Targeting migratory locust cannibalism –

The notorious predatory hunger of locusts can also be directed at certain species. This is why the insects have evolved a protective mechanism, the researchers report: In dense populations, they release a scent that wards off cannibalism. If the locusts lack the pheromone or the ability to perceive it, they will eat each other intensely. Scientists say the findings could have the potential to advance strategies for controlling the notorious pest.

They can cause damage of biblical proportions: the Book of Moses indeed describes how huge swarms of locusts darkened the sky, and when one of the ten plagues devoured everything that grew in the fields. In this way, migratory locusts continue to threaten the food supplies of millions of people in Africa and Asia. Because of this importance, scientists have been studying these insects for some time. The focus is on their complex interaction behavior, which is the basis for swarm formation. Because these locusts mostly live as solitary, non-trouble-making units – only when they band together do they reveal their catastrophic potential.

In the solitary stage, migratory locusts avoid contact with certain species and eat relatively little. However, if population density increases, locusts can change their behavior within a few hours – with disastrous consequences: they become social, travel together and develop enormous appetites. It has already been shown that special scents – collecting pheromones – play a role in this process. It was also known that insects sometimes attack their own species in dense populations. There are indications that cannibalism plays a role in insect migratory behavior: dynamics emerge in swarms as insects flee their own species.

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On the trail of that own appetite

A German-Chinese research team has now devoted itself to further research into the cannibalistic behavior of migratory locusts and the possible role of odorous substances. First, the scientists studied the relationship between population density and propensity for cannibalism in the migratory locust species of migratory locust. The behavior was shown to only start with a certain number of animals in a confined space and get more intense with more concentration. However, they don’t completely eat each other – it seems possible that a pheromone-based regulatory mechanism was used. Therefore, the researchers investigated whether locusts emit specific odors that are not produced in the solitary stage. They recorded, compared and analyzed the volatile substances emitted from solitary and sociable experimental animals.

They come across an odor called phenyl acetonitrile (PAN). Behavioral tests initially indicated that it acted as a deterrent to locusts. “We were able to show that with increasing population density, not only did the size of the cannibals increase, but the animals also produced more PAN,” says lead author Hetan Chang from the Max Planck Institute for Chemical Ecology in Jena. In order to study the function of PAN more closely, the researchers then used genetic engineering methods to develop a line of locusts that no longer produced PAN. “This allowed us to confirm the strong anti-cannibal effect, because cannibalism was greatly increased when the animals could no longer produce the compound,” says Chang.

Detection of the anti-cannibal sense of smell

Then the team went in search of the olfactory receptor corresponding to the PAN. After a long series of tests, it was finally shown that the olfactory receptor OR70a gives insects the “nose” for the signaling substance. Using genome editing again, the researchers then engineered a strain of locust that lacked this receptor. This confirmed the importance of this element: locusts without OR70a can no longer perceive PAN in their specific species and are therefore more susceptible to cannibalism.

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But can this knowledge be used now in practice to combat “biblical” pests? “If you turn off PAN production or receptor function, maybe you can make the locusts act in a cannibalistic way and in that way maybe control yourself,” says senior author Bill Hanson from the Max Planck Institute for Chemical Ecology. Against the background of the potential role of cannibalism in insect migratory behavior, the importance of the system must first be elucidated in more detail. Fundamentally, however, it is clear: “It is very likely that the anti-cannibal effect is of great importance to the ecology of locust populations, and thus the findings could provide opportunities for locust management,” the scientists wrote.

Source: Planck Institute for Chemical Ecology, specialized article: Science, doi: 10.1126/science.ade6155

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