Swarms of locusts, which darken the skies and ravage crops in their path, have been known since biblical times, but they still threaten the food security of millions of people in Asia and Africa.

Scientists announced on Thursday that they had discovered a pheromone that these insects emit to avoid being eaten by other locusts, a discovery that could be used in the future to “tame” them and prevent them from forming migratory swarms, AFP reported.

Bill Hansson, head of neuroethology at the Max Planck Institute and lead author of the study, published in the journal Science, told AFP that it builds on previous research that showed these flocks are driven in part by the threat of cannibalism.

Cannibalism is not uncommon in nature, from lions eating cubs that don’t belong to them to foxes eating their dead mates for sustenance.

In the case of grasshoppers, cannibalism serves a purpose.

Why do swarms go in the same direction?

The migratory locust (Locusta migratoria) goes through such different phases of behavior that until recently they were considered separate species.

In general, these insects are quite solitary and eat relatively little.

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

This is known as the gregarious phase, and cannibalism is thought to cause them to move to areas with more food, according to a 2020 study by Ian Cousin of the Max Planck Institute for Animal Research.

Indeed, explains Bill Hansson, “grasshoppers eat the back of the body. Therefore, if they stop flying, they will be eaten by another.” This explains why the swarms head in the same direction.

Four years of trials

“We thought that every insect evolved some kind of defense,” he continued.

To determine this, the experiments lasted four years. He and his team demonstrated for the first time that cannibalism rates actually increased when the number of “swarming” locusts in a swarm increased (from 50), demonstrating in the laboratory what Ian Cousins ​​had observed in the field in Africa.

They then compared the scents emitted by solitary and gregarious grasshoppers and identified 17 of them as produced exclusively by gregarious grasshoppers.

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

This substance is involved in the production of a toxin (hydrogen cyanide) by locusts, so it seems logical that this is a signal to other individuals to stay away.

Genetic modification

To confirm their discovery, the scientists genetically modified grasshoppers so that they could no longer produce phenylacetonitrile. The result: they became more vulnerable to cannibalism.

The researchers didn’t stop there, testing dozens of grasshoppers’ olfactory receptors, eventually settling on one that was particularly sensitive to the chemical.

Genetically modified locusts that could no longer use this receptor exhibited increased cannibalistic behavior.

In a paper also published in the journal Science, researchers Ian Cousin and Einat Cousin-Fuchs said the discovery sheds light on the “complex balance” of mechanisms that cause migrating locusts to group together or engage in rivalry.

Their methods of population control may use technologies that increase competition between them. At the same time, “we must not eradicate this species,” warns Bill Hansson.

He adds: “If we can reduce the size of the swarms or attract them to areas without crops, then we have a lot to gain.”