The notorious ravenous hunger of the locusts can also be directed at conspecifics. That's why the insects have developed a protective mechanism, researchers report: In dense populations, they release an odor that suppresses the cannibalistic tendency. If the locusts lack the pheromone or the ability to perceive it, they will eat each other more intensively. The scientists say that the findings could have potential for the development of strategies to combat the notorious pests.
They can cause damage of biblical proportions: The book of Moses already 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 supply 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 of swarm formation. Because mostly these locusts live as unproblematic loners - only when they gang together do they unfold their catastrophic potential.
In the solitary phase, migratory locusts avoid contact with conspecifics and eat comparatively little. However, if the population density increases, the locusts can change their behavior within a few hours - with devastating consequences: They become sociable, travel together and develop an enormous appetite. It has already been shown that special odorants – aggregation pheromones – play a role in this process. It was also known that the insects sometimes attack their conspecifics in the dense populations. There are indications that this cannibalism plays a role in the migratory behavior of the insects: dynamics arise in the swarms because the insects are fleeing from their own species.
On the trail of that special 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 investigated the relationship between population density and the tendency to cannibalism in the Locusta migratoria species of migratory locust. It was shown that the behavior only starts from a certain number of animals in a confined space and intensifies with further concentration. However, they don't eat each other completely - it seems possible that a pheromone-based regulatory mechanism is used. Therefore, the researchers investigated whether the locusts emit certain scents that are not produced in the solitary phase. They recorded, compared and analyzed the volatile substances emitted by solitary and sociable experimental animals.
They came across an odorant called phenylacetonitrile (PAN). Behavioral tests initially suggested that it acts as a deterrent to locusts. "We were able to show that as the population density increased, not only did the extent of cannibalism 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 locust line that no longer produced PAN. "This allowed us to confirm the strong anti-cannibalistic effect, because cannibalism increased significantly when the animals were no longer able to produce the compound," says Chang.
Anti-cannibalistic sense of smell revealed
The team then went in search of the corresponding olfactory receptor for the PAN. After a long series of tests, it finally emerged that the olfactory receptor OR70a gives the insects the "nose" for the signal substance. Using genome editing again, the researchers then engineered a strain of locusts lacking this receptor. This confirmed the importance of this element: locusts without OR70a can no longer perceive the PAN in their conspecifics and are therefore more prone to cannibalism.
But can this knowledge now also be used practically for the fight against the "biblical" pests? Senior author Bill Hansson from the Max Planck Institute for Chemical Ecology says: "If you inhibit the production of PAN or the function of the receptor, you could maybe get the locusts to behave more cannibalically and in that way maybe to control yourself". Against the background of the possible role of cannibalism in the migratory behavior of insects, the importance of the system must first be clarified in more detail. Basically, however, it is apparent: "The anti-cannibalistic effect is very likely of great importance for the ecology of the locust populations and the results could therefore offer opportunities for locust management," write the scientists.
Source: Planck Institute for Chemical Ecology, specialist article: Science, doi: 10.1126/science.ade6155