Why Do Locusts Swarm? How Grasshoppers Turn Into Locusts

A locust is a grasshopper that changes in crowds

There is no separate family of insects called locusts. A locust is a short-horned grasshopper that can switch between two forms, or phases, depending on how many of its own kind are around. Only about 20 of the more than 10,000 grasshopper species do this. The best known is the desert locust of Africa and southwest Asia.

In the solitary phase, a locust behaves like any other grasshopper. It is camouflaged, avoids other locusts and, in the desert locust, flies mostly at night. In the gregarious phase it is boldly colored, is attracted to other locusts, and is active by day. Scientists call this ability density-dependent phase polyphenism: one set of genes, two very different ways of living.

Step 1: rain after drought

Swarms start with weather. Desert locusts survive dry years in small, scattered numbers. When unusually heavy rain falls in their breeding areas, the soil stays moist enough for eggs, and fresh plants appear for the young hoppers. Breeding speeds up, and numbers can rise about 10 to 16 times in a single generation.

As long as there is plenty of green vegetation, the locusts stay spread out. The trouble begins when the desert starts to dry again and the plants shrink back into small patches. The growing population is then squeezed together, and each locust bumps into others all day long.

Step 2: touching hind legs releases serotonin

The key trigger is touch. Experiments with desert locusts showed that repeated stroking of the hind legs, the kind of contact that happens in a crowd, makes solitary locusts act gregariously. The sight and smell of other locusts add to the effect.

In 2009 a team of researchers from the UK and Australia published a study in the journal Science showing that this contact causes a rise in serotonin, a signaling chemical in the locust’s nervous system. When they blocked serotonin, crowded locusts stayed solitary; when they gave serotonin to isolated locusts, they started acting gregariously. The behavioral switch can happen within a few hours.

Step 3: new colors, new shape, new behavior

Once locusts behave gregariously, further changes follow over days and generations:

The bold yellow and black colors of gregarious hoppers may warn predators that the insects can be distasteful after eating certain plants. Mothers also pass the gregarious state to their offspring through chemicals added to the foam around the eggs, so the next generation hatches already primed to crowd together.

From hopper bands to flying swarms

Young gregarious hoppers cannot fly, so they gather in bands that march across the ground, sometimes for kilometers. Movement inside a band is partly driven by fear of being eaten: hungry locusts bite the locusts in front of them, and those in front keep moving to avoid being bitten.

After their final molt the adults have wings, and bands become swarms. A typical desert locust swarm holds about 40–80 million locusts per square kilometer. Swarms fly by day, drift with the wind and usually travel about 100–150 km (60–95 mi) a day. During the 1987–1989 plague, swarms even crossed the Atlantic from West Africa to the Caribbean.

Why swarming helps the locusts

Swarming looks destructive, but for the locusts it is a survival strategy. Desert rains are patchy and unpredictable. By leaving a drying area together and riding the wind, a swarm has a good chance of reaching places where rain has recently fallen, where it can breed again. Moving in a huge group also protects each individual, because predators can eat only a tiny share of the locusts.

The cost is enormous competition and some cannibalism, but the payoff is the ability to exploit fresh plant growth across a vast area. An adult locust eats about its own weight in plants every day, as explained in what do grasshoppers eat?

How swarms end

Plagues do not last forever. When the rains fail, breeding slows, and swarms that fly into unsuitable areas such as cold mountains, the open sea or dense rainforest die out. Control operations in affected countries also reduce numbers. As the population thins, locusts meet less often, and the next generations gradually return to the solitary phase.

Locust swarming is different from swarming in honey bees, where a colony splits in two to reproduce. You can compare the two in why do bees swarm?

Myths and records

A popular belief holds that locusts and grasshoppers are different animals. In fact, a solitary desert locust would be called a grasshopper by almost anyone who saw it. Another myth is that swarms appear from nowhere; in reality they build up over several generations after good rains, which is why the UN Food and Agriculture Organization (FAO) can forecast them using rainfall data and field surveys.

The largest swarm ever estimated was formed by the Rocky Mountain locust in North America in 1875. Contemporary estimates suggested about 12.5 trillion insects covering roughly 510,000 square kilometers (198,000 square miles), though the figure is a rough 19th-century calculation. Remarkably, the species had vanished by 1902, probably because farming destroyed its breeding grounds in Rocky Mountain valleys.