Locusts are grasshoppers that change behaviour under crowding. Alone, they are shy and green. Crowded, they turn black and yellow, become highly mobile, and form swarms that can cover hundreds of square kilometres.

The desert locust is the most notorious. Its breeding grounds stretch from West Africa to India, and outbreaks follow heavy rains that allow eggs to hatch in numbers. A single swarm can hold billions of insects and eat its own weight in vegetation every day. The 2019–2021 upsurge in East Africa was the worst in decades, destroying crops in Kenya, Ethiopia, and Somalia.

How outbreaks are fought

• Monitoring by national locust units and the FAO
• Aerial spraying of concentrated insecticides on hopper bands
• Biopesticides such as Metarhizium acridum, a fungal pathogen
• Early warning systems using satellite rainfall data

Control works best before the swarm forms. Once hoppers take flight, options narrow and costs explode. That is why so much of the effort goes into watching deserts during the quiet years.