Locust

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by Jeffrey A. Lockwood


  After dinner, I took the dishes down to the stream for washing. Usually, my job was cooking, but tonight I’d begged off in order to conduct my dissection and ensure that our celebration was in order. Scrubbing the plates by moonlight was peaceful. My hands were soon numb in the icy water, which soothed the cuts and scrapes that had come from working on the jagged ice.

  Over the next few days, we collected 250 bodies from Knife Point Glacier and Bull Lake Glacier, an adjacent body of ice separated by a shared lateral moraine. Many were too crushed or dismembered for definitive identification, but about 100 had the distinct thoracic spine that gave the subfamily its common name of “spurthroated” grasshoppers. Amid these, there were 14 males with well-preserved abdomens and genitalia, all of which were unambiguously the Rocky Mountain locust.

  In the months leading up to our expedition, Larry had invented the “locust body bag,” a contraption for concentrating insect remains embedded in blocks of ice. The device had a folding aluminum frame that supported a thick, black plastic bag—very much like a five-gallon body bag—in which a block of frozen remains was placed. The bag was then sealed and the apparatus set on a sunny rock, so the intense sunlight would heat the bag and melt the ice. A few small holes in the bottom of the bag allowed the meltwater to drain out, and in just a couple of hours the contents were reduced to a soggy mass of jumbled body parts. With his simple invention, we reduced five-pound blocks of frozen detritus to a few ounces of valuable specimens (DNA and the other diagnostic biochemicals are fairly stable for short periods, and we kept a small portion of the samples frozen just to be sure). Although everyone carried their maximum load back to camp each afternoon, Larry’s strength and fortitude made him the human pack-horse at the end of the day. So nobody was more pleased than he with the effectiveness of the “locust body bag” during the 1,300-foot climb back up and over Indian Pass.

  Based on our collections, the sex ratio of the swarms was markedly biased toward females, which outnumbered males by slightly more than three to one. Nowhere in Riley’s studies was this preponderance of females noted. I speculated that this phenomenon might have been characteristic of freshly emerging swarms in the Rockies. If swarming was a survival strategy for spretus, then it makes sense that females would comprise more of the migrants. Let’s face it, if we were sending a rocket into space to establish a colony on a distant planet, it would make a lot more sense to stack the manifest with female colonists. We also know that when swarms of other locust species proceed across the landscape they become increasingly dominated by males, as mature females drop out of the migration to lay eggs. So, perhaps a newly forming swarm—such as would have been blown onto the glacier—would be biased toward females to compensate for their loss on the journey.

  As the sun melted the blanket of snow covering the upper reaches of the glacier, a most remarkable pattern was revealed in the ice. Hundreds of parallel, curved lines stretched across the width of the glacier. Like growth rings in a tree trunk, these darkened bands marked annual deposits. Each winter, the snow created a fresh layer on the glacier, and during the short summer that followed, dust, pollen, and insects would form a thin organic layer on top. This two-toned annual deposit would then be slowly incorporated into the surface of the glacier, creating a distinct record of the year’s events. Trekking up this stratified section of the glacier was like walking along the edge of a gargantuan stack of plates, with the newest plates at the top of the stack. By carefully inspecting each of the darkened bands, we were able to determine the time line of locust deposits on the glacier. These strata did not include whole bodies of locusts, but there were plenty of legs and mandibles. And with Scott’s forensic method for identifying mandibles, we were able to determine that all but one of the strata contained the remnants of spretus. The only other deposit was, not surprisingly, composed of the remains of sanguinipes.

  We meticulously sampled a continuous series of bands, representing three centuries of history. Of course, not every stratum had locust remains. From Riley’s reports we knew that the average interval between outbreaks of the locust across the Rocky Mountain states was six and half years, with a rather wide variance. The average number of bands between those that contained locusts was six and three-quarters, a remarkable match. It seemed that the glacier provided an incredibly effective and accurate record of major outbreaks. Given that the outbreaks originated in the lands just to the northwest of these glaciated peaks and that the prevailing winds were from this direction, perhaps it is not altogether surprising that the glacier managed to trap a sample of the passing swarms during each of the plagues.

  These findings meant that the pattern of locust outbreaks that was documented in the mid-nineteenth century had been ongoing for at least 300 years. Changes wrought by European settlers were not the cause of, and did not increase the frequency of, locust outbreaks. Our analysis also revealed that the temporal pattern of locust remains was random. There were no apparent cycles or regularities in the distribution of swarms. A similar lack of pattern was found when we analyzed Riley’s records of locust outbreaks. If outbreaks of the Rocky Mountain locust were driven by the weather, then randomness made sense; within a span of a few centuries, the occurrence of mid-continental droughts is highly irregular.

  On the next body of ice beyond Bull Lake Glacier, Larry found no insect remains but plenty of evidence that another scientific expedition had recently passed through. Indeed, just a few weeks earlier, a team from the U.S. Geological Survey—Hayden’s old outfit—had taken ice cores from Upper Fremont Glacier to study its history and predict its fate. Through Craig and Charlie’s connections we contacted the federal scientists and let them know of our study. Months later, they sent me a single tibia that they’d extracted from a core 315 feet below the surface. I determined that it was the leg of a grasshopper or locust, and after radiocarbon dating it became the oldest evidence of what we assume was a locust outbreak in North America. When this swarm of Rocky Mountain locusts was preparing to sweep across the Great Plains in the twelfth century, Genghis Khan and his Mongol army were sweeping across the steppes of Asia.

  On the last day on the ice, we set up a drift net in one of the rivulets pouring down the face of the glacier. A drift net is commonly used to sample stream insects. It comprises a metal frame that is anchored to the substrate, in our case by means of ice screws. Attached to the frame is a long, fine-mesh net through which the water flows. Whatever is drifting in the current is caught in the net, and the flow of the water prevents it from being washed back out or otherwise escaping. By extrapolating from our sample catch, we estimated that the remains of at least 4 million locusts were washing out of the glacier during a typical summer’s melting. And according to Charlie and Craig, the terminus of the glacier had receded 250 yards since 1963.

  A vast storehouse of biological specimens, representing centuries of natural history that could only be viewed through this window of ice, was being flushed down the valley. Perhaps a small portion of the lighter fragments would make their way into the Wind River and be carried into the agricultural fields along with irrigation water. I like to imagine that the Rocky Mountain locust might in this way return to the crops of the western farmers, completing its centuries-long journey, but taking nothing from the verdant fields. If not thanked for providing a bit of humus to the soil, at least its arrival is no longer cursed.

  In recent years the rate of melting apparently has accelerated so that immense mounds of decomposing bodies have begun to pile up alongside the receding ice. According to Jonathan Ratner, an environmental consultant and avid hiker in the Wind River Range, hundreds of acres were covered in two to six inches of the rotting peat-moss-like remains of locusts in the summer of 2002. Based on our back-of-the-envelope estimates, the heaps of material included 20,000 cubic yards of corpses, enough to fill 1,200 dump trucks.

  On our final day of the expedition, we packed early because the horses were supposed to show up by mid-morning to take us back out. The
wrangler, stringing only four horses, appeared in camp just after lunch. He muttered an excuse for his tardiness but offered no explanation for why there was only a single riding horse other than his. We loaded the three packhorses, gave Charlie the remaining mount, and resigned ourselves to making the twenty-five-mile hike. The last few miles were traversed in moonlight. Gary and Sue offered us profuse apologies for the wrangler’s lack of planning. Although it was nearly midnight by the time we’d unpacked, Sue prepared a meal featuring melt-in-your-mouth sirloin steaks and butter-slathered baked potatoes crowned with dollops of sour cream. After days of eating rehydrated macaroni and semicrunchy rice dishes (at 12,000 feet water boils at 190 degrees, which made it difficult to fully cook food), the dinner transformed annoyance into deep satisfaction. We bid our hosts farewell, and looked forward to working with them again the following year.

  The next year’s expedition featured a cruel ice storm that generated a couple of tense hours during which hypothermia became a real possibility. The rest of the trip went well, with many more whole bodies recovered from the ice. It would be our last collecting trip to the Wind River Glaciers.6 Funding sources tightened their belts and presumably found projects that kept species alive to be more important than supporting grave robbers.

  The grasshoppers that we brought back to the laboratory from Knife Point Glacier provided valuable material for more refined analyses than had been previously possible. As fate and fortune would have it, we had discovered neither fool’s gold nor twenty-four-carat gold nuggets. Perhaps the best metaphor would be a treasure map, hinting at a solution to our mystery but not quite being the answer itself.

  Although the proteins in the bodies were too degraded for comparative studies, other molecular features were remarkably well preserved. And Dick was the ideal collaborator on the analysis, being as skilled in the laboratory as he was in the field. Dick was still working for the USDA’s research laboratory on campus, and he’d taken personal leave to join the second expedition to Knife Point Glacier. His administration was not fond of his open-ended curiosity and scientific meanderings outside the laboratory’s circumscribed boundaries of vector biology. Unlike many government scientists, Dick had a hard time being a good “company man.” He was a harsh critic of bureaucratic bungling and self-aggrandizing scientists; he did not suffer fools gladly, and he found no shortage of buffoons within the agency.

  Dick began working for the USDA when they maintained a honey-bee laboratory in Laramie, and he’d switched to studies of biting gnats when the feds reorganized and created the Arthropod-Borne Animal Diseases Research Laboratory. In his initial research on bees, he’d used the chemical profiles of the waxes that coat the bodies of insects as a means of distinguishing between the gentle European honeybees and the homicidal Africanized honeybees. Being small, insects are highly prone to desiccation, and they protect themselves from water loss with a veneer of wax, not unlike our use of paraffin as a means of sealing jars of jelly to prevent them from drying out and to keep molds from getting in. Dick had worked closely with Dave Carlson—a USDA colleague and the pioneer of this analytical method. Dave had discovered that there are hundreds of different lipid molecules that insects mix together to create their wax layer, and the particular blend forms a chemical fingerprint for each species. During Gurney’s work on the taxonomic status of the Rocky Mountain locust, this diagnostic feature was not known, and in any case, the sophisticated analytical instruments necessary to separate and identify the lipid cocktails were not readily available to entomologists at that time. So in an effort to close the book on the identity question, Dick, Dave, and I collaborated on a project to determine whether spretus and sanguinipes had different types of waxes.

  We dribbled a few drops of a solvent down the legs of museum specimens of spretus to extract the surface waxes. The same procedure provided wax samples from museum specimens of sanguinipes along with locust remains from our first Grasshopper Glacier expedition and from Knife Point Glacier. The spretus and sanguinipes samples provided distinctly different blends of lipids, each having more than a hundred different hydrocarbons. The extracts from Grasshopper Glacier were frustratingly ambiguous, lying somewhere between our two known blends. The recession of the glacier and the consequent exposure of the insect remains probably led to the slow deterioration of the surface waxes, much as one might expect from an 800-year-old candle. To our delight, however, the samples from Knife Point Glacier, having been entombed deep within the ice, provided a much higher-quality chemical fingerprint. There could be no doubt that these specimens matched those of spretus. But the search for DNA—the ultimate chemical evidence to establish both the taxonomic standing of spretus and the identity of the glacial remains—would not be so simple.

  Bill Chapco is a bespectacled, always-happy-to-see-you professor from the University of Regina in Saskatchewan. This warm and authentic fellow is also the world’s foremost grasshopper geneticist. For years he has worked on methods for extracting and analyzing the DNA from grasshoppers, which, as it turns out, is no mean trick. In 1993, I sent him some of my treasured locust mummies from Knife Point Glacier, knowing that if anyone could eventually tease the genetic code out of these creatures it would be Bill. And I was right, although I had to wait nearly a decade for his deliberate and systematic efforts to bear fruit. In reality, he had developed effective methods some time earlier, but knowing the rarity of spretus specimens, Bill wanted to be absolutely certain of his technique before applying it to the Rocky Mountain locust.

  In November 2002, Bill presented his first findings on the genetics of the Rocky Mountain locust at the national meeting of the Entomological Society of America. Somewhere around 3,000 entomologists converge each winter to share their findings on every imaginable—and a few unimaginable—aspects of insects. Using museum specimens representing a wide range of species, along with the material from the glacier, Bill showed that the standing of spretus as a valid species was fully supported by key regions of their DNA. Much to everyone’s surprise, neither sanguinipes nor even femurrubrum shared the greatest genetic similarity to the Rocky Mountain locust. Rather, the nearest living relative was Melanoplus bruneri, a species with a catholic diet and a propensity for irregular outbreaks in the mountain meadows of the United States and Canada. Bill’s work also left no doubt that the museum and glacial specimens of locusts were the same species. Indeed, much to my delight, the genetic material from the glacial specimens had suffered substantially less degradation than that obtained from the dried museum specimens.

  Molecular analysis can also shed light on the events leading up to extinction. If there was a detectable loss of genetic variation in the Rocky Mountain locust—a genetic bottleneck—this change would indicate if and when the species began a gradual decline leading to its final extinction. Such a narrowing of genetic diversity occurs when a species engages in a high degree of inbreeding in fragmented or reduced populations. We all have a small proportion of abnormal and adverse genes acquired from one of our parents, but the corresponding traits are often not expressed because the alternative, healthy form of the gene from our other parent masks or dominates the deleterious form. So there’s a good reason why we frown on brothers marrying their sisters—and, perhaps, why evolution has predisposed us to find our siblings lacking in sexual desirability. The children of such matings are much more likely to receive a double dose of these rare, harmful genes and to manifest the associated deformities and illnesses. All of this genetic change would have made for an exciting discovery, except the molecular analysis provided no evidence of a bottleneck in the museum specimens from the turn of the last century relative to the older, glacial specimens.

  While Bill had been working on his molecular methods, we had been taking meticulous measurements of glacial and museum specimens looking for evidence of a genetic bottleneck through a trait called bilateral asymmetry. Most animals exhibit bilateral symmetry, meaning that if we were to slice them in half longitudinally the two pieces would be n
ear-mirror images of one another. Of course, this pattern is not found in creatures such as sea stars, which have radial symmetry. Even in bilaterally symmetrical animals, like humans and grasshoppers, the two halves are not perfectly identical. However, in genetically healthy organisms, the degree of symmetry is much greater than in organisms arising from inbreeding. Our measurements of tibial lengths and counts of tibial spines from the right and left hind legs consistently failed to show any differences between Rocky Mountain locusts before and during their decline to extinction. The locust’s breeding pattern during the last few years of its existence was not unusual; no evidence indicated that there were several generations of abnormally diminished or exceptionally isolated populations leading to inbreeding. Our findings agreed with Bill’s—these insects had not been in a prolonged decline.

  The late 1800s were not the last gasp of a dying species. Rather, it seemed that the extinction happened suddenly and without warning to a normal, healthy species. It appeared that the Rocky Mountain locust had been decimated throughout its range within a matter of a few years in an entomological Armageddon. But what sort of force could act in such a manner without leaving evidence of its presence spread across the West?

  13

  Pioneers on Trial

  THE FIRST COURSE THAT I TAUGHT AT THE UNIVERSITY of Wyoming was in the fall of 1986. During the summer, I prepared intensively for my first solo venture into academia, amassing a file drawer of journal articles, reviewing texts, and compiling more than 200 pages of lecture notes. On the first day, I was ready to plunge into the wonderful world of “Insect Population Biology” with a class of graduate students from entomology and zoology and a smattering of senior-level biology students. I had prepared like a football team readies for the Super Bowl, but the students were in preseason form. The experience was traumatic for all concerned.

 

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