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Cannibalism drives locust swarms

Locust swarms are formed because the insect is desperate to literally stay a step ahead of its cannibalistic neighbour, an international research team says.

The team of US, UK and Australian researchers has shown that mass migration in juvenile desert locusts is strongly influenced by the fear of being bitten from behind.

Their findings, published in a recent edition of Current Biology, overturns the theory that swarms are simply a way for insects to move more safely to a new food source.

Instead they have found "the threat of attack by those approaching from behind is the principal factor in the onset of collective movement".

Co-author Dr Greg Sword, of the University of Sydney's School of Biological Sciences, says the findings can help reduce the devastation caused by swarms, which are estimated to affect the livelihood of one in 10 people on the planet.

He says understanding what makes the groups move will help produce models that can predict the direction the swarm will travel.

"If you know where they are going," he says, "you can better target your pesticides."

Simulating a swarm

As part of the study, the researchers placed a large group of juvenile locusts in an 80-centimetre diameter arena, and used motion analysis software to locate the position of every locust.

Juvenile locusts cannot fly, but instead march up to 500 metres per day in swarms that can number "millions".

The researchers reduced the ability of individual locusts to detect the approach of other locusts from behind.

This was done by severing the main nerve in the abdomen of one group of juvenile locusts so they could not feel anything from behind and in a separate group reducing the insects' rear vision using black paint.

Sword says although the surgery did not impede the locusts' movement, the loss of sensation meant the "de-nervated" locusts did not feel other locusts approaching from behind.

This resulted in the locusts being less likely to move, and they were "literally getting their bums chewed off".

"If they are not constantly being touched, they won't move as much," he says.

Catch 22

The defensive movement away from the perceived threat sets up a domino effect, as each individual locusts' movement causes them to touch another locust, which then makes the second locust move away.

"You have millions of individuals all going in the same direction, because if they change direction much, they are likely to come in contact with each other," Sword says.

He says locusts' movement is driven by the need to find nutrients such as protein and salt, and by staying with the swarm, individuals lessen the chance of falling prey to external predators.

"[But] if individuals fail to continue moving they are likely to be attacked and risk becoming another ... source of these essential resources."