Scientists have identified the scent that makes some locusts swarm. They could use the pheromone to trap and kill the insects.

• Large locust swarms have been destroying crops in parts of Africa and Asia this year, which could cause millions of people to go hungry.
• These insects alternate between solitary and social phases.
• Scientists recently figured out what prompts locusts to swarm: a specific pheromone, or chemical scent.
• That trigger could be the key to controlling and killing locust swarms.

A single locust is just bigger than a paper clip.

But when these solitary critters attract others into a growing swarm, billions of locusts wind up flying together, forming a moving carpet that can block out the sun and strip the landscape of plants and crops.

Giant swarms like this have devastated large swaths of crops in Africa and Asia since January, threatening food supplies for millions.

But until now, scientists weren't sure what causes the insects to come together and abandoning their solitary lifestyles. A study published Wednesday in the journal Nature pinpointed the trigger: Migratory locusts respond to a pheromone called 4-vinylanisole (4VA).

4VA is specific to that one type of locust, but the finding could offer a way to control many devastating swarms, including those wreaking global havoc this year. The study authors suggest using 4VA to corral locusts into areas in which they can then be killed en masse with pesticides.

A chemical klaxon

The migratory locust is the most widely distributed locust species on the planet. Like all locust species, these insects can follow one of two paths as they mature: some become solitary creatures, while others gather together in a cohesive mass. Locusts can also transition from solitary to gregarious creatures at any point during their life cycle.

Scientists had long thought this change in lifestyle might be prompted by a pheromone put out by other locusts. Yet until the discovery of 4VA, they hadn't figured out what that chemical klaxon was.

"This study has found the long-anticipated but never-before-described aggregation pheromone that is responsible for bringing solitary locusts together and turning them into gregarious, dangerous swarming locusts," Leslie Vosshall, a neurobiologist who wrote an article accompanying the new study, told Business Insider.

The study authors found that 4VA was equally attractive to male and female migratory locusts, as well as juveniles and adults. Their results also showed that as the density of a locust swarm grew, the amount of 4VA in the air "increased markedly," Voss wrote. That could explain why swarms, once they start, gather more and more solitary locusts over time.

Additionally, the researchers found that once four or five solitary locusts crowd out together, they begin to produce and emit 4VA.

As a substance, 4VA would smell sweet to humans, according to Vosshall.

Trapping locusts with their own scent

Vosshall added that pesticides "are the only effective weapon for fighting such locust plagues," but the size and unpredictability of the swarms force governments and farmers to spray it unnecessarily far and wide.

These chemicals can harm other animals, plants, and possibly people.

But the discovery of 4VA could facilitate a more surgical approach to fighting swarms: The study authors suggest deploying a synthetic version of the scent to lure locusts into traps where they can be killed.

They tried this on a small scale, baiting sticky traps with 4VA. They trapped dozens of locusts.

Another option, they wrote, might be to figure out ways to stop locusts from detecting 4VA at all.

Locusts detect the pheromone via their antennae; the molecules attach to an olfactory receptor. So the researchers genetically engineered locusts to lack that receptor, and found that the mutant locusts were less attracted to 4VA than their wild counterparts.

Based on those findings, the authors think "anti-VA" chemicals could be developed to obstruct the locusts' olfactory process.

"Such molecules could be widely deployed to prevent locust aggregation, in effect making the locusts blind to their own scent," Vosshall said.

'This year is a plague year'

In her article, Vosshall described how heavy rains has helped birth a growing population of swarming desert locusts in Africa and Asia.

"This year is a plague year," she wrote.

Although the desert locust is not the species she studied, Vosshall said that it nonetheless "seems very likely that what was learned from the migratory locust can be applied to the desert locust."

"Probably 4VA is not the aggregation pheromone for these locusts, but this paper provides the blueprint for finding it," she added.