Researchers have identified the pheromone that prevents massive swarms of migratory locusts from devouring each other.
Until now, scientists have long suspected that cannibalism amongst locust swarms contributes to their swarming behaviour, with individuals constantly on the fly to evade members of the same species (called conspecifics) eager to take a bite out of them.
A new study in the journal Science has identified a pheromone named phenylacetonitrile, or PAN, that locusts produce under crowded conditions to deter other locusts, and the olfactory receptor that recognises it.
Because cannibalism has a major impact on locust swarm dynamics, these finding opens up new possibilities for controlling the insects that wreak havoc on crops and threaten food security.
“If you inhibit the production of PAN or the function of the [PAN] receptor, you could get the locusts to behave more cannibalistically and potentially control themselves in that way,” says study leader Professor Bill Hansson, Director of the Department of Evolutionary Neuroethology at the Max Planck Institute, Germany.
Why do locusts swarm?
Under certain circumstances, migratory locusts become more abundant and experience changes in their morphology and behaviour that lead to swarming.
“In most cases, locusts are in the solitary phase, where they avoid physical contact with conspecifics and eat comparatively little food,” explains first author Dr Hetan Chang, a postdoctoral researcher at Max Planck Institute for Chemical Ecology.
“If the population density increases due to rainfall and sufficient food, the locusts change their behaviour within a few hours; they can smell, see, and touch each other. These three types of stimulation increase serotonin and dopamine levels in the locust brain, causing solitary locusts to become aggressive gregarious locusts that are very active and have a large appetite.
“They also release aggregation pheromones, which eventually leads to swarming and poses a huge threat to agricultural production. Cannibalism does only occur in the gregarious phase.”
Pheromones are chemical signals secreted by organisms for communicating with members of the same species. They can trigger an immediate effect on behaviour or can affect physiology and produce an effect after a period of time.
“We wondered how these insects influence each other’s behaviour within huge swarms, and whether olfaction plays a role,” says Hansson.
In a series of behavioural experiments with the migratory locust Locusta migratoria, the team showed that the rate of cannibalism increased with the number of gregarious locusts kept together in a cage.
By analysing and comparing all of the odours emitted by solitary and gregarious juvenile locusts, they identified that, of the 17 odours produced solely in in the gregarious phase, only PAN deterred other locusts.
“We showed that as population density increased, not only did the level of cannibalism rise, but the animals also produced more PAN,” says Chang.
“Using genome editing, we were able to knock out an enzyme responsible for the production of this compound. This allowed us to confirm its strong anti-cannibalistic effect, because cannibalism was again significantly increased when the animals were no longer able to produce the compound.”
The most challenging part of the whole process was actually identifying the olfactory receptor responsible for recognising PAN – since locusts have more than 140 olfactory receptor genes. Tests on 49 different olfactory receptors, using more than 200 relevant odours, eventually revealed the olfactory receptor OR70a.
Genetically modifying locusts so that their OR70a receptor was no longer functional resulted in a much greater rate of cannibalism because they could no longer perceive the anti-cannibalism signal.
In their paper, the researchers suggest that “the system is very likely to be of major importance in locust population ecology, and our results might therefore provide opportunities in locust management”.