surprising result," says project leader Professor Kenneth Wilson from Lancaster University. "We have also found that the viral loads (number of virus particles) of both adults and larvae vary considerably within and between outbreaks."

Community-based armyworm forecasters monitor African armyworm moths using pheromone traps and alert their community to impending outbreaks. Image: Wilfred Mushobozi

Community-based armyworm forecasters monitor African armyworm moths using pheromone traps and alert their community to impending outbreaks.
Image: Wilfred Mushobozi

Caterpillars 'inherit' their viral load from their parents, via vertical transmission through the egg, which is probably topped up when caterpillars in high-density outbreaks ingest virus from caterpillars that have already died (horizontal transmission). Virus levels gradually increase through the armyworm season which coincides with the annual rainy season.

Wilson thinks that because there is very little vegetation for the caterpillars to feed on in the dry season and so armyworm densities decline, horizontal virus transmission declines, and overall viral loads decline. "So we see a 'cycling' of viral loads between the rainy season when it is high, and the dry season when it is low," says Wilson.

Therefore, the best time to seed the armyworm population with large amounts of baculovirus is during early-season outbreaks which are rarely infected with virus. The armyworm season starts with the first rains – just after the last of the dry season when viral loads are low. "This control strategy should get the 'topping up' process started earlier and faster, and reduce armyworm numbers before they have a chance to expand and migrate into neighbouring countries," says Wilson. "These (early season) outbreaks provide the main breeding grounds for moths that will initiate outbreaks elsewhere in eastern Africa. If we can control these outbreaks, we can put a break on this progression and outbreak expansion." Moreover, Wilson thinks that the early season rains will probably help spread the virus too.

Massive attack

The work has uncovered some other telling details that could make all the difference between controlling armyworm outbreaks before they spiral out of control.

Combining sound biological knowledge and the right tactics in the field techniques could bring armyworms under control. Image: Wilfred Mushobozi

Combining sound biological knowledge and the right tactics in the field techniques could bring armyworms under control.
Image: Wilfred Mushobozi

First, armyworm mortality increases if it is infected with a cocktail of viral genotypes, rather than a single one, and if they are infected with a more virulent late-season isolate. This is good news for culturing the virus locally by harvesting infected larvae, because a single, most efficacious strain does not have to be isolated which would be expensive, time consuming and technically challenging.

Furthermore, using a baculovirus cocktail of strains means that the likelihood of the armyworms evolving resistance is vastly reduced. "This 'cocktail' approach to microbial biopesticides may be appropriate for other biopesticides," says David Grzywacz, a project partner from the University of Greenwich who specialises in biopesticide production.

Second, mortality can be increased by nearly 10 times if the larvae are also infected with particular species of bacteria, which is a new focus for research. If bacteria and the virus could be combined and integrated in a comprehensive control strategy, then managing outbreaks could be a feasible and realistic goal within a decade.

Sustainable agriculture overseas

The SARID programme (Sustainable Agricultural Research for International Development) aims to support relevant high-quality basic and strategic biological and biotechnological research in crop science and sustainable agriculture that has the potential to contribute to the achievement of the Millennium Development Goals, and which will establish productive partnerships between scientists in the UK and developing countries.

12 projects conducted over five years are funded from a pot of £7.5M and DfID are the main co-funders. The project involves 32 collaborations between UK universities and institutions across the globe, and other research initiatives include reducing arsenic levels in rice, tackling pests and pathogens of bananas, coconuts, kale, cabbage and sweet potatoes, as well as efforts against pests such as invasive nematodes and the African witchweed menace.

References

  1. Emergency Transboundary Outbreak Pest (ETOP) situation report for June with a forecast till mid-August, 2011 - US-AID, June 2011
  2. Njuki J, Mushobozi W, Day R K. (2004). Improving armyworm forecasting and control in Tanzania: Report of a socioeconomic survey. Nairobi, Kenya: CAB International, Africa Regional Centre

Contact

Arran Frood

tel: 01793 413329