For most of the year, and sometimes for several years in a row, the desert locust is an unremarkable insect. It lives in scattered numbers in the dry scrublands that stretch from West Africa across the Arabian Peninsula to north-west India, an area of around 16 million square kilometres. In this solitary form it is green or sandy brown, moves about mainly at night and actively avoids other members of its species. Yet the same insect is responsible for some of the most destructive outbreaks known to agriculture. When conditions change, solitary locusts can be transformed within a few generations into dense, mobile swarms of yellow and black insects that travel by day and feed on almost any green vegetation in their path. Scientists refer to these two forms as the solitarious and gregarious phases, and for a long time they were believed to be entirely separate species.

The process that turns one form into the other begins with rain. Locusts lay their eggs in moist sandy soil, and an unusually wet season allows desert plants to flourish and far more young insects to survive. As the land dries out again, the green patches shrink and the growing population is forced together on the remaining plants. It is this crowding, rather than the abundance of food itself, that sets off the change. Laboratory experiments showed that repeated contact on the hind legs is sufficient to start it: when researchers gently tickled that part of a solitary locust's body for a few hours, the insect began to seek out others rather than avoid them. A study published in 2009 linked the shift to a sharp rise in serotonin, a chemical messenger in the nervous system. When the action of this chemical was blocked, locusts remained solitary even when they were crowded together.

Changes in behaviour come first; changes in appearance follow. Crowded young locusts, known as hoppers because they cannot yet fly, develop the bright colours of the gregarious phase and begin to march across the ground together in bands that can stretch for several kilometres. The adults that emerge from these bands have longer wings in proportion to their bodies, which suits them to long-distance flight. Once airborne, a swarm may contain tens of millions of insects in every square kilometre and, carried by the wind, can cover 100 kilometres or more in a single day. Because each adult eats roughly its own weight in fresh vegetation daily, the quantities involved are enormous: according to the Food and Agriculture Organization of the United Nations (FAO), a swarm covering just one square kilometre can eat as much food in a day as 35,000 people.

Since swarms cross borders with ease, no single country can deal with them alone. For several decades a forecasting service based at the FAO's headquarters in Rome has gathered reports from national locust teams across the insect's range. Survey officers record the condition of the vegetation, the number and colour of the locusts they find and whether hoppers are beginning to form bands, and their observations are combined with satellite estimates of rainfall and soil moisture. The aim is to issue warnings early enough for governments to act while locusts are still concentrated in relatively small breeding areas, before any swarms have formed. This approach is known as preventive control, and its logic is largely economic: treating a few thousand hectares of hopper bands is far less expensive than pursuing swarms across millions.

Preventive control, however, depends on access. Many breeding areas lie in remote deserts, and some are in regions affected by conflict, where survey teams cannot travel safely. The upsurge that began in 2018 illustrated the problem. Two unusual cyclones brought heavy rain to an uninhabited desert in the south of the Arabian Peninsula, where several generations of locusts bred largely unobserved. From 2019, swarms spread into the Horn of Africa and beyond. Kenya experienced its worst outbreak for 70 years, and a number of countries that had not seen swarms for decades found that they had few trained staff and little pesticide in reserve.

Control itself has traditionally relied on chemical pesticides, sprayed from vehicles, aircraft or hand-held equipment. These act quickly but can harm other insects, birds and the people who apply them, and they are unsuitable for use near water sources or on farms that grow crops for export markets with strict limits on chemical residues. One alternative is a biological pesticide made from a fungus that infects locusts and grasshoppers but appears harmless to other animals. Its main drawback is speed: infected insects take between one and three weeks to die, and during that time they continue to feed. For this reason, the fungus is best suited to hopper bands in breeding areas, where there is time to wait, rather than to swarms that are already damaging crops.

Some researchers hope that a better understanding of the switch between the two phases will eventually make it possible to stop swarms forming at all. They suggest that substances interfering with the locust's own chemical signals might keep crowded insects in their solitary state. Others are more cautious. Any such treatment, they point out, would still have to be applied in the right place at the right time, which brings the problem back to where it started: finding the locusts before they find the crops.