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Newswise — Current thinking about the desert locust, the swarming African insect that destroys crops and wreaks havoc on economies, has been that it originated in the western hemisphere and migrated to Africa. A new genetic study of this insect of biblical fame says it's actually the other way around. Like humans, this voracious species of locust probably came out of Africa.
In a paper in the January '06 issue of the Proceedings of the Royal Society of London, lead authors Sean Mullen, University of Maryland post-doctoral fellow, and Nathan Lovejoy, an assistant professor at the University of Toronto, Scarborough, indicate that these locusts -- Schistocerca -- originated in Africa. From there they probably migrated to the western hemisphere, between three and five million years ago, by flying over vast expanses of Atlantic Ocean.
The findings reverse the conclusions of an earlier morphological study (Song 2004), which tried to determine why there is only one species of these locusts in the eastern hemisphere, when all of its more than 50 species of relatives live in the west. That study theorized that the desert locust colonized Africa from North America.
But some aspects of that theory didn't sit right with Mullen and his co-researchers. "Past outbreaks demonstrate that these locusts can fly great distances," Mullen says. "They can fly for hours and hours in lab studies, and we know that in 1988, a swarm of the desert locusts flew from Africa to the Caribbean " a distance of over 4500 kilometers. In addition, it is widely recognized that the prevailing wind currents blow from Africa towards the west."
By comparing a portion of the mitochondrial genome of 20 locust species from different geographic locations, the researchers found that, indeed, it was the desert locust lineage that gave rise to the western hemisphere species.
A Bug of Biblical Proportions
By itself, the desert locust is pretty harmless. But, like teenagers, things change dramatically when they get into a crowd. When they congregate in one place where the food is abundant, the locusts go through what's called a phase change. That's when the trouble starts.
"Normally, they are solitary creatures," Mullen explains. "But when they become densely populated, they become gregarious. They actually go through developmental and behavioral changes because of the physical contact."
First, their looks change. A soft green when it's on its own, the desert locust turns a more ominous dark brown and yellow in a crowd.
Then their behavior changes. The harmless loner becomes a monster when millions " even billions -- of them break out into a swarm that can literally block the sun.
"There can be 80 million locusts in a square kilometer," says Mullen. "The swarms can be hundreds of kilometers wide, with billions of locusts."
Fortunately, most of the western hemisphere species do much less damage than the desert locust and rarely form large swarms. "This may be due to environmental differences," says Mullen, "but more work needs to be done to clearly demonstrate a link between swarm formation and environmental conditions during development."
The study was funded by National Geographic and the National Science Foundation.
Read the paper
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