What Is a Locust Swarm? A Shy Insect That Changes Personality

The change between the solitary and gregarious forms is so complete that the two were classified as separate species until 1921, when Boris Uvarov showed they were the same insect. The trigger is mechanical: repeated touching of the hind legs, which happens when population density rises and individuals are forced together on the remaining patches of vegetation after rain. That stimulus releases serotonin in the thoracic nervous system, and an increase of around threefold within a couple of hours flips the behaviour from avoidance to attraction. The physical changes follow more slowly and partly across generations, since a gregarious female produces eggs primed for the gregarious form, transmitted through a chemical added to the egg foam. The reverse transition happens when density falls, which is why a plague ends. The discovery of the serotonin mechanism, published in 2009, is one of the clearest examples of a single neurochemical switching a behavioural phenotype.

The sequence from harmless insects to continental emergency follows a recognisable progression:

• •Unusually heavy rain in normally arid breeding areas produces vegetation and damp sand, which locusts need for laying
• •Breeding success rises sharply, and because a generation takes only a few months, numbers can multiply roughly twentyfold per generation
• •Vegetation dries out as the rain ends, concentrating the growing population onto shrinking green patches, which supplies the crowding that triggers the change
• •Wingless young form marching bands that move across the ground together, which is the stage at which control is most effective
• •Adults form flying swarms, which travel with the prevailing wind, up to a hundred and fifty kilometres a day, crossing seas and deserts
• •A swarm of a square kilometre contains around forty to eighty million insects and eats roughly the quantity of food that thirty-five thousand people would, and swarms have exceeded a thousand square kilometres

The control problem is one of geography and timing. The recession areas where desert locusts persist between plagues cover about sixteen million square kilometres across thirty countries, much of it remote, sparsely populated, and in several cases politically unstable or actively at war, which is where surveillance is weakest and where outbreaks are consequently missed. Control depends on finding populations while they are still on the ground and treating them with insecticide from vehicles or aircraft, since spraying a flying swarm is far less effective. That requires functioning national locust units, trained survey teams, serviceable vehicles and stocks of pesticide, all of which decay during the decades between plagues when nothing appears to be happening, which is the chronic underfunding problem the specialist agencies describe. Newer approaches include a biological pesticide using a fungus that infects only grasshoppers and locusts, which is slower than chemicals and usable in environmentally sensitive areas, and satellite monitoring of soil moisture and vegetation that identifies where breeding conditions have developed.

Plagues have been recorded for as long as there has been writing, appearing in Egyptian, Mesopotamian, Chinese and biblical sources, and they have caused famine repeatedly. The last major desert locust plague before recent years ran from 2003 to 2005 across West and North Africa, affected more than twenty countries, and cost over half a billion dollars to control against crop losses estimated at around two and a half billion. The upsurge beginning in 2018 originated with unusually heavy rains and two cyclones in the Arabian peninsula, spread to the Horn of Africa where several countries had no effective control capacity, reached Kenya at a scale not seen for seventy years, and moved on into Iran, Pakistan and India, arriving in the middle of a pandemic that restricted the movement of pesticide and personnel. Other species cause comparable problems elsewhere, including the Australian plague locust and the migratory locust across Africa and Asia. The Rocky Mountain locust of North America, which formed the largest swarm ever recorded in 1875, estimated at over three hundred thousand square kilometres, went extinct within thirty years, apparently because ploughing and irrigation destroyed the river valley habitat where it bred between outbreaks.

Desert locusts exist in two forms of the same species: a solitary green insect that avoids others, and a gregarious yellow and black swarming form, and the switch is triggered by physical contact on the hind legs raising serotonin within hours. Heavy rain in arid breeding grounds produces rapid multiplication, and the subsequent drying concentrates the population and flips it. A square kilometre of swarm holds tens of millions of insects and eats what thirty-five thousand people would, and control depends on treating them before they fly, across sixteen million square kilometres of frequently inaccessible territory.