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Ancient Locust Swarm Crossed the Atlantic, Study Says

Stefan Lovgren
for National Geographic News
December 28, 2005
 
Somewhere between three to five million years ago a giant swarm of
locusts left West Africa and flew across the Atlantic Ocean to colonize
the New World, a new study shows.

Using DNA evidence, researchers have reconstructed the evolutionary relationships among insects of the genus Schistocerca, a diverse group of locust species found throughout the Americas, Africa, and Asia.

"Our results show that the first Schistocerca species to evolve was the African desert locust," said Nathan Lovejoy, an assistant professor at the University of Toronto at Scarborough in Canada.

"This suggests that Schistocerca originated in Africa and somehow dispersed across the Atlantic."

How the locusts made that trans-Atlantic journey remains a mystery, however, since the insects can't store enough fat to power a trip that would last several days.

African Migration

A single African desert locust (Schistocerca gregaria) is harmless to humans. But when the insects congregate in one place where food is abundant, their look and social behavior alters in a phenomenon known as phase change.

Phase change causes the locusts to swarm over vegetation, behavior that has wreaked havoc on crops in Africa and the Middle East for centuries.

A 2004 locust outbreak in West Africa caused significant crop losses, contributing to a food shortage in Niger earlier this year. (Read "Food Crisis In Niger Will Strike Again, Experts Say.")

Scientists studying this economically infamous insect have long wondered why the closest relatives of the desert locust are found in the Americas and not in Africa.

A 2004 study led by Hojun Song, a graduate student at Ohio State University, used locust anatomy to reconstruct the evolutionary history of the insects. Song's work suggested that the desert locust colonized Africa from North America.

Lovejoy and his colleagues used DNA sequences from 20 Schistocera species to make a similar reconstruction. The National Geographic Society and the National Science Foundation funded the team's work.

But their results imply a different theory: The African desert locust was the very first Schistocera species to evolve.

"This suggests that the origin of the genus was in Africa, and that the other [more than 50] species [found in the Americas] are the product of a dispersal from Africa to the Americas," Lovejoy said.

The researchers also used DNA sequences to estimate when this dispersal occurred. "Our best guess is that it happened three to five million years ago," Lovejoy said.

Rafts of the Dead

But how did the locusts get all the way across the ocean?

"Swarms take flight during the day, increasing the possibility that thermal updrafts will carry the insects to high altitudes where they can be transported by fast-moving, upper-level wind currents," Lovejoy said.

"Despite this, we know locusts simply don't have enough stored fat to sustain flight long enough to cross the Atlantic," he said.

One possibility is that among the many millions of swarming locusts were a few truly exceptional insects with sufficient fat stores to somehow survive the trip.

But in October 1988 a swarm of desert locusts crossed the Atlantic traveling from Africa to the Caribbean. Island residents said the locusts were flying in huge swarms and came in multiple waves—apparently ruling out this "few lucky individuals" hypothesis.

Another possibility is that locusts flying at the front of the swarm may have became exhausted and died in the ocean, forming floating mats of dead insects. Other members of the swarm could have landed on these mats.

"Locusts are quite cannibalistic, so it seems very likely that they could have fed upon the corpses below, thereby obtaining enough energy to sustain additional flight," said Greg Sword, a research ecologist with the U.S. Department of Agriculture.

Sword co-authored the study, which will appear in the January 2006 issue of the Proceedings of the Royal Society of London.

Because a single swarm can contain billions of locusts, it could create a series of "rafts of the dead" and still contain enough live insects to reach the Americas in large numbers.

Sword interviewed people in Barbados who said they saw piles of locusts washing up on beaches for days in 1988, consistent with the idea that the surviving locusts could have rested on such rafts.

"Although it seems extraordinary, this is perhaps the most likely hypothesis" to explain how the locusts could cross the Atlantic, Sword said.

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