locust plagues -- 1/4/23

Today's selection-- from Cats vs Dogs. Whatever happened to the locust plagues that once swarmed the center of America?
"In the summer of 1873, black clouds drifted east from the foothills of the Rocky Mountains towards the newly settled farms of Nebraska, Iowa, Minnesota and the Dakotas. The pioneer families had no warning: the sky went dark at midday, the air filled with a sound like a thousand scissors. Then the clouds fragmented and locusts fell like hail onto crops of corn and wheat. In a few hours, the insects had devoured months of work. Locusts had been invading farms on the American frontier on and off for decades, but the irruption of the mid-1870s entered into legend. Many families gave up farming and fled to the cities. On 26 April 1877, John Pillsbury, the governor of Minnesota, called for a day of prayer to plead for deliverance from the locusts. A few days later, the insects rose up and left as inexplicably as they had come.

"When the Rocky Mountain locusts swarmed, they darkened the skies over vast swathes of the western and central USA, from Idaho to Arkansas. The number of insects was mind-boggling: one reliable eyewitness estimated that a swarm of locusts that passed over Plattsmouth, Nebraska in 1875 was almost 3,000 kilometres long and 180 kilometres wide. And they were devastating. 'You couldn't see that there had ever been a cornfield there,' one farmer said after a swarm passed through his land. Yet between these episodes of frantic fecundity, the locusts seemed to disappear.

"The Rocky Mountain locust (Melanoplus spretus), a big, beefy species of grasshopper, was considered the greatest threat to agriculture in the West. So entomologists tried to learn everything they could about the insects -- what trig­gered them to swarm, what they ate and how they reproduced. But after the spring of 1877, the locusts vanished and never plagued western farmers again. Within thirty years of Minnesota's official day of anti-locust prayer, the insect was extinct. The last live specimen was found by a river on the Canadian prairie in 1902.

"No one mourned the loss, and scientific interest in the locust waned. In the 1940s and 1950s, when farmers began to wage war on their enemies with insecticidal chemicals, a few researchers began to speculate about what could possibly have seen off the locust so spectacularly in those pre-pesticide days.

"During the disastrous outbreaks of the 1870s, farmers fought back with every tool they could find or invent. They deliberately set their fields on fire. They dragged tar-coated hunks of metal through the ground, hoping to trap locust hatchlings in the sticky goo. Nothing helped much. When desperate pioneer women tried to protect their vegetable gardens by draping blankets over them, the locusts ate the blankets before moving on to the vegetables.

"Whatever had done for the locust, it seemed, was some event far beyond the capabilities of nineteenth-century farmers. As the extinction coincided with a time of dramatic environmental change across the West, there were plenty of plausible explanations. Perhaps the locusts had depended on the fires that Native Americans had routinely set to keep the prairies open. Or maybe their most crucial habitat had been shaped by the huge herds of bison that were now all but extinct.

"Most standard entomology texts claimed that extreme fluctuations in population, like those that took place when the locusts swarmed, were a sign of a species in trouble, fighting to recover a balance with its environment. The sweeping changes that came with settlement, some scientists suggested, pushed the locusts through cycles of population explosion and collapse, and in the end wiped the species out.

"When Jeffrey Lockwood, an insect ecologist at the University of Wyoming, was hired to explore the biology of grasshoppers in 1986, the post-mortem on the Rocky Mountain locust had not gone beyond this sort of general speculation. Lockwood wanted to know more, and he hoped that somewhere there were still a few specimens of the long-vanished locust to study. Among the high peaks of the Rockies in Montana and Wyoming were glaciers where swarming insects had fallen, become immobilised by the cold, and died. Some of these glaciers might still hold frozen remains of the Rocky Mountain locust.

"Lockwood and his students spent summers searching in the ice at remote spots high in the Rockies. They began their hunt at Grasshopper Glacier in Montana, hoping it might live up to its name. Sure enough, they found some scattered body parts that might have once belonged to Rocky Mountain locusts, but without whole bodies there was no way to prove these bits had not belonged to some other, still living, species of grasshopper.

"'Finally, after four years of fruitless searching, we found the mother lode,' says Lockwood. On Knife Point Glacier in the Wind River Mountains of Wyoming, they recovered 130 intact bodies of Rocky Mountain locusts, the legacy of a swarm that had risen out of the river valleys of western Wyoming in the early 1600s. The antiquity of the frozen insects -- confirmed by radiocarbon dating -- proved that locusts had irrupted long before European settlers changed the face of the West. The reproductive frenzies, which at times produced enough insects to blanket the entire state of Colorado, were normal events in the history of the locust. Further study of Knife Point Glacier revealed deposits of locust parts throughout the layers of ice, indicating that swarms passed over the mountains at regular intervals during the centuries before the locust's extinction.

"To find more clues to what killed off the locust, Lockwood began to scour the scientific literature of the late 1800s. There he found the writings of Charles Riley, an entomolo­gist who had spent much of the 1870s and 1880s searching for ways to kill the locusts.

"Riley had mapped what he called 'the permanent breeding zone' of the locust, the territory where mating adults and eggs could be found every summer, regardless of whether the locusts were swarming. For a species that could spread across much of the continent during outbreaks, this home base was surprisingly small. Between swarms, the locusts lived only in the river valleys of Montana and Wyoming, where they buried their eggs in the damp ground along the banks of streams. These fertile spots were the same places the incoming settlers chose to farm.

"Riley had experimented with ways to control the number of locusts. Ploughing, he discovered, could push locust eggs so far down into the soil that they would fail to hatch. Flooding the ground where eggs had been deposited also killed many of the young. Riley concluded that agriculture itself -- the processes of ploughing and irrigation -- were the strongest weapons against the locust. But because less than 10 per cent of the land in the western USA was arable, he doubted that farming would ever have had a significant impact on the locust.

"A century after the locust disappeared, Lockwood took Riley's map of locust egg-laying areas in the 'permanent zone' and superimposed it on a map of land under cultiva­tion for corn, wheat or hay in 1880. He found that he had charted the geography of an extinction. In the 1880s, when the locust population had shrunk during an intermission between outbreaks, every corner of its breeding grounds was being farmed. The settlers, Lockwood suggests, had destroyed their nemesis without ever knowing it, simply by ploughing the land and watering their crops. 'The most spectacular "success" in the history of economic entomology -- the only complete elimination of an agricultural pest species -- was a complete accident,' he says." 



Cats vs Dogs: 99 Scientific Answers to Weird and Wonderful Questions about Animals


Nicholas Brealey Publishing


Copyright New Scientist 2020


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