Scientists announced Thursday that they have discovered a pheromone that these insects emit so as not to be devoured by their congeners -- a find that could be used in the future to tame these voracious beasts.

Bill Hansson, director of the neuroethology department at the Max Planck Institute and lead author of the study, published in the journal Science, told AFP he had built on previous research, which showed that these swarms were partly governed by the threat of cannibalism.

Cannibalism is not uncommon in nature -- from lions eating lion cubs that do not belong to them, to foxes eating their dead congeners to satiate.

For locusts, cannibalism serves a specific purpose.

Migratory locusts (locusta migratoria) go through such different phases of behaviour that until recently they were thought to be separate species.

In general, these animals are rather solitary and eat relatively little.

But they transform when their population density increases, during temporarily optimal rains and breeding conditions -- followed by a period of food shortage. A surge of hormones then makes them roar, forming these swarms and making them more aggressive.

This moment is known as the "gregarious" phase, and cannibalism would keep them moving in order to lead them to areas richer in food, according to a 2020 study conducted by Iain Couzin, of the Max Planck Institute for Animal Research.

Indeed, explains Bill Hansson, "locusts are eaten from behind. So if you stop moving, you're eaten by someone else." This explains why the swarm is heading in the same direction.

Four years of testing

"We figured that every animal develops some kind of tusk," the researcher continued.

To determine this, the experiments lasted four years. He and his team first proved that cannibalism rates did increase when "gregarious" locusts were more and more numerous in a cage (from 50 onwards), demonstrating in the laboratory what had been observed in the field in Africa by Iain Couzin.

Then they compared the odors emitted by solitary and gregarious locusts, and identified 17 of them produced exclusively by gregarious phase locusts.

One chemical in particular, phenylacetonitrile, has been shown to repel other locusts in tests.

This substance is involved in the gregarious locust's production of a toxin (hydrogen cyanide), so it seems logical that it is the signal sent to other individuals to keep their distance.

Genetic modification

To confirm their discovery, the scientists used CRISPR molecular scissors to genetically modify the locusts, so that they can no longer produce phenylacetonitrile. As a result, they were more vulnerable to cannibalism.

The researchers did not stop there, testing dozens of olfactory receptors of locusts, finally stopping on one of them, particularly sensitive to this chemical.

Genetically modified locusts, no longer able to use this receptor, exhibited enhanced cannibalistic behavior.

In a commentary article also published in the journal Science, researchers Iain Couzin and Einat Couzin-Fuchs said the discovery shed light on the "complicated balance" between the mechanisms that drive migratory locusts to cluster together, or compete.

Methods to control their population could use technology that increases competition between them. At the same time, "we must not eradicate the species," warns Bill Hansson.

He added: "If we could decrease the size of the swarms, or attract them to places without crops, then we will have gained a lot."

© 2023 AFP