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Halting the armyworm march

December 2009

Estimates vary, but around 25% of crops can be lost to pests and diseases. Fungi and other plant pathogens can be devastating, but many farmers fear outbreaks by hordes of insects.

New pest outbreaks can, and do, appear – they are not the stuff of Biblical legends – but the results are as devastating. In early 2009, a state of emergency was declared in Liberia after it was invaded by a new species of caterpillar. When the problem spread to neighbouring Guinea it sparked fears of a regional food crisis. 

Armyworms aggregate en masse.
Image: Lancaster University

The pest was at first misidentified as the African armyworm, Spodoptera exempta, which are ravenous feeders of the early stages of cereal crops such as corn, rice, wheat, millet and sorghum, as well as pasture grasses. Like Desert Locusts, the moth larvae change form when they aggregate to form a gregarious ‘marching army’ phase (hence the name).

Armyworms are a serious pest in Kenya, Tanzania and surrounding countries in most years, but when numbers are really high, as seen in 2005, their impact can be catastrophic, with larval densities exceeding 1000 per square metre and crops being destroyed in a matter of hours. There are even reports of armyworms causing the deaths of grazing cattle.

However, current work by UK and Tanzanian scientists offers a potential solution. The work, funded by BBSRC and the Department for International Development, is exploring the potential of a new biological pesticide against armyworms using a natural virus of the insect.

Dr Wilson of Lancaster University, who leads the project, says the armyworm nucleopolyhedrovirus (NPV) can kill more than 80% of the caterpillars infected. “But it is totally harmless to humans, livestock and even other insects.” This could make it an environmentally-friendly and cheap alternative to the imported chemical pesticides that are currently used by the farmers that can afford them.

Dr Kenneth Wilson tackles African Armyworms.
Image: Lancaster University

“Our research shows that the virus is highly effective and can be locally produced,” says Wilson. “Now we want to understand how best to use the virus, as well as to identify the mechanisms for its sustainable production in Africa.”

The caterpillars in Liberia struck 65 towns, and the Ministry of Agriculture reported that up to 20,000 people left their homes, the fields empty and markets devoid of food that had more than doubled cost in surrounding areas. The pest was later identified as Achaea catocaloides, another moth which occurs throughout West Africa and typically feed on the Dahoma tree but can develop in large numbers and attack agricultural crops.

Currently, the only control for these Liberian caterpillars is imported chemical insecticides, but soon a locally produced biological pesticide may offer a solution against armyworm caterpillars in East Africa.

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