Sweet-Smelling Locust Pheromone Could Be Key to Stopping Their Swarms

Swarming locusts in Kenya, February 1, 2020.  (Image: Ben Curtis, AP)
Swarming locusts in Kenya, February 1, 2020. (Image: Ben Curtis, AP)

Scientists have identified the pheromone responsible for turning individual locusts into the swarming variety. They also found a way to “turn off” locusts’ ability to respond to this pheromone, in a breakthrough that could lead to new control strategies for preventing the spread of these voracious and extremely destructive insects.

Migratory locusts, or Locustia migratoria, are, quite literally, a blight of biblical proportions. Just as in the past, these swarming insects pose a significant threat to crops and food security around the world. Earlier this year, for example, hundreds of millions of locusts ravaged the Horn of Africa, damaging crops in Ethiopia, Kenya, Somalia, Sudan, Uganda, and other countries, threatening millions of people in the region with a food crisis.

The damage inflicted by these bugs isn’t slight. Locusts swarming across one square kilometre of land can consume the equivalent of what 35,000 people would eat in a single day, according to the Food and Agriculture Organisation of the United Nations.

Migratory locusts, a species closely related to grasshoppers, start their lives as solitary individuals. Their attraction to one another, however, leads to a snowball effect, in which small groups quickly expand into massive conglomerations consisting of billions of individuals. Interestingly, the pigments of migratory locusts change from green to black as they transition into their swarming mode (it’s the locust version of the Jolly Roger pirate flag, I suppose), and they begin to produce a noxious chemical that’s basically cyanide, which they use to ward off predators.

Like I said, biblical.

The Kenyan sky darkened by locusts on January 24, 2020. (Image: Ben Curtis, AP)

And as a new Nature paper shows, migratory locusts also produce a sweet-smelling chemical that causes them to swarm. The discovery of this chemical, an organic pheromone, is a big deal, as it could lead to new mitigation strategies to control the size of locust outbreaks. The new paper was led by Le Kang from the Chinese Academy of Sciences in Beijing, China.

Figuring that a scent of some sort was causing gregariousness in migratory locusts, the researchers analysed 35 different chemical compounds produced by these swarming insects. Only one, a pheromone named 4-vinylanisole (4VA), was found to do the trick. In lab tests, this small molecule was shown to pack a big punch, acting as a powerful social lubricant; the chemical’s attractive force brought locusts together regardless of age, sex, or their mode of life, whether solitary or social.

As subsequent lab tests showed, the presence of just four or five solitary locusts was enough to kickstart the process, in which the insects began to produce and emit 4VA. This process triggers the feedback effect in the wild, enabling a small group of migratory locusts to quickly increase in size, according to the new research.

Immature desert locusts in Somalia on February 5, 2020. (Image: Ben Curtis, AP)

Diving deeper, Kang and his colleagues identified the specific sensory cell in the locusts’ antennae that makes it possible for the locusts to detect the pheromone. These cells, called basiconic sensilla, have a specific olfactory receptor called OR35, which binds to 4VA.

Not content to stop there, the researchers conducted an experiment to see how migratory locusts might behave without the OR35 receptor. To do this, they used the CRISPR-Cas9 gene editing system to remove the gene that encodes for this receptor in locusts which, in theory, should remove their sensitivity to 4VA.

This theory was then confirmed in practice, as Leslie Vosshall, a researcher from the Medical Institute at Rockefeller University in New York who wasn’t involved in the new study, explained in an associated News & Views article:

These mutant locusts did not have antennal responses to 4-vinylanisole and were unable to detect the pheromone and respond behaviourally. This finding is exciting, because it indicates that a locust can be engineered to be immune to the effects of the pheromone. In principle, such an insect would not be expected to convert into the gregarious form.

Indeed, the new research could lead to any number of viable mitigation strategies to prevent the onset and spread of swarming locusts.

A possibility tested in the study was the setting of bait traps laced with 4VA, which in the future could be mass-produced in the lab in its synthetic form. In tests, this strategy resulted in the capture of dozens of locusts. That’s a promising result, but realistically speaking, the number of traps required to truly affect locust populations would have to be enormous; locust swarms can contain upwards of 4,000 million to 8,000 million individuals.

Other possible strategies include chemicals to impair the OR35 olfactory receptor or a gene drive to reduce the number of individuals capable of expressing the receptor. But as Vosshall pointed out, many unanswered questions remain:

It is not clear whether 4-vinylanisole is responsible solely for the initial aggregation of locusts, or whether it also triggers the pigmentation change and subsequent aggressive swarming behaviour seen after locusts gather. It is possible that the aggregation pheromone merely brings locusts together and that other, secondary mechanisms, and perhaps extra volatile, then induce further changes to the morphology and behaviour of the insect. Further investigation is needed to determine whether desert locusts also respond to 4-vinylanisole.

This paper is really promising, but future research is needed to fully explore this effect in locusts, to assess the feasibility of any proposed population mitigation strategies, and, of course, to evaluate the potential ecological implications of these strategies. Lots of work still needs to be done, but humanity’s battle against locusts will hopefully get a major upgrade soon.