Why locusts swarm

IN SOME PARTS of the world, covid-19 is not the only plague that 2020 has brought. In parts of Asia and east Africa, swarms of locusts have stripped fields. The UN reckons the swarms in India and Pakistan are the largest for a quarter of a century, and that the numbers in Kenya are the highest for 70 years. One swarm in northern Kenya was estimated to be 25 miles (40km) long and 37 miles wide.

Locusts are usually inoffensive, solitary creatures that do not stray far from the place that they were born. But under the right circumstances—namely heavy rain, and a subsequent boom in plant growth—they can become “gregarious”. When that happens the insects change colour and gather in ravenous swarms which can fly more than 100km in a day.

In a paper published in Nature, Xiaojiao Guo, of the Institute of Zoology in Beijing, and a group of other researchers, shed light on part of the biochemical machinery that drives that transformation. They think they have identified the specific pheromone that attracts the insects to each other, and thus causes them to swarm.

Dr Guo and her colleagues collected 35 chemicals collected from the bodies and faeces of the migratory locust, the most widespread species. Six of those showed significantly higher production among gregarious locusts than among solitary...