By Catherine Brahic

Scientists have pinpointed the trigger that turns a disordered assembly of locusts into a co-ordinated marching army capable of devastating vast expanses of crops and natural vegetation in Africa.

One of the researchers, Emma Despland, now an assistant biology professor at the University of Concordia in Canada, calls the desert locust "the world's most notorious agricultural pest".

The crucial factor is a particular population density. Once that is achieved, the insects fall into line with each other and begin to walk in the same direction, according to a paper published in the respected peer-reviewed journal Science by seven scientists from Australia, the United States Canada and the United Kingdom.

Locusts start their lives as solitary wingless 'nymphs' that tend to avoid each other.

But if resources get scarce, they are forced to interact. They can then form co-ordinated local bands that are able to move in unison into neighbouring habitats and recruit increasing numbers of insects, eventually creating huge armies.

Predicting the onset of group movement is therefore vital to efforts to control locust outbreaks. The study is one of the first examples in animal biology of a group dramatically changing its behaviour as it reaches a critical mass.

The researchers put progressively larger numbers of locusts in a circular arena, filmed their behaviour and tracked their movements with computer software.

At medium densities of about 25 to 62 insects per square metre, the locusts fell into line and began moving in the same direction, even changing direction suddenly as one.

When there were more than 74 insects per square metre, the co-ordinated marching bands ceased to change direction and kept on marching in the same direction for the full eight hours of the experiment. A video is available online at www.scidev.net.

The observations confirm what computer models known as self-propelled particle models had predicted.

"Better understanding the process by which they form is the key to the control of these swarms,' said Sydney University Professor Stephen Simpson, who worked with colleague Dr Jerome Buhl of School of Biological Sciences.

The researchers conclude that using similar computer models could help devise ways of controlling locust outbreaks.

Their findings also back the UN Food and Agriculture Organisation's definition of the kind of juvenile locust group likely to cross the tipping point and trigger an outbreak.

The researchers are now expanding their computer models to simulate the environments that the locusts live in and predict the behaviour of much

larger swarms. - SciDev.Net