Why do locusts swarm? © Getty Images

Why do locusts swarm?

We’ve teamed up with the folks behind BBC World Service’s CrowdScience to answer your questions on one topic - this week it's all about why locusts swarm.

Locusts have caused famine and starvation for as long as humans have been farming. Swarms can appear suddenly, and proceed to decimate crops over huge areas. Since June 2019, one of the worst locust plagues in decades has been devastating parts of East Africa and surrounding regions, consuming up to 1.8 million tonnes of vegetation a day.

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Locusts are actually a group of short-horned grasshoppers. They are usually solitary, fairly bland-looking insects, but when conditions are right, they can switch into a ‘gregarious’ mode, becoming social, multicoloured eating machines that sweep across the landscape in swarms of up to 80 million locusts per square kilometre.

This swarming behaviour is triggered by high rainfall. When there’s plenty of lush vegetation for the wingless nymphs (called ‘hoppers’) to feast on, their numbers swell, and the insects are no longer able to avoid each other. The sight, smell and touch of other locusts causes a flood of serotonin in their brains, which in turn causes genes that control their gregarious phase to switch on, and ‘solitary’ genes to switch off. The result is a Jekyll-to-Hyde transformation. The gregarious nymphs form into large bands, before taking to the air once they reach their winged, adult stage. As they swarm, any solitary locusts they meet swiftly join the throng.

Why locusts evolved to swarm in the first place is still not fully understood, but a 2008 study suggested that it’s because swarms close the gaps between separate patches of locusts, and this makes it harder for predators to move among them and pick the insects off.

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