A desert locust is a shy green grasshopper that spends most of its life alone in the sand. Crowd it for two hours and its brain chemistry flips: it turns yellow and black, seeks out its own kind, and joins a flying mass that can darken 2,400 square kilometres of sky and eat a nation's daily bread before dusk.
Solitary and swarming locusts look so different that entomologists once filed them as separate species. They are the same insect in two settings. Crowding — jostling, specifically against the hind legs — triples serotonin in the thoracic ganglia, and the animal rewires its colour, shape, appetite and social behaviour.
With enough rain and green vegetation, a locust population multiplies roughly twenty-fold each generation, and a generation takes about three months. Four generations — one calendar year — turn a manageable infestation into a plague. The vertical scale below is logarithmic; on it, catastrophe is a straight line.
Multiplication rates and the 160,000× annual figure: FAO Locust Watch. The 8,000× marker is the real increase estimated across three generations in Arabia's Rub' al Khali after cyclones Mekunu (May 2018) and Luban (October 2018) filled the Empty Quarter with lakes and vegetation — in terrain too remote to survey or spray.
In recession — the normal state — desert locusts live in arid country receiving under 200 mm of rain a year, a belt running from Mauritania through the Sahara and Arabia into western India. In plague, that belt roughly doubles and spills across a fifth of the Earth's land surface.
Each adult locust eats about two grams a day — roughly its own body weight. Multiply that by the insects in one square kilometre of swarm and you get the daily food of about 35,000 people. In January 2020 Kenya recorded a swarm measuring up to 60 km by 40 km. Do the multiplication and the comparison stops being entomology.
People-equivalents of one day's food. The swarm bar is derived, not measured: 2,400 km² (60 × 40 km, FAO field report) × 35,000 people per km² (FAO). At the lower density of 40 million locusts/km² that swarm holds roughly 96 billion insects eating about 192,000 tonnes of vegetation a day. National populations are World Bank 2020 estimates, rounded.
The 2019–21 upsurge landed on a region already in drought. The World Bank's conservative estimate of what uncontrolled swarms could cost East Africa, Djibouti and Yemen in 2020 alone — staple crops, livestock production, damaged assets — reached USD 8.5 billion. That number never fully materialised, because of what came next.
Every USD 1 invested in at-scale, risk-informed desert locust control averted an estimated USD 15 of losses in the greater Horn of Africa (FAO, 2023).
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