Bugs on drugs - chemical cause of locust swarms identified

30 January 2009

Professor Steve Simpson, from the School of Biological Sciences, with scientists at Oxford and Cambridge Universities, have found the neurochemical cause of one of earth's most devastating biological phenomena - locust swarms.

Professor Steve Simpson's neurochemical work on locusts is featured on the front cover of the journal Science.
Professor Steve Simpson's neurochemical work on locusts is featured on the front cover of the journal Science.

In research featured on the front cover of the journal Science on 30 January 2009, the group have pinpointed a single neurochemical - serotonin - as the cause of swarming behaviour in locusts.

Locusts are one of the world's most destructive insect pests, affecting the livelihoods of 1 in 10 people on earth. However, whether or not a locust joins a devastating swarm depends entirely on its environment.

Unlike other grasshoppers, the usually shy locust can switch from a non-swarming (solitarious) phase where they actively avoid other locusts to a swarming (gregarious) phase depending on the density of locusts in their vicinity. After a few hours of crowding, locusts that were initially shy and solitarious will change behaviour to become gregarious locusts that form disastrous swarms of millions.

'Phase change' - the ability of locusts to change their behaviour - is at the heart of the locust pest problem, and has consequently been the focus of research by University of Sydney scientist Professor Stephen Simpson and his team for almost 20 years.

"To effectively control locust swarms, we must first understand exactly how it is that a single shy locust becomes a highly social animal that swarms," said Professor Simpson.

After first discovering that phase change was caused by stimulation of sensory hairs on the hind leg of locusts, Professor Simpson's team began to investigate the neurological and neurochemical basis of this effect.

Dr Michael Anstey, who completed the research as part of his PhD at the University of Oxford supervised by Professor Simpson, and Dr Stephen Rogers, a postdoctoral fellow who previously completed his PhD with Professor Simpson and is now part of Professor Malcolm Burrows' team at Cambridge, led the research investigating the effect of neurochemicals on the change in locust behaviour.

The group measured levels of 13 neurochemicals in locusts that were gregarious (swarming) and solitarious (non-swarming). The only neurochemical that showed a relationship with social behaviour was serotonin.

"It was clear that as locusts switched from solitarious to gregarious, the amount of serotonin in their central nervous systems also increased," explained Professor Simpson.

"The next step was to determine if this relationship actually meant that serotonin was the cause of gregarious, and thus swarming, behaviour in locusts."

To do this, the researchers either added serotonin or prevented the production of serotonin in locusts. The results show unequivocally that serotonin is responsible for the behavioural transformation of locusts from solitarious to gregarious. Withholding serotonin means that locusts will not become gregarious, while adding serotonin will cause the phase change from solitarious to gregarious.

Interestingly, the neurochemical serotonin is involved in social behaviour of species across the animal kingdom, including crustaceans, rats and humans.

"The fact that serotonin causes the transition from a shy, antisocial animal into a party animal means that pharmacologically, gregarious locusts are on Ecstasy or Prozac," explained Professor Simpson.

"I like to think of the analogy of a shy person who goes to a party and takes an ecstasy tablet. Their serotonin levels kick up and they talk toeveryone and smile and dance and are really social."

Since the neurochemical serotonin causes the switch from a solitarious and harmless locust into a swarming one, could a product that inhibits serotonin be used to prevent locust swarms?

"It's a very good idea, but in reality it would be difficult to create a locust control agent that interferes with serotonin," explained Professor Simpson.

"Because social behaviour in so many animals depends on serotonin, if we used serotonin antagonists in the environment that were not specifically targeted, we run the risk of affecting other processes in locusts, as well as severely impacting animals other than locusts.

"We would need to be sure that locusts have a unique serotonin receptor that causes phase change, which we haven't identified yet. Any locust control agent would have to be specific for this serotonin receptor in locusts."

The significant discovery that serotonin causes the change from shy to gregarious behaviour, is a vital clue to our complete understanding of the biology underlying locust swarms.

"Thanks to my group's research at Oxford and Sydney, and with neurophysiology colleagues at Cambridge, we now understand the critical links in the phase change process - from the habitat conditions that encourage initial congregation of solitarious locusts, to cues from crowding that elicit the switch from solitary to gregarious behaviour, to detailed neural and chemical events within the nervous system that produce the behavioural transition, to inter-individual interactions between gregarious locusts that trigger and direct mass migration.

"No other biological system is understood from neurons to populations in such detail or to such effect: locusts offer an exemplar of the how to span molecules to ecosystems - one of the greatest challenges in modern science."

Contact: Katynna Gill

Phone: 02 9351 6997

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