BioEd Online

The menace of locusts lies in their numbers: a vast army of billions of insects chomping their way through anything in their path. Researchers have now pinpointed the tipping point at which these insects transform from wandering loners into a 'marching band' that seems to function as a gestalt eating machine.

The discovery of how swarming is triggered could inform efforts to control locust swarms that destroy crops in drought-ridden regions such as northern Africa and Australia.

A loose collection of locusts can chomp its way through a field. But the really devastating damage is done when locusts begin to move in unison. Then it only takes a single individual to spot some food and the entire swarm could wheel around to descend on it as one, allowing them to cut a swathe through vegetation. "Information about the location of food can propagate very effectively through the group," says David Sumpter of the University of Oxford, UK.

Sumpter and his colleagues, led by Jerome Buhl of the University of Sydney, Australia, studied juvenile locusts, known as nymphs, in the lab to investigate how this collective behaviour is triggered. They placed varying numbers of nymphs in an 80-centimetre-wide, ring-shaped arena and captured their behaviour on video.

At low densities, the locusts milled around randomly. But when the density reached about 25 nymphs per square metre, they began to march as one around the ring (see video), with occasional spontaneous changes in direction.

This is very similar to the behaviour of real locust bands in the wild, in which densities are typically around 50 locusts per square metre, the researchers say. They report their results in this week's Science¹.

Forward as one

Such collective behaviour is not just confined to locusts, says Sumpter. Swarming behaviour might follow similarly simple rules in a range of animals, including fish, birds and even humans.

"People walking down the street do not show any patterns if the density is low," Sumpter explains. "But at higher densities you start to see specific patterns, such as lane formation."

Sumpter is also confident that the uniform swarming behaviour, with sudden mass changes in direction, are produced not only in wild locust nymphs, but in the flying adults too.

It's not clear exactly how the locusts fall into step with one another, he says. Locusts are also known to be cannibals, so they might all move in the same direction to avoid bumping into one another, Sumpter suggests. "If you want to avoid being eaten, it might be better to keep parallel," he says.

The researchers are now filming real locust swarms in Australia to see whether their behaviour can be predicted.

Interestingly, at densities greater than about 75 individuals per square metre, the locust nymphs in the study stopped making spontaneous changes in direction. In those extra-crowded conditions, they kept to a single-minded forward march. So it may prove easier to predict the path of a super-dense locust swarm than a less dense one.

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