Trigger for locust swarming identified

A locust marching in a swarm Copyright: Gabriel A. Miller

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Researchers have pinpointed the trigger that turns a disordered assembly of locusts into a coordinated marching army capable of devastating vast expanses of crops and natural vegetation.

In a paper published in Science today (2 June), Jerome Buhl and colleagues show that the crucial factor is a particular population density at which the insects fall into line with each other and begin to walk in the same direction.

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

At medium densities (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 coordinated marching bands ceased to change direction and kept on marching in the same direction for the full eight hours of the experiment.

The observations confirm what computer models known as self-propelled particle models had predicted, so Buhl’s team and his colleagues concludes that using such models could help devise ways of controlling locust outbreaks.

Their findings also back the UN Food and Agriculture Organization’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.

Link to full paper in Science

Video of locusts marching in the circular arena [avi file] [3358KB]

This shows six minutes (accelerated six times) of video of an experiment with 120 locusts. The counter-clockwise group motion that can be seen in this movie began after five minutes and lasted for the remaining eight hours of the experiment.

Reference: Science 312, 1402 (2006)