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Biblical plague explained? Israeli study suggests why locusts form massive swarms

Researchers find a bacterial species that proliferates in the insects’ guts when they join a group; the finding could lead to new means for combating outbreaks

Illustrative: Swarms of desert locusts fly up into the air from crops in Katitika village, Kitui county, Kenya January 24, 2020. (AP Photo/Ben Curtis)
Illustrative: Swarms of desert locusts fly up into the air from crops in Katitika village, Kitui county, Kenya January 24, 2020. (AP Photo/Ben Curtis)

Researchers at Tel Aviv University recently decided to address an ancient mystery: Why do locusts form destructive swarms?

Specifically, researchers were interested in understanding what causes the usually harmless and solitary insects to radically change their behavior and form huge migrating swarms — a question that has baffled both scientists and farmers.

The destructive swarms have ravaged crops and caused famine since ancient times.

In Exodus, locusts were the eighth of 10 plagues inflicted on Egypt, where the Israelites were enslaved.

“They will cover the face of the ground so that it cannot be seen. They will devour what little you have left after the hail, including every tree that is growing in your fields,” the text says.

The Tel Aviv University study found that a locust’s bacterial composition in its gut, called a microbiome, goes through drastic changes when the host insect joins a larger group.

Bacteria called Weissella, almost completely absent from the microbiome of solitary locusts, become dominant in the insect’s “gregarious phase,” when it swarms, the study found.

Researchers then used a specially developed mathematical model to track changes to the Weissella bacteria and found that swarming allowed the bacteria to spread and infect large numbers of locusts, a clear evolutionary advantage.

Solitary Male and female Locusts. (Keren Levi)

“Our findings do not prove unequivocally that the Weissella bacteria are responsible for the swarming and migration of locusts. The results do however suggest a high probability that the bacteria play an important role in inducing this behavior — a new hypothesis never previously proposed,” said Prof. Amir Ayali, who led the study.

The study may have significant implications for “countless people, animals, and plants all over the globe” still threatened by locust outbreaks, Ayali said. “We hope that this new understanding will drive the development of new means for combating locust outbreaks.”

Locust swarms can decimate crops and cause famines. In the past three years, swathes of Africa, Indian and Pakistan have been hard-hit by locust swarms.

The Desert Locust (Schistocerca gregaria) “is the most destructive migratory pest in the world,” according to Locust Watch, a division of the United Nation’s Food and Agriculture Organization.

“They are ravenous eaters who consume their own weight per day, targeting food crops and forage,” Locust Watch says.

A swarm has “the capacity to consume the same amount of food in one day as 35,000 people,” the group says.

A swarm entered Israel from Egypt in 2013.

The Tel Aviv University study was published last month in the peer-reviewed science journal Environmental Microbiology.

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