2023-05-28 15:00:12
Of the 10 plagues of Egypt, the eighth, the “locusts,” as the translators of the Bible named it, is probably the one best reproduced in reality. Entire portions of the African continent are regularly deprived of crops by swarms of desert locusts and migratory locusts, the two most fearsome species of the insect. Let’s recall the principle: Under certain environmental conditions. this insect, solitary and quiet in normal times, transforms itself into a gregarious and perpetually hungry monster. The swarms it forms can cover several hundred square kilometers and contain several billion individuals – yes, billions. Their passage leaves entire regions devastated.
For 15 years, British researcher Iain Couzin, from the Max Planck Institute in Konstanz (Germany), has believed he knows the reason: cannibalism. During its molt, Dr. Jekyll the grasshopper would become both Mr. Hyde and Hannibal Lecter. In the huge cages of his laboratory, the researcher observed the continuous race of the insects, each running at the same time to catch the one who precedes it and to avoid the one who follows it.
But how does this curious dynamic work? Another German team, this time at the Max Planck Institute in Jena, decided to follow this idea. In a paper published in the journal Science on May 5, it showed that placing five migratory locusts (The migratory locust), or even 25, in a 9.5-liter cage kept them peaceful. But at 50 the fourth- and fifth-stage nymphs began to bite each other, a phenomenon further accentuated at 250 per cage. It should be noted that before reaching adulthood, locusts go through five juvenile stages interspersed with molts, during which they walk, sometimes jump, but do not fly.
On the trail of pheromones
Above all, these researchers specializing in insect odor focused on the volatile compounds emitted by the animal. They found 17 of them, produced by the nymphs during the gregarious phase alone. Three of these pheromones were already known to attract locusts. Thirteen others appeared to be neutral. The last one, phenylacetonitrile (PAN), was also an old acquaintance, since male desert locusts emit it to repel competitors while mating.
So the researchers tested it. By genetically modifying a grasshopper so that it does not produce this pheromone, they transformed the poor beast into an immediate prey for its congeners. Conversely, by suppressing the specific olfactory receptor of PAN in individuals, they started to attack their comrades without distinction, with or without PAN.
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