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.

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.”