Published online 30 January 2009 | Nature | doi:10.1038/news.2009.72
Updated online: 3 February 2009

News: Briefing

Halting the African armyworm

Liberia prepares for second plague of caterpillar pests.

ArmywormsArmyworms eat through crops and pollute water supplies.FAO

A plague of crop-eating caterpillars has struck Liberia and a second wave could spread across West Africa in the next few weeks, the Food and Agriculture Organization of the United Nations (FAO) has warned. Ellen Johnson-Sirleaf, Liberia's president, has declared a state of emergency and appealed for international aid.

What are armyworms?

The African armyworm is the caterpillar of the night-flying moth Spodoptera exempta, and is a major crop pest, usually found in eastern Africa. From October to December — one of the rainy seasons — the moths lay their eggs on grasses and crops in Kenya and Tanzania. Their hatched larvae, which grow to 2–5 centimetres long, march in groups, devouring any food sources they come across. They subsequently pupate to form moths, each of which can fly up to 1,000 kilometres and lay 1,000 eggs in its 10-day lifetime.

Armyworms have attacked, with varying intensity, 48 times in the past 55 years, says David Grzywacz, an expert on the caterpillars at the Natural Resources Institute of the University of Greenwich, UK. The second wave of moths generally fly north or south of eastern Africa, to countries such as Ethiopia or Zambia. Liberia, in western Africa, was simply not prepared to deal with the threat, he says.

What damage have they caused?

The caterpillars have munched through cocoa, bananas and maize (corn), and are defecating in water supplies. So far more than 100 villages and around 500,000 people have been affected, says Arthur Tucker, of Liberia's Ministry of Agriculture.

"This is an emergency on the scale of a major locust outbreak," says the FAO's Christopher Matthews.

Many of the armyworms have now bored into the ground, FAO entomologist Winfred Hammond reported on Thursday. When they re-emerge as moths in a week to 12 days, he says, a second wave may spread into neighbouring countries such as Guinea and Sierra Leone.

Worms have already been seen in six towns in Guinea and moths have been spotted in some regions of Liberia, Tucker says.

How can they be controlled?

The caterpillars can be sprayed with pesticides as soon as they are detected. Tanzania has a network of pheromone traps set up as monitoring devices to attract the moths as soon as they appear. The country also stockpiles pesticides to deal with the caterpillars.

But because it was taken by surprise, Liberia was unable to deal with the threat fast enough. Initially, farmers who could not get hold of pesticides set fire to worms and crops, says Tucker. Government teams are now spraying pesticides imported from Accra in Ghana, he says.

Unfortunately, the pupae have now burrowed underground and are out of reach of pesticides, so Liberia must prepare for a second wave of moths. The idea of using pheromones to corral the moths together for destruction won't work, says Grzywacz — there will be too many to control cost-effectively. Neighbouring countries are already spraying pesticides in preparation.

Are synthetic pesticides the best we can do?

Grzywacz is working on spraying a biological control agent, a nucleopolyhedrovirus, which has been tested in Tanzania. The virus attacks the worms every year, but usually occurs too late in their outbreak cycle to prevent serious crop damage. The chances of spraying the virus in the current outbreak are zero, says Grzywacz, because the treatment hasn't been proved on the ground, it isn't available in large quantities (although it could be mass produced) and it is not registered for anything other than experimental use.

What happens next?

"The Liberians are right to be worried that the next wave of outbreaks could be even worse," says Ken Wilson, of Lancaster University, UK, who works with Grzywacz. If the weather conditions continue to be good for armyworms (generally scattered showers and warm), he says, then the next wave of moths will be initiating a second generation of outbreaks in a few weeks, which could be even worse than the last one. "Of course, if they are lucky, the weather conditions may turn against the armyworms and things may gradually subside." 

Updated:

The caterpillars swarming across Liberia have been re-identified by entomologists as _Achaea catocaloides_, not armyworms as first suggested by the country’s ministry of agriculture and the UN’s Food and Agriculture Organization. The forest insects feed on the Dahoma tree but can develop in large numbers and attack agricultural crops, says the Liberian ministry of agriculture. They occur throughout West Africa.

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