EATING MACHINE: The migratory grass-hopper, Melanoplus sanguinipes, is the closest living relative of America's extinct plague locust. Can it also swarm?
One summer in the Oklahoma panhandle thegrasshoppers were everywhere. Every patch of weeds along the alleywould erupt like a pan of popping corn if I set foot in it. When wedrove the highway, we inadvertently slaughtered dozens. The collisionsspeckled our windshield with hemolymph. Their wings, coffee-coloredfans striped with yellow at the outer edges, lodged in our wipers andfluttered in the onrushing air. Sometimes an entire grasshopper, ormost of one, would lodge there as well, struggling to get free as thewind tore it to tatters.
They could be found in unaccustomedplaces that summer. For several mornings running I saw two or threeswimming in the dog's water dish. The rosebushes took on the riddledlook of lace, as though the grasshoppers had tasted the leaves andfound them unappealing but serviceable. In the country, the cedar postsof barbed wire fences would seem at a glance to be shimmering withheat, like a water mirage on the highway, but a second glance wouldshow the effect was not an optical illusion. The posts were simplycrawling with grasshoppers moving up or down for no apparent reason.They seemed to be moving with great caution, edging past each other.When a stationary grasshopper got bumped, it would draw its legs intighter and shift its footing, like a person uncomfortable on a crowdedbus.
Then there was the jackrabbit. We found it beside a dirt roadon the way to the mailbox. It was dead, probably road-killed.Grasshoppers were thick in the weeds and grass along that road, anddozens clustered on the carcass. When someone poked at itexperimentally, a few of the hoppers jumped off and opened their wingsand were carried away by the wind. Others crawled off sluggishly. Somestayed put. With the carcass now more exposed, we could see that it wasbald in patches, and that its hide was wounded in shallow divots, as ifit had been hit all over with buckshot that failed to penetrate. Itseemed that the grasshoppers had been eating it.
As the seasonwore on, the grasshoppers grew absurdly thick. Among the metallic greenones there were others, some yellow and spotted, others a brightergreen. All these I was familiar with, though I had never made anyparticular study of them. But I began to see things utterly new to me.One grasshopper was black and flecked with gray, like burned charcoal.Another was black but flecked with a Tabasco red. This variety has beenexplained to me thus: In outbreaks, grasshoppers are so plentiful thatthey overwhelm their usual predators, offering them more food than theycan use. Other grasshopper species, rare enough to go unnoticed most ofthe time, get relief from predators in this circumstance, and thereforeare more likely to be around for people to notice.
Other thingsseemed different too—there were a great many large grasshoppers, thickas a lipstick. One morning on my driveway I found the largest specimenI had ever seen, a yellowish creature longer than a soda can. It wasdead—a fact that gave me some comfort. Streams of black ants led up toits carcass. Their presence was the first thing that convinced me I wasseeing a once-living creature rather than a toy. I turned it belly-upwith a stick. Its head and thorax were intact, but its abdomen wasriddled with holes. I had not seen this damage at first because itslong wings concealed it from above. Through the holes I glimpsed antsworking at the grasshopper's half-hollow hull. I returned with a rulerand measured the monster at just less than six inches.
SWARMSAND INFESTATIONS: Locust plagues long haunted American farmers, andthey may do so again. In the 19th century, black clouds of RockyMountain locusts swept across the plains almost every summer, leavingonly stubble where crops once stood. Inset map: The 1874 swarm (shownin red) was the largest ever recorded: 1,800 miles long and 110 mileswide, it caused the equivalent of $650 million of damage. Othergrasshopper species probably did not swarm, although they experiencedmajor infestations in 1855, 1864, and 1866. Large map: The U.S.Department of Agriculture releases a "Grass-hopper Hazard Map" everyyear showing where infestations are most likely to occur in the comingsummer. In some areas, grasshopper populations can reach densities ofmore than 200 per square yard. (Map by Matt Zang)
WhatI've been describing is an infestation, a localized population suddenlygrown orders of magnitude beyond its usual numbers. The causes are notthoroughly understood. In the United States, hot, dry weather hassomething to do with it—the heat lets grasshoppers grow faster, and thedryness discourages the fungi that would otherwise check thepopulation's growth.
Swarms of locusts—giant flying species ofgrasshoppers—are a traveling variation on this phenomenon and dominatea wide swath of this planet almost every year. Moving in groups ofmillions, the locusts migrate over great stretches of territory,settling down periodically to eat every bit of vegetable matter insight. They are hunger on the wing.
In the United States migratoryswarms of locusts are presumed to be a thing of the past. But a locustis really just an oversize grasshopper in a gregarious mood. Whengrasshoppers of certain species gather in great numbers, they begin tochange their behavior. Normally, they are somewhat solitary. If forcedtogether they seem uncomfortable, leaping away from each other. Buthunger often forces them together when a bumper crop of grasshoppersencounters a meager supply of food and they must compete for it. If thecrowding persists, the younger insects begin to change. The changesvary with the species, but in general their bodies grow to massivesize. Their wings become clear and strong. Their colors shiftdramatically—for example, from green and yellow to solid black. Theirproportions alter, their shape essentially changing to accommodateflight. So profound is this change that scientists in the past havemislabeled the two phases, solitary and gregarious, as distinctspecies.
Wheatgrassflourished in a Wyoming pasture (top) before an infestation of bigheadedgrasshoppers reached densitiesof 40 per square yard inthe 1980s. The species of grasshopper that decimated this field (below) hasa particularly nasty habit of clippinggrass stems off before nibbling at them and passing on.(Courtesy of Jeff Lockwood)
The creatures behave differently too. They eat withshocking voracity. They whirl into the air in groups, forming swarmclouds. The swarms fly long distances, disrupting ecosystems forhundreds of miles. In the 1870s, one swarm was tracked from Montana toTexas, a distance of 1,500 miles. Polluted layers of glaciers high inthe Rockies show that their flight sometimes takes them to altitudesbeyond the normal range of grasshoppers. In 1874 a Nebraska doctor usedtelegraphs to find the far edges of a swarm he observed flyingoverhead, establishing that its area exceeded that of Colorado.Factoring in their rate and the depth of the swarm cloud, he arrived atan estimate of 12.5 trillion grasshoppers. The Guinness Book of World Recordslists this swarm as the "Greatest Concentration of Animals" yetobserved. More rigorous methods were used on a swarm in Kenya in 1954,yielding the figure of 10 billion grasshoppers in a swarm, whichhappened to be only one of 50 swarms in that country at the time.
Themost vivid firsthand account of swarm behavior is surely that of LauraIngalls Wilder, the children's writer who chronicled the life of herfamily on the American frontier. By 1874 Wilder's restless father hadmoved the family to a homestead near Walnut Grove, Minnesota. Wildervividly paints the heat of that summer: The edge of the prairie "seemedto crawl like a snake" with heat shimmer, and the pine boards ofbuildings dripped their viscous sweat. The family's wheat crop, whichpromised to yield generously, was head-high. Then a cloud dimmed theday, moving in without wind. Its individual particles glittered. Thefalling insects sounded like a hailstorm, and this sound was succeededby the multitudes chewing (like the working of thousands of scissorblades, some witnesses said). Prairie grasses and wheat and oat cropsvanished; beets, beans, potatoes, carrots, and corn were razed; willowand plum trees were shorn. (Although the Ingalls family didn't growthese crops, others noticed the insects preferred to start with tobaccoand onions when these were available.) "Not a green thing was in sightanywhere" after a few days, Wilder concludes, echoing the Book ofExodus.
Only the family's chickens benefited, snapping up thewindfall of easy prey. (Some writers of the period noted that thechickens took on the flavor of grasshoppers, and that they and theireggs became inedible. Others wrote that the grasshoppers themselveswere edible, though the Ingalls family does not seem to have taken aninterest in this option.) The family fell on hard times, and at nightthey could hardly sleep for the sensation of crawling on their skin. OnSunday they arrived at church with their best clothes crawling withgrasshoppers and stained with their brown spittle. The meager creekthickened with scum, and the land twitched with dust devils. The cow'smilk went bitter and nearly dried up.
Today's Americaninfestations are minor compared with swarms like those, but they stilldevastate wide areas from the Pacific Northwest to the Great Plains.About 2 million acres of Colorado have been eligible for treatmentagainst grasshoppers in a single year, and it is not uncommon forseveral counties at a time to be affected by infestations. In anordinary season, grasshoppers eat about 20 percent of the fodder onrangeland; in areas of infestation, the percentage can increase to 100,affecting millions of acres at $5 to $10 per acre. On cropland, thedevastation is even greater.
Although the Rocky Mountain locust,the species that wreaked havoc here in the 19th century, does seem tohave disappeared, its ecological niche may be only temporarily vacant.
Atrest, the red-legged grasshopper, a close cousin of the Rocky Mountainlocust, looks vaguely mechanical. Two immense eyes take up the sides ofits head. Roughly between these are three smaller eyes and a pair ofshort antennae. The hard, yellow green underside of its thorax ismarked with deep indentations that resemble smiley faces doubled anddistorted in mirrors. The abdomen is segmented. The rear of it ends infour blunt appendages closed together like pinching fingers. When theinsect takes flight, its vitality is revealed. The camouflagedforewings open to show the vivid hind wings, which flutter loudly andtoo fast to be seen distinctly. The specialized hind leg is a marvel ofcomplexity. It possesses sensory equipment as well as a comblikeprojection used by the male to coax music from his wings, and itsmuscles are powerful enough to accomplish some of the most impressiveleaps, proportionally speaking, in the animal kingdom. The other day,for example, a grasshopper leaped up to pelt me in the chest. Acorresponding vertical leap for me would be a jump of 180 feet.
Muchof the grasshopper's insides are taken up by reproductive equipment.The female's ovaries produce rows and rows of eggs, which are attachedby stemlike parts to each other. The effect is something like anorderly bunch of grapes, lined up mostly in neat rows, and glisteningwith moisture. However, the naked eye is impressed mainly by the fatblack strand of digestive tract. Narrower in spots and girdled withknotty, fibrous gastric ceca near the middle, it is essentially a tube.The dark color is that of chewed vegetation. At any given time, asubstantial proportion of the grasshopper's body weight is itsunconverted food. When a grasshopper is eaten by a mantis, the mantistypically eats around this unattractive vegetation. It is left holdingthe digestive tract, which resembles the stick at the center of a corndog.
That summer in Oklahoma, I caught some grasshoppers in jarsand fed them on weeds and grasses. They ate avidly, leaving nothing ofmy offerings. Their mouths had toothed jaws with multiple points ofarticulation, a complex arrangement that appeared to constitute two orthree mouths working at once. In fact, this equipment allowed them tochew both vertically and horizontally. Typically, they chewed along ablade of grass, creeping up the blade several inches before goingdeeper. Or they chewed a hole through the blade, then expanded the holeuntil the blade was cut in two and it collapsed.
The jars didn'tsuit them: They were heavy, humid creatures, and in a day or two theglass was fogged with condensation, the bottom of the jar laden withtheir conical black feces and their spit. They died off quickly, evenwhen placed in more commodious accommodations. I tossed a grasshopperinto the web of a black widow spider, assuming the spider woulddispatch it quickly. Instead, the grasshopper's energetic struggleswrenched it loose from the web, though at the cost of a hind leg. Irepeated this experiment with a different individual. This time thegrasshopper's leaps freed it easily, and further leaping knocked thespider from its web. The spider lay on its back, kicking frantically,as the grasshopper chewed its front leg.
It's such hunger thatdrives a locust swarm. A single desert locust can eat its own weight ina day. Multiplied by a billion, this hunger may be the most demandingour planet has known. The mechanisms behind the behavioral change fromsolitary grasshopper to swarming locust are not well understood. In thelaboratory, scientists have been able to provoke grasshoppers into aphase shift by pelting them with wads of paper for hours on end. Thisresult suggests that the jostling the grasshoppers experience in agroup prompts the phase shift. Stephen Simpson of the University ofOxford has recently located the hardwiring for this mechanism moreprecisely on the insects' hind legs. A spot there (which Simpson calls"the G-spot—G for gregarization") is the trigger that somehow provokesmorphological changes. (Other research, now largely disproved, pointedtoward pheromones found in the feces, most likely produced bygut-dwelling bacteria, as the stimulus to change.) It may be thatmultiple cues are involved, and perhaps different locust species usedifferent cues.
Population explosions occur in various animalspecies, from rabbits to mayflies. Vast migrations occur in creaturesas diverse as monarch butterflies and wildebeests. But the phase shiftand swarming of grasshoppers appears to be unique. It may provide a wayto survive and reproduce when food supplies dwindle. In Africa, thedesert locust's eggs can lie dormant in arid soil for several yearsuntil rain triggers their hatching. The hatchling nymphs thrive on thebrief lushness that desert rains bring. When they've devouredeverything in an oasis, they swarm to reach green regions beyond thedesert. In the case of the Rocky Mountain locust, however, the swarmingis more mysterious. These locusts never seemed to establish permanentpopulations beyond their home base in the Rockies.
When the youngof swarming locusts hatch, they, too, are swarming locusts. The abilityof grasshoppers to inherit traits their parents acquired was once apuzzle, for it seems to bypass the gradual genetic change that thetheory of evolution predicts. Some entomologists describe thisphenomenon as "cultural": The mother locust supplies her eggs with aheavier dose of nutrients and a chemical—called a maternal gregarizingagent—that encourage her offspring to develop toward the gregarious endof its potential. The environment into which it hatches (the jostlingswarm itself) also constitutes a cultural influence. Such nongeneticinheritance is not unique; it has been observed in human populationswhen, for example, consistently good nutrition over several generationsprompts an increase in average stature. The offspring of locusts bornin a subsequent season may or may not develop into locusts; crowding isthe deciding factor.
One afternoon I looked in on the six or sevengrasshoppers I had in separate jars and found that they had donesomething interesting. Wet orange strands, about the thickness andtexture of a braided bootlace, lay in their jars. A day or two later, Isaw one of my captives in the act of producing such a strand. Itpressed its hind end against the floor of the jar and arched its back,as if exerting downward pressure, and an orange strand squeezed outfrom the rear, like toothpaste from a tube. This strand looked slimierand smoother than the others I'd seen but was recognizably the samething. By the next day it had dried to look exactly like the otherhoppers' strands, the pattern of its texture emerging as it dried. Aday later it was dry enough to see that what had appeared to beindividual strands woven together were merely dozens of pieces, shapedlike sesame seeds, arranged in an orderly overlap—eggs, of course.
InMissouri in the 1870s, the egg masses of Rocky Mountain locusts lay sothick in the beds of rivers and creeks that authorities offered afive-dollar bounty per bushel of them. This species, the onlygrasshopper in North America that typically shifted into the locustphase, had swarmed for hundreds of years, as proved by layers oflocusts in glaciers dated at around 750 years old. Presumably, they hadswarmed for millennia before that. But around 1880 the swarms abruptlyceased, and the species went extinct. The last live specimens werecollected in 1902.
No one knows why the Rocky Mountain grasshoppervanished, but changes in habitat are the most likely reason. In thelate 19th century, the bison was virtually exterminated, as were thenative peoples. Settlers drastically reduced the numbers of beaver inthe Rocky Mountains, removing an important control on flooding. Cattlebrought in by ranchers grazed and trampled the riversides, and farmersplowed up their fertile soil. To fight off the locusts, farmers triedall sorts of control measures, from contraptions called "hopperdozers"and controlled fires to fasting and prayer. What actually worked was tocarry on farming. Plowing devastated the Rocky Mountain locustpopulation, as did the planting of exotic trees, which brought in manynew predatory birds.
Hundreds of other grasshopper species thriveddespite, or even because of, these habitat changes. But according toJeff Lockwood, an entomologist at the University of Wyoming, the RockyMountain grasshopper's nesting habits made it particularly vulnerable.(Many species prefer grassy hillsides for nesting sites.) The RockyMountain locust's boom-and-bust population cycles also put it at risk.Despite the vast areas attacked by swarms—from Manitoba to Texas andfrom Wisconsin almost to the West Coast—the locust's home base, thearea where it could always be found in plague years as well as othertimes, was a much smaller area in the northern Rockies. Whitesettlement there quickly dispatched the species. It's the only knowncase of a pest species exterminated by human action, and it was anaccident.
RockyMountain locusts, once so common that farmers had to strip them fromtheir fields with tar-smeared "hopperdozers," now exist only as museumspecimens. The last locust was found in 1902; its extinction remains amystery.
Theextinction went unnoticed for a few decades—that such a fecund creaturecould abruptly vanish was counterintuitive—and unmourned. The focus of19th-century science was killing pests, not appreciating them. We stilldon't know what effect the extinction has had. The swarms causedwidespread nutrient recycling and large-scale habitat disruption.Charles Bomar, an insect ecologist at the University of Wisconsin atStout, has speculated that they served as some sort of cyclic groundclearing, much like forest fires. But specific effects are difficult tosubstantiate, and no one has proved any related extinctions.
Eventhe basic facts are in dispute. Daniel Otte, the curator of entomologyat the Academy of Natural Sciences in Philadelphia, suggests the RockyMountain locust is not extinct at all but has simply refrained fromswarming in recent times, perhaps because of encroaching agriculture.Otte points out that almost no one can distinguish closely relatedgrasshopper species by sight. In fact, the integrity of the RockyMountain locust, Melanoplus spretus, as a distinct species has only recently been demonstrated through genetic analysis. The characteristics that distinguish M. spretus from its close, and still living, cousin Melanoplus sanguinipes (the migratory grasshopper) are its proportions. Identifying an M. spretusinvolves taking its measurements—the length of the various legsegments, for example—and comparing them with published figures derivedfrom statistical analysis. To complicate this problem, no one is surewhat the solitary phase of M. spretus looked like. The very idea of grasshoppers shifting phases came about just as M. spretuswas dying out. It's possible, Otte says, that Rocky Mountain locustsare nibbling at your lawn right now, unrecognized. More likely, they'rehidden in remote river valleys, reduced in number but still thriving.
Thisapparent extinction, far from creating a domino effect of furtherlosses, may have created an opportunity for other grasshopper species.The red-legged grasshopper (Melanoplus femurrubrum), which wasresponsible for newsworthy outbreaks in Idaho two years ago, thrives onground broken by agriculture and other human endeavors. Its numbershave grown much larger since the extinction of its cousin. Last summer,patchy outbreaks of clear-winged grasshoppers (Camnulla pellucida)in Colorado attained densities of 200 per square yard; a tenth of thisnumber is considered a danger to crops. Scientists have had somesuccess in developing toxins and parasite-laden baits to combatgrasshopper outbreaks. But applying pesticides on broad stretches ofland has rarely proved cost-effective. Some pesticides seem to makefuture outbreaks worse, because the predator and parasite populationsthey affect don't recover from the poisoning as fast as thegrasshoppers do.
Even though existing North American grasshopperspecies don't migrate as readily as the Rocky Mountain locust did, someof them do swarm in less dramatic migrations. And their swarmingpotential may have more to do with circumstances than with any inherentlimitations. Bomar suggests these other species, especially thered-legged grasshopper and the migratory grasshopper, are stepping intothe niche the Rocky Mountain locust vacated. His suggestion brings backuncomfortable memories of the giant I found in my driveway. "Thepotential for swarms is there," Bomar says. "Eventually, one of thesemicropopulations is going to move out." He's intrigued by thescientific opportunity such an event would present. For the rest of us,it may be the return of an ancestral nightmare.
Almost all things grasshopper can be found on the University of Wyoming's grasshopper Web site.
Reporting by Lauren Gravitz, Illustration by Don Foley