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Plague on the Prairie

Locusts have an apocalyptic quality. No other insect pest, perhaps no other animal, has the capacity for such remorseless destruction. Even those pests that, in aggregate, cause more economic damage, do not appear as a cloud that darkens the sun and devour every green thing in a matter of hours, leaving famine in their wake. And not just famine, but guilt; since the time of Pharaoh and Moses, they have been seen as the instrument of divine wrath: a rod with which the Lord chasteneth those whom he loves.

Jeffrey A. Lockwood's Locust is aimed at the general reader with an interest in science and natural history, and it tells the story of a mystery. Who killed the Rocky Mountain locust? In the latter half of the nineteenth century, the resourceful settlers embarked on their prairie schooners and headed west. The death rate among them was appalling. Cholera, smallpox, typhus, hostile Indians and drought took their toll. But the survivors, who reached the western states, Kansas, the Dakotas, Nebraska, Utah and Minnesota, and established their homesteads, were suddenly confronted with an enemy against which they had no weapon and which threatened the whole enterprise of settling the West of the United States. As one witness wrote:

The voracity of these insects can hardly be imagined by those who have not witnessed them, in a solid phalanx, falling upon a cornfield and converting, in a few hours, the green and promising acres into a desolate stretch of bare, spindling stalks and stubs.... Imagine hundreds of square miles covered with such a ravenous horde, and you can get some realization of the picture presented last year in many parts of Kansas.

Brigadier General E. O. C. Ord saved the lives of many settlers in 1874 by distributing Army rations. He wrote: "There is a famine prevailing in Western Nebraska and Kansas, and... probably thirty thousand persons and their animals are in danger of starving unless food be sent to them speedily."

Lockwood gives a systematic account of the impact of the locust plagues and the various responses that it evoked. By presenting them as the instrument of God's wrath, Brigham Young tightened his grip on the Mormons of Utah, who had been backsliding. Elaborate horse-drawn locust-killing machines were designed. Criddle's Mixture, consisting of Paris Green (an arsenical compound originally used as a pigment in wallpaper), molasses and horse manure, was developed as an early chemical insecticide. Locusts in their early wingless stages (nymphs) were driven into pits and buried. State governors called Days of Prayer, laws were passed, the Army was called out, and a Commission established.

The locust plague brought an extraordinary cast of characters to the fore: artists, preachers, as well as doctors turned entomologists. The stage was eventually dominated by one man, Charles Valentine Riley, the illegitimate son of an Anglican clergyman in London. After training as an artist in Germany, he emigrated to America, worked as a farmhand, fought in the Civil War and, on the basis of a childhood interest in collecting insects, was appointed state entomologist of Missouri. His beautifully illustrated reports contain almost everything we now know about the Rocky Mountain locust.

And then it was gone. In 1902, Norman Criddle, of the Mixture, collected two specimens of Melanoplus spretus, as the Rocky Mountain locust was known to science. This is the last record of an insect that had covered the West in swarms numbering billions. This extraordinary disappearance is the mystery that Lockwood sets out to solve. He takes us on a journey of investigation from the shores of the Caspian Sea to the glaciers of the Rocky Mountains.

Locusts are grasshoppers that occur in dense migrating swarms. There are about ten species worldwide and they are now found in all the inhabited continents except temperate North America (one species from Tropical Central and South America just extends into southern Mexico). They also characteristically occur in periodic plagues, disappearing during the intervening recessions. The discovery that was central to understanding how this happens, and from which all modern attempts to manage locust problems derive, was made by a Russian entomologist, Boris Petrovitch Uvarov, in the early years of the twentieth century. By an extraordinary piece of luck, for he would almost certainly otherwise have perished in one of Stalin's purges, he was plucked from the shores of the Black Sea by an officer in the British expeditionary force which was unsuccessfully trying to suppress the Bolshevik Revolution. He was then employed by the Imperial Bureau of Entomology in South Kensington, where he published the results of his researches on locusts in southern Russia. Uvarov became the leading world expert on locusts, and determined policy on locust control within the British Empire, ending his life as a Fellow of the Royal Society and a Knight Commander of the Order of St Michael and St George.

Observing an infestation of the Migratory locust, Locusta migratoria, Uvarov was surprised to find among it specimens of another grasshopper, quite different in shape, colour and behaviour, Locusta danica, which had not been present before the infestation. More surprising still was that there were intermediate forms between the two. From his detailed observations and the experiments of a colleague, Vassily Plotnikov, Uvarov deduced that the two species were one, with the capacity to transform itself from a sedentary, solitary grasshopper to a gregarious, migrating locust and back. The stimulus for this is physical crowding, which occurs in areas, now known as "outbreak areas", which may be very restricted, but have particularly favourable ecological conditions for the species. During recessions, the locusts survive in the solitary phase, but successful breeding leads to higher densities and transformation to the gregarious phase. The locusts then migrate in swarms to cause a plague in the much wider "invasion area". Experiments by the South African entomologist Jacobus Faure showed an identical phenomenon in the Brown locust, Locustana pardalina. Uvarov's theory not only provided an explanation of the periodicity of locust outbreaks, but also offered a strategy of preventive control by which the solitary phase could be monitored and transformation to the gregarious phase nipped in the bud. Unfortunately the account of Uvarov's work is the weakest part of the book.

Uvarov's theory thus allowed for the possibility that the Rocky Mountain locust was still holed up in disguise like, in Lockwood's words, "an ageing gang of bandits". Faure came to America to try to repeat his success with Locustana and re-create Melanoplus spretus in the laboratory by rearing the closely related species Melanoplus sanguinipes under crowded conditions. He convinced himself that Melanoplus spretus and Melanoplus sanguinipes were phases of the same species, but the evidence of his own data suggests otherwise. Modern biochemical techniques show pretty clearly that none of the presently recognized species of Melanoplus are the solitary phase of Melanoplus spretus. It appeared that the locust species that threatened the western states of the US with famine was truly extinct, and not through any deliberate control measures taken by man. There has been no shortage of theories, from poisoning by alfalfa to ecological changes caused by the extermination of the bison.

Research on the Rocky Mountain locust was rather inhibited by a lack of material. There are only 487 dried, pinned specimens remaining in museums. But the possibility of an inexhaustible supply of deep frozen specimens entombed in the glaciers of the Rocky Mountains themselves stimulated Lockwood to mount a series of expeditions to search for them. In spite of the retreat of the glaciers, most likely caused by climate change, he and his colleagues were able to find evidence that intermittent locust plagues had occurred for at least 800 years.

So what happened to the Rocky Mountain locust? Some ecological detective work enables Lockwood to offer a plausible explanation for its disappearance. The work of Riley and his colleagues had traced the origins of the swarms to a "Permanent Zone" in the Rocky Mountains. Lockwood argues that the outbreak area of the locust was a specific ecological zone within this, namely the well-watered valleys on the eastern slopes. This was precisely where the settlers chose to develop their farms. Moreover, they depended heavily on irrigation, which is known to be an extremely effective method of killing locust eggs. When the plague receded, as it had always periodically done, the locust's refuge was destroyed. The ecology of the outbreak area had been transformed by the plough. Even so, one would expect a few isolated populations to have persisted, and Jeffrey Lockwood teases the reader with the possibility that Melanoplus spretus may still persist in Yellowstone National Park. An identifiable specimen would set the seal on this remarkable piece of acridological detective work.

A. W. Harvey is an biologist who has worked on projects to control locusts and other migratory pests in Africa and the Middle East and is currently managing the UN coordinated locust control programme in Afghanistan. He has also carried out research on the swarming locusts of Central and South America.