Locust
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When Larry DeBrey came into my office a few weeks after our fateful discussion regarding monarch butterflies and locusts, I could tell that he’d struck gold. He’d found a map showing the western lands under cultivation in 1880. The blotches representing croplands in the Rocky Mountain region were squarely within the Permanent Zone—the overlap was nearly perfect. The vast, windswept steppes of Idaho, Montana, and Wyoming were bare of wheat, corn, and hay fields. The pioneer farmers were courageous souls, but they were no fools. They avoided the alkali flats and sagebrush steppes, settling in the montane valleys. High above, the snowpack fed the perennial streams that cascaded down the flanks of the Rockies, joining together in the foothills to sculpt broad, sandy river courses that trapped fertile soil in their meandering bends.
But even with this coincidence of locusts and humans in time and space, the coarse scale of the map couldn’t reveal whether these ancient adversaries had actually collided. Was it possible that the locust population that gave rise to the swarms that blanketed two-thirds of a continent could be so concentrated as to succumb to the invasion of a few thousand pioneers? Surely even the most worn-out laggards of the plague from the 1870s would occupy far more land than a ragtag bunch of farmers could destroy. However, I’d learned that what seemed obvious was not always so.
It was time to let the numbers speak for themselves. How much land would the Rocky Mountain locust be likely to occupy between outbreaks? To begin, our best estimate of the peak population of the locust during a typical outbreak was somewhere around 10 trillion insects. The challenge, however, was to work backward from this population to estimate the number of individuals that would have been lying low between outbreaks. Based on the fecundity of the locust and natural mortality rates, it was safe to say that these creatures had the capacity to increase their numbers by 10- to 100-fold in each generation of an outbreak. So, let’s assume that the population had increased at these rates over the course of three generations, which was the typical period from the onset to the apex of an outbreak cycle. Given this range of reproductive rates, the result would have been 1,000 to 1 million times more locusts at the peaks than in the valleys—both mathematically and geographically. Working backward, then, we can estimate that a plague of 10 trillion locusts originated from a population of 10 million to 10 billion individuals.
From what we know of modern-day locusts, grasshoppers are a sensible surrogate for the solitary phase of spretus. And so, if we assume that the Rocky Mountain locust in its recession areas lived in a manner similar to that of existing rangeland grasshoppers, we can generate a reasonable approximation of the land area required to support a few million or billion creatures. On productive pastures and grasslands similar to those occupied by spretus in the Permanent Zone, grasshoppers typically occur at a population density of about one individual per square yard. Much higher densities are found during outbreaks, but we’re interested in the mundane times between these biological spectacles. Thus, a population of 10 million locusts would have required an area of about 2,000 acres, and a population of 10 billion locusts would have needed a couple million acres. With this line of reasoning, we can estimate that the actual habitat of the Rocky Mountain locust during its recession periods was as little as 3, and as much as 3,000, square miles. In other words, all of the individuals would have comfortably fit into a circle of land between 2 and 60 miles in diameter. Of course, this is quite a range, and we know that the montane river valleys were not laid out in neat circles, nor were the locusts uniformly distributed. However, no matter how we parcel out the locusts and their habitats, the implications are the same—this species was squeezed into a tight ecological bottleneck.
The startling conclusion that emerges from these mathematical extrapolations is that the Rocky Mountain locust was extremely vulnerable to even small-scale human disturbances. But this inference suggests that the locust was far more concentrated than would be apparent from maps of the Permanent Zone, shown to cover parts of five states and provinces. Further ecological evidence gleaned from the historical record supports the notion that the insect was restricted to very particular conditions within the region—habitats that corresponded with the arable lands.
I was not the first to recognize that the immense swaths of land representing the Permanent Zone on Riley’s maps were an extremely crude depiction of the Rocky Mountain locust’s habitat. In 1959, when Ashley Gurney was struggling to explain the locust’s disappearance, he noted, “Within the [Permanent] Region, spretus did not breed everywhere, but instead it did so in favorable places. River bottoms, sunny slopes of uplands, and subalpine grassy areas among the mountains were considered to be favored egg-laying sites.” But Gurney was seeking an ecological change on a continental scale, so this clue was set aside without his recognizing that it was crucial to solving the mystery. Norman Criddle also harbored suspicions about the actual hideouts of the locust within its montane distribution. Although in 1917 he was still of the opinion that the creature was lurking about, it was clear that he didn’t believe that the locust would be found just anywhere in the Rockies:At present, however, the insect seems to have vanished completely. Indeed, there are some who would place it with the Passenger Pigeon as an object of the past. It seems almost incomprehensible, however, that such can be the case. More probably the real permanent breeding grounds are more restricted than was supposed, and the locust will yet be located either by the discovery of its real haunts or by a new invasion following favorable weather conditions for breeding purposes.
Riley drew the maps of the Permanent Zone with a very broad brush, and it was not until delving into the particulars of the insect’s biology that I found the keen observations for which he was so highly acclaimed. He narrowed the ecology of the locust somewhat in describing it as “a denizen of high altitudes, breeding in the valleys, parks and plateaus of the Rocky Mountain region of Colorado, and especially Montana, Wyoming and British America.” But this panoramic allusion was too vague for a natural historian with Riley’s acute sense of ecological detail. And the more specific his description, the more intriguing this line of evidence became. His attention focused explicitly on a crucial landscape:The insects hatch in immense quantities in the valleys of the three forks of the Missouri river and along the Yellowstone, and, when fledged, they move on from there in a southeast direction at about the rate of 10 miles per day. . . . Prompted by that most exigent law of hunger—spurred on for very life—it rises in immense clouds in the air to seek for fresh pastures where it may stay its ravenous appetite.
Riley was not the only scientist to note that the locusts were highly restricted to particular habitats. In August 1882, another renowned entomologist was leading a scientific expedition into the Permanent Zone. Lawrence Bruner, the namesake for the Rocky Mountain locust’s nearest relative, Melanoplus bruneri, was working his way across Montana in an effort to track down the locust. He’d already taken the time and effort to name an imposing edifice near Fort Buford “Riley Peak” in honor of his entomological colleague. As Bruner came across a remnant population of spretus, he noted in his journal, “We crossed a vast expanse of nearly level prairie, well grassed, with a loose, sandy, clay soil . . . admirably fitted for the rearing of locust swarms. In fact the entire country . . . is one vast hotbed, calculated to produce the largest and healthiest swarms in America [forming] cradles of the ravaging pest.”
In contrast to the effusive claims that would typify economic entomology in the later era of DDT, Bruner offered a most humble interpretation of the power of human ingenuity:The question now arises, Has there been any plausible and practical means discovered during any of these extended tours by which the locusts can be exterminated? My answer to this, if direct, is no; if indirect, yes. In the first place we are to make an answer in accordance with the definition of the word exterminated. As I understand it, it signifies blot out, and that would require every individual insect o [sic] this species to be killed, which would be an utter impossibility. But, by
various methods already described in the annual reports of this Commission created by Congress, their numbers could be and will be so killed off from time to time as to bring them under the control of their natural enemies.
The possibility of killing the ravaging pest while it lay in its cradle—the entomological version of the mythical infanticidal Medea—never occurred to Bruner or Riley.
What did occur to Riley was that reproduction was the key to understanding the pockets of habitat that supported spretus in the Permanent Zone. He knew from his own observations and the reports of his contemporaries that female locusts were very particular about where they laid their eggs. In various documents he noted that oviposition was restricted to sandy and gravelly habitats, where the soils were firm but not hard-packed. The females avoided very loose soil and moist ground. Like insectan Goldilocks in search of “just right” conditions, the locusts buried their eggs in the well-drained soils of the montane valleys. The egg beds lay between the overly wet, silty streamsides and the sere, rocky hillsides. During their plundering of the prairie farms, the locusts laid their eggs in soils that mimicked the fertile river valleys of the Rockies.
The well-drained, sandy cradles in which the six-legged mothers deposited their eggs were so essential to the life of the locust that Riley argued for this being the limiting factor in the species’ ability to persist in its eastward migrations. It was clear that the Rocky Mountain locust could not indefinitely perpetuate itself in the habitats of the Midwest, but nobody knew why. After “fully digesting all the facts,” Riley concluded that the most plausible constraints were the warm, wet weather, which induced a combination of “debility, disease, and deterioration”—and the locusts’ evident “need for their native sandy and gravelly soil.” Spretus was reluctant to lay its eggs in rank vegetation, hard-packed clay soils, and rich loamy fields because these nurseries produced fewer, weaker nymphs.7
The Rocky Mountain locust’s initial decline had been part of the natural ebb and flow of the species. Outbreaks typically lasted for three to five years, and so the 1874-1877 plague had essentially run its course and a period of remission was to be expected. The collapse of the locust back into its Permanent Zone in the river valleys of the Rockies was ecologically unremarkable. Rather than a tidal wave of vitality washing over the continent, it had been reduced to a trickle of life restricted to its favored habitats. There were no indications that this recession was a prelude to extinction because we had not paid attention to the ecological bottleneck.
In the Rocky Mountain region, farmers and locusts crowded into the arable and irrigable lands of the broad, fertile river valleys. The opportunity for conflict over this rich habitat was undeniable. However, just because the accused and the victim were in the same place at the time of the murder does not mean that a guilty verdict is certain. A prosecutor also must demonstrate that the perpetrator had the means to dispatch the victim. Did the pioneers have the capacity to drive the locust to extinction, and what ecological weapons could have been wielded by these simple folk to slay the mightiest species on the continent?
THE MEANS
Lawrence H. Keeley is a professor of anthropology at the University of Illinois, and his work focuses on the most human and least humane of all behaviors: warfare. His rigorous analyses of the nature and costs of waging war have yielded some surprising results. Although we might well imagine that bombers, machine guns, and other modern weapons have dramatically increased the death rate on the battlefield, Keeley paints a radically different picture in War before Civilization: The Myth of the Peaceful Savage. Taking part in primitive warfare is often far deadlier on a proportional basis than participation in contemporary conflict.
Perhaps the epitome of the modern battlefield was the grand campaign of the Normandy landing in World War II. Between D-Day and the liberation of Paris, a period of seventy-nine days in 1944, about 290,000 soldiers died. Guns, bayonets, grenades, artillery, tanks, bombs, mines, and the other instruments of modern warfare killed about 3,700 men during each day of this campaign. Compare these numbers to the massacres of Tutsis and Hutus in 1994. The killings began with the assassination of the presidents of Rwanda and Burundi on April 6 and continued for ninety days, ending with the capture of Kigali. Although the death toll is not fully known, between 500,000 and 1 million people died. No fewer than 5,000 lives were taken during each day of this conflict. There were no bombers, no cannons, no tanks. Yet 1 out of every 14 people in these two African nations was killed in a matter of three months. The weapons were machetes and sharp sticks, with a smattering of small arms. The capacity of humans to destroy each other—and other species—is not contingent upon advanced technologies.
Recent excavations of fossil and subfossil bones on Pacific Islands have revealed an incredible story of extinction. With the arrival of the Polynesians in the outer Pacific, from Tonga in the west to Hawaii in the east, at least half of the species of endemic birds disappeared. Hopscotching from island to island in single outrigger or double canoes, these first settlers left a record of environmental destruction in their wake. The eminent conservation biologist Edward O. Wilson summarizes this period of human expansion quite simply: “The voyagers ate their way through the Polynesian fauna.” For example, on Eua (in present-day Tonga) the colonists were greeted by the squawking and twittering of 25 bird species, of which 17 were permanently silenced. Of about 100 species of birds that lived on Hawaii before the arrival of humans, the native people extinguished half before the arrival of Captain Cook in 1778, and another 15 or so have disappeared since. But islands are not the only landforms subject to the devastation of primitive people.
Shortly after the arrival of humans in North America, nearly three-quarters of the large mammal genera became extinct. Mammoths and ground sloths had flourished for 2 million years, but within 1,000 years of our arrival these creatures disappeared. The story is the same for the largest birds that once roamed the continent. Climatic shifts might have played some role in these extinctions, but the general consensus is that global warming during the Pleistocene was, at best, an accomplice of the human hunters. Chain saws, synthetic chemicals, and hydroelectric dams might be the modern, high-tech weapons in the war against nature, but it appears that canoes, snares, and spears were entirely adequate to the task.
The means of environmental disruption available to the European settlers of the Rocky Mountain region were not much more sophisticated than those of the first people who colonized this land. Wherever the locust might have begun to increase in numbers along the river valleys, the farmers probably employed direct means of suppressing the insect. But from the accounts of the settlers and the records provided by Riley, it appears that such battles were scattered and the skirmishes were minor. Rather, the farmers waged a sort of unwitting guerrilla warfare, an insidious assault on the supply lines and homelands of the locust.
In 1866, General John Pope held out little hope for agriculture in the arid West. The only cause for limited optimism was in the Rocky Mountain region, where, “the streams being more numerous, and the timber more abundant, it is practicable to form settlements and to cultivate the valleys of the streams by irrigation.” This glimmer of hope soon became a radiant beacon for the settlers seeking prosperity. A century later, the renowned western historian Gilbert Fite described precisely this course of events: “Whenever possible, settlers located on or near a creek or river in order to have water for livestock and irrigation.” By the 1880s, the competition for the West’s key resource was becoming downright nasty: “Water was becoming scarce at critical times during the growing season, valleys were becoming crowded, and conditions for newcomers were discouraging.”
Irrigation was the lifeblood of agriculture in the Rocky Mountain region. Of the 50,000 farms in 1890, 70 percent were operated by irrigators—about 2 million acres were regularly flooded. A decade later, more than 5 million acres in the Rockies were being irrigated by 70,000 operators. Our most generous estimate of the locust’s distribu
tion during its recession periods was 2 million acres in the montane river valleys of the West. In this context, it seems entirely possible that the vast majority of the insect’s sanctuary was saturated by pioneer farmers on a yearly basis. But would this have been lethal?
In combating the locust, Riley understood that habitat modifications could have far greater effects than desperate acts of direct confrontation, such as thrashing, poisoning, or crushing the invaders. He advised, “When irrigation is practicable, as it is in some of the ravaged parts of Colorado, let the ground be thoroughly inundated for a few days, and the eggs will lose vitality and rot. . . . Experiments prove how soon they succumb to excess moisture.” As he reviewed the various primitive tools available to defend agriculture from these rapacious creatures, he became increasingly convinced of the value of water.
Sixteen years later, in one of his last publications on the locust, Riley remained convinced that irrigation was one of the most economical and effective methods for suppressing spretus. By this time, he understood that water represented both life to the farmer and death to the locust:This [irrigation] is feasible in much of the country subject to locust ravages, especially in the mountain regions, where, except in exceptionally favorable locations, agriculture can be successfully carried on only by its aid, and where means are already extensively provided for the artificial irrigation of large areas. Where the ground is light and porous, prolonged and excessive moisture will cause most of the eggs to perish, and irrigation in autumn or spring may prove beneficial.