When it comes to this odorous substance, locusts literally go into raptures: Researchers have discovered a pheromone that locusts emit in order to form armies that devour entire areas of land. The discovery could help develop strategies to fight the plague, the scientists say.
Every year they cause devastation according to biblical measurements: Huge swarms of migrating locusts haunt parts of Africa and the Middle East in particular and eat everything short and small in the fields. The swirling formations can cover vast areas and contain many millions of insects. So far, the only effective weapon against this threat to agriculture has been airborne pesticide spraying. But these measures are often insufficient and have harmful side effects. Better and at the same time gentler control strategies are therefore in demand.
What leads to swarming?
Opportunities to suppress swarming would be ideal, because it represents the basis of the catastrophic potential of insects. Migratory locusts begin their lives as unproblematic loners. But then the animals can suddenly mutate under certain circumstances: They begin to unite and form larger and larger groups. They also change their color from green to black. The growing units then show a synchronized behavior and finally form the huge armies that move through the landscape, buzzing and munching.
But what triggers this transformation from loner to swarm animal? Researchers have long assumed that an odorous substance – a so-called pheromone – plays an important role in this. But so far no molecule has been identified that meets the criteria for such an aggregation pheromone. As part of their study, the researchers led by Xiaojiao Guo from the Chinese Academy of Sciences in Beijing have now examined 35 compounds from the migratory locust species Locusta migratoria be delivered. They first carried out experiments to check whether one of these substances influenced the behavior of the animals.
It turned out that a volatile substance called 4-vinyl anisole magically attracted male and female grasshoppers in both the juvenile and adult stages of development. It also became clear that the concentration of the substance in the air increased as the density of the grasshopper groups increased. It was thus suggested that 4-vinyl anisole played a role in triggering the positive feedback loop that leads to the accumulation of locusts. In a further experiment, the scientists were able to show what is at the beginning of the process: When four to five grasshoppers come into close contact with one another, the production of the pheromone is triggered.
An aggregation pheromone with potential
Next, the scientists investigated how the locusts perceive 4-vinyl anisole. By examining the reactions of the animals’ antennae, they were able to show that certain sensory hairs react to the odorous substance. Then they set out to find the specific olfactory receptor protein that is responsible for this reaction. They found evidence that the receptor called OR35 is strongly and selectively activated by 4-vinyl anisole. To support this, the scientists then used Crispr / Cas9 technology to generate a locust line that lacks the gene for the formation of the OR35 odor receptor. Investigations of these animals then showed: their antennae no longer reacted to the 4-vinyl anisole and thus the effects on the behavior of the locusts were also absent.
Overall, the results show that the substance is actually an aggregation pheromone. However, there are still some questions that need to be clarified. For example, how exactly 4-vinyl anisole works and whether cofactors still play a role in swarm formation. In addition, further studies must show how widespread this substance is as a pheromone among the various species of migratory locusts.
However, as the authors point out, there is considerable potential in the discovery for developing strategies to combat the insect pests. Synthetically produced 4-vinyl anisole could be used to lure grasshoppers into traps in order to decimate their population. In addition, promising agents appear to be those that block the odorant’s receptor or its activity. This could possibly prevent the locusts from joining together in the devastating swarms, the scientists write. So it will be interesting to see if this approach can one day put this ancient plague of humanity in its place.