Cannibalism and olfaction in the formation of locust swarms

Massive swarms of migratory locusts pose a threat to the food supply of millions of people in Africa and Asia, with their behavior resembling a natural disaster. This phenomenon was also described in the Old Testament’s Book of Moses as one of the ten plagues, where locusts would swarm and devour all crops and trees.

Scientists believe that cannibalism plays a role in the swarming behavior of locusts, as individuals are constantly on the move to avoid being pursued by others. To understand how these insects influence each other’s behavior and if olfaction plays a part, a study was conducted by Bill Hansson, director of the Department of Evolutionary Neuroethology at the Max Planck Institute, and his team, building on previous research by Iain Couzin.

Locusts exist in different phases, with solitary locusts living alone and avoiding contact with others, while gregarious locusts exhibit swarming behavior. Population density, rainfall, and food availability can cause locusts to shift from the solitary phase to the gregarious phase. Stimuli like touch, smell, and sight can also trigger serotonin and dopamine release, leading to an increase in aggression and appetite, and eventually, the release of aggregation pheromones that result in swarming behavior.

Cannibalism only occurs in the gregarious phase, and behavioral experiments have shown that the rate of cannibalism increases with population density. The research team also compared the odors emitted by solitary and gregarious locusts during their juvenile stage and found that gregarious locusts emit particular odors not produced in the solitary phase.

Out of the 17 odors that were exclusively produced in the gregarious phase of locusts, only one, phenylacetonitrile (PAN), was found to be an odor signal that deterred other locusts in behavioral tests. To confirm this, scientists used genetically modified locusts that were unable to produce PAN.

As the population density increased, the level of cannibalism rose, and the locusts produced more PAN. By knocking out the enzyme responsible for producing the compound, the researchers were able to confirm its anti-cannibalistic effect. When the locusts were unable to produce PAN, cannibalism increased significantly. According to Chang, this study shows that PAN plays a critical role in preventing cannibalism in locusts.

Hetan Chang, Markus Knaden and Bill Hansson from the Max Planck Institute for Chemical Ecology discuss the new findings. Credit: Anna Schroll

The research team faced a significant challenge in identifying the olfactory receptor that recognizes PAN, as locusts have over 140 olfactory receptor genes. They had to clone as many genes as possible and test them one by one, which involved conducting tests on 49 different olfactory receptors using more than 200 relevant odors. Eventually, they identified the olfactory receptor OR70a as a highly sensitive and specific detector of PAN in the migratory locust Locusta migratoria.

Further behavioral experiments using genetically modified locusts whose OR70a receptor was no longer functioning revealed a strongly increased cannibalism rate. This is because without the corresponding receptor, the cannibalism stop signal cannot be perceived by the locusts.

The discovery of a pheromone that controls cannibalism in locusts is entirely new. Since cannibalism significantly impacts locust swarm dynamics, understanding the population ecology of these animals, particularly the effect of PAN, provides new possibilities for locust control. “If you inhibit the production of PAN or the function of the receptor, you could potentially get the locusts to behave more cannibalistically and control themselves in that way,” explains Bill Hansson.

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