Locust

• 1. Introduction

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  The locust is a grasshopper with a second life, and in 1875 that life produced a swarm known as Albert's swarm. It was estimated to contain 12.5 trillion insects. It covered about 198,000 square miles, an area larger than California, and was thought to weigh 27.5 million tons. The insects behind it belonged to a species of the American West. Yet a locust is not a separate animal from a grasshopper in any strict sense. Many species spend most of their lives as quiet loners that farmers barely notice. Then something changes them into a devouring crowd. What flips that switch inside the insect? Where in the world do these crowds form, and what have people done when they arrived? And how did the greatest swarm-maker of North America disappear altogether?

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• 2. The Switch

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  The word locust comes from the Latin locusta, which could mean either a locust or a lobster. In biology it names a behavior rather than a family tree. Locusts are short-horned grasshoppers of the family Acrididae that are able to swarm. That ability evolved separately in at least 18 genera spread across 5 subfamilies.

  Boris Uvarov first described the change while studying the migratory locust in the Caucasus. Before his work, the solitary and crowded forms had been counted as two species, Locusta migratoria and L. danica. Uvarov named the two phases solitaria and gregaria. He later helped set up the Anti-Locust Research Centre, and Charles Valentine Riley and Norman Criddle also played parts in understanding and controlling these insects.

  The trigger is touch. When crowding increases contact with the hind legs, serotonin levels rise in the insect. The locust changes colour, eats far more, and breeds more readily. Several contacts per minute over a four-hour period are enough to start the transformation. Among desert locusts, meeting one another releases serotonin that makes the insects attracted to each other, and swarming cannot begin without that attraction.

  The crowded form is not just a darker version of the loner. Gregarious nymphs show strongly contrasting yellow and black markings, grow larger, and take longer to mature. The adults have different body proportions, less difference between the sexes, and higher metabolic rates. They begin reproducing earlier than solitary adults, though they lay fewer eggs.

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• 3. Bands and Swarms

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  By the second instar, gregarious nymphs are already drawn toward one another. Within a short time they form bands of many thousands, and the band behaves like a single animal. It moves across the landscape, mostly downhill, curling around barriers and merging with other bands. Sight and smell hold the insects together, and the bands appear to steer by the sun. They stop to feed, then press on, and can cover tens of kilometres over a few weeks.

  The forward push has a grim mechanism. In young desert locusts, individuals bite the rear of the insect ahead, which prompts it to move. When researchers cut the nerve that lets a locust feel those bites, the group moved less and cannibalism rose.

  Adults keep the same bond. A locust that drifts away from the swarm flies back into it, and stragglers rise to rejoin it when it passes overhead. As the front of the swarm settles to feed, those behind fly over and land in turn. The whole mass rolls forward with an ever-changing leading edge. It lingers on the ground until the vegetation is gone, then flies a long way to a place where passing rain has produced fresh green growth.

  A big swarm can hold billions of insects over thousands of square kilometres, at densities of up to 80 million per square kilometre. Specialists use three terms for the stages. The first gregarious bands are an outbreak, their merging is an upsurge, and regional upsurges from separate breeding grounds make a plague. During the desert locust plague that ran across Africa, the Middle East, and Asia in 1966-69, numbers grew from two billion to 30 billion over two generations, while the area they occupied shrank from over 100000 square kilometres to 5000.

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• 4. Locusts Around the World

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  Swarming grasshoppers live on every continent except Antarctica. The desert locust, Schistocerca gregaria, is probably the best known, thanks to its range across North Africa, the Middle East, and the Indian subcontinent. From 2003 to 2005 it overran much of western Africa after unusually heavy rain. The first outbreaks came in 2003 in Mauritania, Mali, Niger, and Sudan. Swarms then moved north to Morocco and Algeria and threatened croplands. They even reached Egypt, Jordan, and Israel for the first time in 50 years. Handling the infestation cost an estimated US$122 million, and crop damage reached as much as $2.5 billion.

  Across Australia the main swarmer is the Australian plague locust, Chortoicetes terminifera. The migratory locust, Locusta migratoria, is sometimes split into as many as 10 subspecies. It swarms in Africa, Asia, Australia, and New Zealand, though it has grown rare in Europe. In March 2013 the Madagascan form reached plague status, with many swarms of over a billion insects covering about half of the country.

  In the Sahel, the Senegalese grasshopper and the African rice grasshopper often behave like locusts and change in body form when crowded.

  North America is the only sub-continent besides Antarctica with no native locust. The High Plains locust reached plague proportions in the American Midwest during the Dust Bowl of the 1930s. It is rare now, so the continent has no regularly swarming species.

  The lineage is old. A fossil wing from Early Oligocene sediments of the Pabdeh Formation in Iran appears to belong to a locust crossing the ancient Paratethys Sea. That points to ocean-crossing locust flights going back at least 30 million years.

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• 5. Plagues in Scripture

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  Egyptian artists carved locusts into tombs made between 2470 and 2220 BC. Few insects have left such a long paper trail. Locust plagues appear in the Book of Exodus, the Indian Mahabharata, and the Quran. The Iliad describes locusts taking to the wing to escape a fire, and the Book of Revelation imagines human-headed locusts. In the ninth century BC, Chinese authorities went as far as appointing anti-locust officers.

  The records agree on one point. The insects came without warning, often after a change in wind or weather, and the damage was severe.

  The Greek philosopher Aristotle studied locusts and their breeding. The Roman historian Livy recorded a ruinous plague at Capua in 203 BC. He also noted epidemics among people afterward, which he blamed on the stench of rotting locust corpses. Many writers made the same link between locust plagues and human disease.

  A pestilence in the northwestern provinces of China in 311 AD killed 98 percent of the local population and was blamed on locusts. The true cause may have been a rise in rats, whose fleas spread disease after the rats fed on locust carcasses.

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• 6. Later Plagues

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  Chinese records list 173 locust outbreaks across 1924 years. Elsewhere, desert locust plagues came and went across Africa, the Middle East, and Europe over two millennia. The Bombay locust, Nomadacris succincta, was a major pest in India and southeastern Asia during the 18th and 19th centuries. It has seldom swarmed since its last plague in 1908.

  In the spring of 1747, locusts reached the countryside outside Damascus and ate most of the crops and plants. A local barber named Ahmad al-Budayri recalled that they "came like a black cloud. They covered everything: the trees and the crops. May God Almighty save us!"

  The worst human toll came in the First World War era. A locust infestation across Palestine and Syria in 1915 was one of the main causes of the famine in Lebanon from 1915 to 1918, in which around 200,000 people died. Plagues grew less common in the 20th century, but they still appear whenever the conditions line up.

  The most puzzling story is one of absence. The last Rocky Mountain locust seen alive was in Canada in 1902. Recent research points to the insect's breeding grounds in the Rocky Mountain valleys. A flood of gold miners brought sustained farming there, and that development destroyed the eggs the locusts had laid underground.

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• 7. Watching and Fighting

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  For most of history, people could do little to shield crops from locusts. By the early 20th century they were cultivating soil where eggs lay, collecting hoppers with catching machines, burning them with flamethrowers, trapping them in ditches, and crushing them with rollers. In the 1950s the organochloride dieldrin proved extremely effective as an insecticide. It was later banned in most countries because it lingered in the environment and built up in the food chain.

  Spraying from tractors can kill hoppers early, but it is slow and labour-intensive. Ultra-low volume spraying, which uses concentrated pesticides applied in overlapping swathes from aircraft or specialised ground equipment, covers large areas quickly. GPS, GIS tools, and satellite imagery now help plan the work.

  In 1997 the multinational LUBILOSA team tested a biological alternative across Africa. Dried spores of the fungus Metarhizium acridum are sprayed in breeding areas. They pierce the locust's outer shell, grow inside the body, and kill it. The fungus passes from insect to insect and persists, so repeat treatments are unnecessary. In 2009 Tanzania used it on around 10,000 hectares of Iku-Katavi National Park, and the local elephants, hippopotamuses, and giraffes were unharmed.

  Monitoring decides how well any of this works. Acting early is far more successful than acting after swarms build up, but organisational, financial, and political problems get in the way. Morocco and Saudi Arabia can afford to kill bands before they take wing. Their neighbours Mauritania and Yemen lack the resources and may breed swarms that threaten the whole region.

  Several bodies issue forecasts. The Australian Plague Locust Commission works well partly because Australia is a defined area with no invasions from elsewhere. The International Locust Control Organization serves Central and Southern Africa. A Food and Agriculture Organization commission coordinates desert locust work in West and Northwest Africa, and the FAO also watches the Caucasus and Central Asia, where over 25 million hectares of farmland are at risk. In February 2020, India decided to use drones and special equipment to track locusts and spray insecticides.

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• 8. Table and Laboratory

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  The Torah forbids most insects as food but allows certain locusts, namely those that are red, yellow, or spotted grey. The New Testament says John the Baptist lived in the wilderness on locusts and wild honey. Some have argued the text meant a vegetarian food such as carob beans, but the Greek word akrides plainly means locusts. The Prophet Muhammad was reported to have eaten locusts on a military raid with his companions, and Islamic jurisprudence deems them halal.

  Today locusts count as a delicacy in many African, Middle Eastern, and Asian countries. In the Haouran region, poor farmers during famine removed the head and guts before eating them, while Bedouins swallowed them whole. In 2014, consumption spiked around Ramadan in Saudi Arabia's Al-Qassim Region, because many Saudis believed them healthy. The Saudi Ministry of Health warned that pesticides made them unsafe. Yemenis eat them too and objected to government plans to spray them. ʻAbd al-Salâm Shabînî recorded a locust recipe from Morocco.

  Nutritionally, locusts are treated as meat and are often fried, smoked, or dried. They yield about five times more edible protein per unit of feed than cattle. Their feed conversion rate is 1.7 kilograms per kilogram, against roughly 10 for beef. Adult locusts hold 13 to 28 grams of protein per 100 grams fresh weight, while beef holds 19 to 26. A 100-gram serving of desert locust has 11.5 grams of fat, of which 53.5 percent is unsaturated, and 286 milligrams of cholesterol.

  The insects are also useful in science. They are large and easy to breed, so they serve in evolutionary biology and in testing whether findings from fruit flies and houseflies hold more widely. Their robustness suits school laboratories. At Tel Aviv University, scientists have borrowed the sharp sense of smell in locust antennae to detect odors in robotic applications.

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