The ravenous swarm stretches as far as the eye can see. It has no commanding general or strategic plan; its only goals are to eat, breed, and move on— a relentless advance that transforms pastures and farms into barren wastelands. These are desert locusts— infamous among their locust cousins for their massive swarms and capacity for destruction. But these insects aren't always so insatiable. In fact, most of the time desert locusts are no more dangerous than garden-variety grasshoppers. So what does it take to turn these harmless insects into a crop-consuming plague?
Desert locust eggs are laid in the damp depths of desert soil, in arid regions stretching from North Africa to South Asia. During the dry weather typical in these ecosystems, desert locusts live a solitary lifestyle. Adolescent hoppers will spend a few lonely weeks foraging for plants, before growing wings, reproducing, and dying. But when a region receives an abundance of rain, the scene is set for a startling transformation. Increased moisture supports more vegetation for newly hatched hoppers to eat, leading large groups to feed in close proximity. The frequent contact stimulates their leg hairs, triggering the release of a hormone that causes them to actively cluster even closer. Gluttonous crowds of locusts produce huge amounts of poop, which carries a pheromone that furthers their transformation. The hopper’s diet shifts to include plants with toxic alkaloids. Soon, the locusts take on a striking pattern that warns predators of their newly poisonous nature. Smaller groups merge into bands of millions, which mow down virtually all plant life in a kilometer-wide swath. Roughly every week they shed and expand their exoskeletons, growing to roughly 50 times their hatching weight in just one month. Finally, the metamorphosis is complete. The adults beat their translucent wings and take flight as a full-fledged locust swarm. In this gregarious phase, these long-winged, brightly colored creatures appear so different from their solitary counterparts that they were long thought to be a separate species.
A typical swarm contains more locusts than there are humans on the planet, covering hundreds of square kilometers in a dense cloud. At these numbers, desert locusts easily overwhelm their predators. A large swarm can match the daily food intake of a city of millions, and flying with the wind, the insect invasion can travel up to 150 kilometers a day. This living tornado can also cross large bodies of water. In 1988, a swarm even managed to traverse the Atlantic Ocean. The locusts likely formed rafts to rest at night, before fueling up in the morning with a nourishing breakfast of their dead kin. While flying over land, they seek out moist soil to lay eggs. Swarming mothers transfer their gregarious condition to their offspring, making it likely that the next generation will form another swarm. This means that while an individual desert locust lives only three months, a plague can last up to a decade.
The potential for a years-long plague isn’t unique to desert locusts, but the region they inhabit makes the prospect particularly deadly. Their habitat spans some of the world’s poorest countries, largely populated by people who grow their own food. By consuming crops and pastures, these insects directly endanger 10% of humanity.
Fortunately, a desert locust plague doesn't last forever. When a wet period ends, vegetation becomes scarce and egg laying conditions decline. As existing swarms die off, new hatchlings spread out in search of food, creating enough distance to prevent solitary locusts from transforming. Human intervention can also help. Researchers use satellite imagery to identify regions at risk of becoming locust hotspots and alert local governments. While most countries fight back with chemical insecticides, some regions have found success using fungal diseases that are lethal to locusts but safe for people and the environment. Unfortunately, other modern practices are exacerbating the threat. Fields densely packed with a single crop are like a table set for locusts. And erratic weather caused by climate change makes swarms harder to predict. If we plan to discourage lonely locusts from becoming catastrophic crowds, humans need to cut carbon emissions, rethink our agriculture, and generally reconsider our own ravenous appetites.
