Dare the ten plagues of Egypt, the eighth, namely the “grhoppers”, as the translators of the Bible into French have somewhat improperly named it, is undoubtedly the one that reality best reproduces. Regularly, entire portions of the African continent are deprived of crops by swarms of desert locusts or migratory locusts, the two most formidable locust species.
Let us remember the principle: this insect, solitary and quiet in normal times, is transformed, under certain environmental conditions, into a gregarious and perpetually hungry monster. The swarms it then forms can reach several hundred square kilometers and several billion individuals – yes, billions. Their page leaves entire regions devastated.
For fifteen years, the British researcher Iain Couzin, from the Max-Planck Institute in Constance (Germany), thinks he knows the reason: cannibalism. During his moult, Dr Jekyll the cricket would become both Mr Hyde and Hannibal Lecter. In the huge cages of his laboratory, the researcher described the constant race of insects, running both to catch the one in front and avoid the one following.
But how does this curious dynamic work? Another German team, at the Max-Planck Institute in Jena, this time, decided to follow this path. In a published article in the review Scienceon May 5, she showed that by placing five migratory locusts (Locusta migratoria), or even twenty-five, in a 9.5 liter cage, they remained peaceful. But that at fifty the nymphs of stage 4 and 5 began to bite each other, a phenomenon even accentuated at 250 per cage. It should be noted here that locusts go through five juvenile stages – during which they walk, sometimes jump, but do not fly – interspersed with moults, before reaching adulthood.
On the trail of pheromones
But it is above all the volatile compounds emitted by the animal that these researchers specializing in the sense of smell of insects had in their sights. They thus found seventeen, produced by the nymphs during the gregarious phase alone. Three of these pheromones were already known to attract locusts. Thirteen others appeared neutral. The last one, phenylacetonitrile (PAN), was also an old acquaintance, since in locusts the males emit it to repel competitors during their antics.
The researchers therefore tested it. By genetically modifying a cricket so that it does not produce this pheromone, they have transformed the poor beast into immediate prey for its fellow creatures. Conversely, by suppressing the specific olfactory receptor of PAN in individuals, they began to attack their comrades indiscriminately, with or without PAN.
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