Page last updated at 19:11 GMT, Tuesday, 27 January 2009

Q&A: Caterpillars ravage Liberia

African armyworms (Image courtesy of Lancaster University / K.Wilson)
The unidentified caterpillars have forced 20,000 to flee their homes

Liberian President Ellen Johnson-Sirleaf has declared a state of emergency in response to a plague of mysterious crop-destroying caterpillars.

The invaders were thought at first to be armyworms - but field investigators have now cast serious doubt on this diagnosis.

So what exactly are these creatures and what can be done to halt them?

How serious is the situation in Liberia?

Up to 20,000 people have now fled their homes, according to the Ministry of Agriculture.

The unidentified caterpillars have struck 65 towns, leaving in their wake wells contaminated by faeces, fields empty of crops and markets devoid of food.

The cost of some foods has more than doubled in many areas.

The last time insects attacked on this scale in Liberia was in the late 1970s.

And to make matters worse, the mysterious "worms" are now spreading to neighbouring Guinea and towards Sierra Leone.

So what is causing the plague?

Nobody knows, yet. The government originally identified the bugs as armyworms, but their diagnosis has now been overturned by investigators in the field.

Samples of the unidentified insects are now being sent for testing, to experts at Cardiff University and the International Institute of Tropical Agriculture in Benin.

What exactly are armyworms?

The menace known as the "African armyworm" is in fact a caterpillar. It gets its moniker from its habit of "marching" in large numbers into crop fields.


During the rainy season, the insects emerge in sudden swarms, devour crops, and then move on once fields are barren.

They are the larvae of nocturnal moths, of the species Spodoptera exempta.

The moths are capable of long-distance migration - more than 100km (60 miles) per night - allowing the swarms to spread rapidly.

Each female lays between 500 and 1,000 eggs in her 10-day lifetime.

Once hatched, the larvae migrate through grasslands in snake-like colonies. On reaching crop fields, they begin feasting.

At full size (5cm - 2in) the caterpillars can lay waste to whole farms within days.

So how do the caterpillars in Liberia differ from armyworms?

There are several key differences, according to Tim Vaessen, a UN Food and Agriculture Organisation emergency co-ordinator.

While armyworms feed on ground cereal like millet, rice or sorghum, the unidentified insect favours munching the leaf of the Dohama tree, he said.

And whereas armyworm casings are buried in the soil at a depth of several centimetres, the unknown species' cocoons are found on the surface beneath ground leaves.

The mystery caterpillar appears to rear up, making half circles in the air as it moves forward, which the armyworm does not do. The wings of its moth also appear different.

Can anything be done to halt their march?

At present, the best weapon against the pest is to spray crops with chemical insecticides.

Armyworm (Liberia's agriculture minister website)
Armyworms are among the world's most destructive agricultural pests

These are too expensive for most farmers, so the government is stepping in and calling for international assistance, to begin aerial crop spraying.

Forecasting systems for pest attacks do exist in some African countries, to warn farmers about possible outbreaks in time to spray crops, where they can afford to.

But these warnings do not always work because attacks often affect isolated communities that are difficult to contact.

In the case of armyworms, a more radical approach to prevention - a virus known as NPV - is being investigated by researchers from the UK, Canada and Tanzania.

The naturally occurring virus is specifically lethal to armyworms, but it appears too late in most years to prevent them causing serious crop damage.

Scientists hope to study and harness the virus, to create an effective pest control technique.

Their goal is to create a pesticide which will be harmless to humans and other wildlife, but will destroy armyworms and thus keep Africa's staple food crops intact.

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