Locust upsurge

Why does a scattering of harmless solitary insects become a plague crossing continents in a single season?

In an ordinary year the desert locust is a shy green grasshopper, thinly scattered across the dry belt from West Africa to India, actively avoiding the company of its own kind. Two years later, swarms are crossing the Red Sea and stripping fields in Kenya.

The tempting reading is that something arrived. But it is the same species, Schistocerca gregaria, with the same genome it had when it was harmless. So let me ask you the awkward version of the question: what would have to be true of an animal for it to have two completely different careers without becoming a different animal?

Start with the arithmetic, because it does more work than people expect.

A female lays her eggs in a pod of roughly eighty, buried in damp sand, and lays several pods in her life. Not many survive. The FAO's working rule of thumb, drawn from decades of field survey, is that under good conditions a locust population multiplies about twentyfold from one generation to the next, and a generation takes something like three months.

So follow it. Twentyfold once is 20. Twice is 400. Three times — one year of favourable weather — is 8,000. A thousand insects nobody would report become eight million.

Now the question that matters: why is this not happening constantly? What is the limiting term? Eggs need moist soil; hoppers need green vegetation to eat. In a desert, both arrive only with rain, and rain there is rare and patchy. Most of the time the multiplier is well below one, and the population is quietly shrinking. The exponential is always running. Only its sign changes.

That reframes the puzzle. A plague does not need a new mechanism; it needs an unusually long run of wet seasons in the right places. In 2018 two cyclones, Mekunu and Luban, dumped rain into the Rub' al Khali of the Arabian Peninsula — terrain almost nobody surveys. Roughly three generations bred there undetected, which is exactly the 8,000-fold arithmetic above, before swarms appeared in the Horn of Africa in 2019.

But numbers alone would give you a lot of grasshoppers, not a swarm. Something else has to happen. Do you notice what rain does after it stops?

It concentrates. As the flush of vegetation dries back, the surviving green patches shrink, and the insects that were spread over the desert are pressed together onto them. And density — not abundance, not heat, not hunger — is the trigger for the second mechanism.

Crowding is sensed mechanically. Repeated contact on the outer face of the hind femur, sustained for a couple of hours, is enough to flip a solitarious locust into gregarious behaviour; serotonin in the thoracic ganglia was identified as the chemical carrier of that switch (Anstey and colleagues, 2009). Behaviour changes within hours. Colour and body shape take longer, and mothers pass a gregarizing factor into the foam around their eggs, so the next generation starts part-way across already.

Here is the part I would ask you to sit with. Solitarious locusts avoid each other. Gregarious ones are attracted to each other. So crowding produces gregarious insects, and gregarious insects produce crowding. That is a second self-amplifying loop, running not on numbers but on aggregation — and it converts a large population into a single moving object. Bands of hoppers march; adults form swarms that ride the wind up to about 150 km in a day, at densities of tens of millions per square kilometre. A swarm covering one square kilometre eats in a day roughly what 35,000 people would.

Attributing any particular upsurge between the two loops is genuinely hard, because the early generations are the ones nobody sees.

Two corrections are worth making out loud.

First, locusts are not a separate species that appears in bad years. Solitarious and gregarious are two phases of one animal, and an insect can travel between them in either direction. That is called phase polyphenism, and the desert locust is its textbook case.

Second, and more practically: the swarm is not the event. By the time a swarm arrives over a farm, the multiplication that produced it happened two or three generations earlier, hundreds of kilometres away, on ground that was green at the time. This is why locust control is organised as continuous desert survey rather than as emergency spraying, and why the FAO vocabulary separates an outbreak (local), an upsurge (several regions, growing), and a plague. Effort scales with the population you must kill, and that quantity is the one growing twentyfold per generation while the decision is being made.

The same arithmetic explains the ending, too. Plagues collapse quickly because when the rains fail the twentyfold multiplier inverts, and a declining exponential is just as fast as a rising one.

• How the FAO's Desert Locust Information Service turns rainfall and vegetation imagery into forecasts of where to survey.
• Why gregarious hopper bands march at all, and the role cannibalism pressure plays in keeping a band moving.
• Whether biopesticides such as Metarhizium fungus can be deployed early enough to matter, given how fast the multiplier compounds.

Every term the collection defines is gathered in the glossary.