How solitary locusts turn maurading bands

Sydney, Aug 30 (IANS) Vast locust swarms, spread over many square kilometres, can blot out the sky and destroy standing crops whenever they land. The scourge has been well recorded in history and science.

The irony is that locusts live a solitary life, avoiding contact with each other when the density of their numbers is low in a given area. At the tipping point – when that density reaches a critical level – this repulsion declines and the locusts make a rapid transition to being ‘gregarious’ or attracted towards each other.
The ultimate result, if that behaviour continues, is a swarm which can contain millions of insects travelling thousands of kilometres while devastating vegetation and crops, says new research by the University of Sydney, the journal “Ecology Letters” reports.
Previously scientists at the Sydney University uncovered what triggers the change. However, precisely what evolutionary advantage there was in locusts having two forms, solitary and social, remained unknown until this current research, according to a statement from Sydney University.
“While the locusts’ change has been extensively studied, the ultimate driving factors have, until now, remained unclear,” said Stephen Simpson, professor from the Sydney School of Biological Sciences, who led the study.
“By modelling the evolution of locusts’ behaviour, we discovered that the density-dependent change in their behaviour may be caused by trying to minimise the threat of cannibalism. Cannibalism is a common characteristic of all locusts, especially when they are deficient in protein,” he said.
In their solitary phase, the locusts avoid each other entirely except when they are ready to mate. When their numbers increase, however, they become sociable and move away from locusts who approach them in order to avoid being eaten. At the same time, they are attracted to locusts moving away from them. Earlier research had shown that the locusts form swarms as they fear being bitten from behind.
This escape-pursuit behaviour results in individuals aligning the direction of their movement and the formation of large mobile bands or swarms, the researchers said.