A single chemical may draw lonely locusts into a hungry swarm

Locusts are usually harmless loners. But in a mass, they become plagues. A swarm can contain hundreds of millions of locusts! These hordes can cross continents, eating through crops along the way. A new study has now found what may make a single locust decide to turn social.

Groups of locusts pump out a chemical, scientists now find. And it might explain how individuals of one locust species go from loners to crowd-lovers. The finding might also help scientists develop new ways of controlling or breaking up locust swarms. They might even be able to use those scents to bait the insects into traps.

When solitary locusts get together, they do more than hang out. They transform into what scientists call their “gregarious” form. That’s a fancy word for social. A gregarious locust is a larger, hungrier locust than before. And gregarious groups tend to grow bigger and bigger.

A swarm of desert locusts (Schistocerca gregaria) the size of the city of Rome eats as much food in a day as 51 million people. That’s the entire population of Kenya. This year, East Africa is experiencing its worst locust plague in decades.

Scientists weren’t sure what coaxed solitary migratory locusts of another species, Locusta migratoria, to congregate. But they suspected it might be chemicals known as aggregation pheromones (FAIR-uh-moans). Released by the insects, these airborne scents could act as an beacon. They could summon other locusts to morph into a gregarious swarm.

Le Kang is an an entomologist, someone who studies insects. He and his colleagues at the Chinese Academy of Sciences in Beijing scouted for aggregation pheromones. They started by identifying chemicals that gregarious locusts pump out. The team puffed six of these scents into arenas with lone locusts. They also tested control scents — those not made by gregarious locusts. The goal was to see if any of the six scents attracted the insects. One did. Called 4VA (for 4-vinylanisole), it drew in locusts of all sexes and ages, whether solitary or gregarious.

That’s important, says Baldwyn Torto, who was not involved in the study. As a chemical ecologist, he studies chemical interactions between organisms and their environment. He works at the International Centre of Insect Physiology and Ecology in Nairobi, Kenya. Attracting both loner and gregarious locusts, he says, means 4VA could function to both bring solitary locusts into the swarm and keep that swarm together over time.

Come together, right now

Solitary locusts start spewing 4VA once they gather in groups as small as four or five, Kang found. As group size grows, 4VA levels rise. That could broadcast a larger signal and contribute to an exponential growth, leading to swarms. 

Kang and his colleagues also confirmed 4VA can attract locusts in the real world. To test this, they baited sticky traps with the chemical and put them on artificial grass and a natural breeding area of migratory locusts in northern China. Traps with 4VA attracted more locusts than did traps without the pheromone. The effect was small, here. But the researchers tested the traps only on locusts a short distance away.

The researchers described their findings August 12 in the journal Nature.

“It’s a significant and exciting study,” says Torto. “This [compound] has potential.” But while 4VA is clearly a player, he notes, it may not be the whole story.

Chemical communication among insects isn’t always controlled by a single pheromone. Multiple compounds often work together. And this study didn’t look into that. He says there’s a chance that other odors of gregarious locusts could interact with 4VA. The combo might make intensify the call to turn social.

Still, the prospect of baited traps for locust control excites Torto. “We don’t have a good way of attracting locusts,” he says. Many regions manage outbreaks by dumping pesticides onto swarms from planes. This chemical dump can harm livestock and the environment. Traps laced with 4VA could concentrate locusts. When they’re all in one place, killing them might become easier. Such baits would be especially helpful if they attract other types of locusts, too, like the desert species.

This study also could help scientists change the locusts themselves. Kang and his colleagues identified the locust’s protein that detects 4VA. It’s nestled on specific sensory hairs that extend from the insect’s antennae.

Coming: Gene editing for swarm control?

A gene-editing tool known as CRISPR/Cas9 lets scientists tweak an organism’s DNA. Using CRISPR, Kang and his colleges were able to change locust DNA so that the insects no longer made a 4VA detector. Without it, the chemical no longer lured in the locusts.

A chemical that blocks antennae from sensing 4VA could be sprayed on locusts to prevent swarming, the researchers suggest. Or, scientists could engineer locusts so they don’t make the 4VA sensor. Such locusts would be less likely to swarm.

Any research that offers new ways to manage pests without poisons is very exciting, says Arianne Cease. She studies sustainability — how to use resources so they are available in the future — at Arizona State University in Tempe. Such technologies are still a ways off. CRISPR might have off-target effects, such as editing DNA that scientists don’t want to change.

Changing the DNA of a species might also change locusts in ways scientists don’t yet understand. Changes to a locusts’ genes would get passed on to any offspring. That might make any harmful changes permanent. People affected by locust swarms would need to weigh issues like these, before anyone edits wild-locust DNA.

Cease also questions whether turning off one gene would prevent swarms. Becoming a swarm involves a whole set of changes. Locusts change their behavior, body function and body size. Tweaking just one aspect of this may not prevent swarming, she says. “I’d be surprised if there were just one smoking gun.”

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