The Terrifying Science Behind the Locust Plagues of Africa
Feb 5, 2020 8:00 AM
With hundreds of billions of locusts tearing through farmland, it's the worst outbreak to strike the region in decades.
Photograph: Tony Karumba/Getty Images
Tearing across East Africa right now is a plague of biblical proportions: Hundreds of billions of locusts in swarms the size of major cities are laying waste to the crops in their path. It's the worst outbreak in 25 years in Ethiopia. In Kenya, make that the worst in seven decades.
Fueling the locusts' destruction is a bounty of vegetation following unusually heavy rains. All that food means the landscape can support a huge number of rapidly breeding insects. And the problem is about to get a lot worse—the insect population could boom by a factor of 500 by June. The Food and Agriculture Organization of the UN is calling the situation in the Horn of Africa "extremely alarming," and estimates that a swarm covering one square kilometer can eat as much food in a day as 35,000 humans. Farmers throughout East Africa now face food shortages, as the plague consumes both crops in the field and in storage.
Locusts are actually special kinds of grasshoppers known for their gregariousness, and not in a good way. Around 20 species of the 7,000 known grasshopper varieties transform into what's known as a gregarious phenotype, which means their bodies actually change as they socialize into swarms. Normally solitarious (a word that locust biologists made up, by the way), they change color and grow bigger muscles as they gather into massive clouds, rolling across landscapes and devastating crops. "They have this sort of Dr. Jekyll and Mr. Hyde switch," says Arianne Cease, director of the Global Locust Initiative at Arizona State University.
(The kind of desert locust currently plaguing East Africa is in fact named for this tendency to socialize: Schistocerca gregaria.)
But why does the desert locust go gregarious, when the vast majority of grasshopper species remain solitarious? That might have something to do with the dry environments these species call home. Desert locusts only lay eggs in moist soil, to keep them from drying out. When heavy rains come in to saturate the desert, locusts—ever the opportunists—breed like mad and fill the soil with their eggs, perhaps 1,000 per square meter of soil. When those eggs hatch, they'll have plenty of vegetation to eat, until things dry up once again.
As soon as things start getting crowded, desert locusts become gregarious and migrate away in search of more food. "If they were to stay locally, one potential is that there are too many of them and they would run out of food," says Cease. "And so they migrate to find better resources." By doing so in swarms, the locusts find safety in numbers—any individual is less likely to get eaten. But for farmers in surrounding countries, the locusts' newfound mobility can spell ruin.
To adapt to this new social life, the locusts' bodies transform, inside and out. They change color from a drab tan to a striking yellow and black, perhaps a signal to their predators that they're toxic. Indeed, while solitarious locusts avoid eating toxic plants, the gregarious locusts are actually attracted to the odor of hyoscyamine, a toxic alkaloid found in local plants. Sure, by eating those plants and assuming their toxicity and changing color to yellow and black, the insects make themselves more conspicuous, but that isn't such a big deal when there's millions of them barreling across a landscape—no one's trying to hide. Being bright and alone, especially in a barren desert, probably isn't a good strategy for the solo locust, so they stay drab.
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And speaking of food, you might assume that to fuel their epic migrations—an individual locust might travel over 90 miles in a day, consuming its own weight in plant matter—the insects would need to load up on protein, especially since their new bodies come with extra muscle mass. To put it in human terms, says Rick Overson, research coordinator of the Global Locust Initiative, "If your friend told you that they were going to become a vegan, one concern you might have for them is to make sure to get enough protein."
But that's not how locusts seem to operate. Cease and Overson have shown that for South American locusts, at least (they haven't yet done field tests on desert locusts in Africa), it's more about loading up on carbohydrates, especially as they're transforming into their gregarious phenotype.
And it's precisely this physiological quirk that turns a locust swarm into a plague: These swarming grasshoppers love grains, a staple of the human diet. This is particularly threatening to farmers with depleted soils, because overgrazed lands tend to harbor more carbohydrate-rich species—grasses in particular are sapped of their protein as nitrogen washes out of overworked soil. It all but guarantees a swarm is going to make itself at home on somebody's farm. "Going back to the Bible and the Koran, humans have perceived themselves as passive victims of these locust swarms that appear from nowhere and darken the skies," says Overson. "And this connection to nutrition sort of illuminates a different dimension to this, in that we might be more active players as humans in the complex dynamics of locusts swarming."
Water, another critical factor of locust biology, also helps explain why things are so bad in Africa right now. In 2018, the heavy rains that locusts crave came with two cyclones, in May and October, that made landfall in nearly the same place in the southern Arabian Peninsula. The May storm alone dumped enough water for desert vegetation to grow for six months, which is long enough for two generations of locusts to appear and explode their populations—fast. "Mind you, there's an exponential increase of about 20-fold for each generation," says Keith Cressman, senior locust forecasting officer with the UN's Food and Agriculture Organization. "So that means after six months—since each generation is three months—you've got about 400-fold increase." Then the October cyclone added several more months' breeding time.
This insect population boom unfolded in the remote deserts of Oman, far from humans who might see the growing threat. Cressman's organization, the FAO, helps coordinate a vast network of human observers and satellite data to forecast locust plagues. All told, the network includes operators from two dozen front-line countries between West Africa and India with national locust control programs, patrolling the wilds in trucks, looking for the first sign of trouble. Everyone's in touch, monitoring in real time, coordinating with Cressman at the FAO headquarters in Rome.
But this outbreak eluded the monitoring network. "Nobody knew what was going on because this was just in one of the most remote places on this planet," says Cressman. "There's nothing there—there's no roads, no infrastructure, no Facebook, no anything. All you have is towering sand dunes that are as tall as skyscrapers."
It wasn't until observers found locusts in southern Oman at the end of 2018 that Cressman could raise the alarm. The following January, the region started to dry, and you can imagine how things went from here. Like armies in search of conquests, locust populations started spreading north into Iran and south into Yemen in search of food. "As those weeks kept continuing, and more and more swarms are coming out of that area, you start to appreciate the magnitude of what was in that area to begin with," says Cressman.
Yemen, ravaged by war, no longer had the means to deploy the specially trained crews that spray common pesticides that kill the insects in a matter of hours. (It's too dangerous for farmers and other regular folks to spray the pesticides themselves.) Then, catastrophically, heavy rains hit the country, providing yet more breeding opportunities for the invading locusts. Early last summer, the plague jumped the gulf and landed in Somalia, then continued its march into Ethiopia and Kenya.
In an ideal world, Cressman and his colleagues would catch and quash the threat early. They can project where the locusts might head over a month ahead of time, and alert those countries to mobilize their forces—distributing pesticides from a central repository, prepositioning aircraft for aerial control operations, and readying the professional locust hunters. "A locust plague is much like a wildfire," Cressman says. "If you can find it when it's just a tiny campfire, and you can put it out, you're good. No problem." But if they can't detect and obliterate the locust plague early, it will grow and grow, really only stopping when the swarm runs out of food.
Once the pesticide operation begins, people occupying infected lands have to vacate for 24 hours until the chemicals break down. And if the pesticides aren't sprayed precisely, other insects in the environment become collateral damage. A new biocontrol method, though, is showing promise, says Cressman: The killer fungus Metarhizium acridum, which only torments locusts and grasshoppers, could more selectively target the menace.
It's a menace that may only grow stronger, because locusts will likely be winners on a warming planet. They need a lot of vegetation to fuel their swarms, and that requires rain. The highly active cyclone seasons the past few years may be a sign of things to come. Warmer seas spawn more cyclones, and more cyclones—especially sequential ones that give locusts wet soils to breed in as they march across the landscape—could mean more locusts.
On the climatic flip side, locusts are highly adapted to a life of heat and drought: The Global Locust Initiative's experiments have shown that Australian plague locusts can survive up to a month without water. So while other species struggle to adapt to a rapidly-warming planet, the locusts will have an advantage both in their heat-tolerant physiology, and potentially from a decrease in competition from less fortunate insects. "If climate change does accelerate aridification and temperature—as it's predicted to do in many areas—it would be very easy to imagine that some locust species could expand their range," says Overson, of the Global Locust Initiative. "For the desert locust, this would increase the already daunting geographic area that needs to be monitored."
If these are the end times, Planet Earth certainly isn't being subtle about it.
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Matt Simon is a science journalist at WIRED, where he covers biology, robotics, cannabis, and the environment. He's also the author of Plight of the Living Dead: What Real-Life Zombies Reveal About Our World—And Ourselves, and The Wasp That Brainwashed the Caterpillar, which won an Alex Award.
TopicsBiology
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