(An expanded version of a story from The Book of Deadly Animals. Go to the beginning of this story.)
The most vivid firsthand account of swarm behavior is surely that of Laura Ingalls Wilder, the children's writer who chronicled the life of her family on the American frontier. By 1874 Wilder's restless father had moved the family to a homestead near Walnut Grove, Minnesota. Wilder vividly paints the heat of that summer: The edge of the prairie "seemed to crawl like a snake" with heat shimmer, and the pine boards of buildings dripped their viscous sweat. The family's wheat crop, which promised to yield generously, was head-high. Then a cloud dimmed the day, moving in without wind. Its individual particles glittered. The falling insects sounded like a hailstorm, and this sound was succeeded by the multitudes chewing (like the working of thousands of scissor blades, some witnesses said). Prairie grasses and wheat and oat crops vanished; beets, beans, potatoes, carrots, and corn were razed; willow and plum trees were shorn. (Although the Ingalls family didn't grow these crops, others noticed the insects preferred to start with tobacco and onions when these were available.) "Not a green thing was in sight anywhere" after a few days, Wilder concludes, echoing the Book of Exodus.
Only the family's chickens benefited, snapping up the windfall of easy prey. (Some writers of the period noted that the chickens took on the flavor of grasshoppers, and that they and their eggs became inedible. Others wrote that the grasshoppers themselves were edible, though the Ingalls family does not seem to have taken an interest in this option.) The family fell on hard times, and at night they could hardly sleep for the sensation of crawling on their skin. On Sunday they arrived at church with their best clothes crawling with grasshoppers and stained with their brown spittle. The meager creek thickened with scum, and the land twitched with dust devils. The cow's milk went bitter and nearly dried up.
Today's American infestations are minor compared with swarms like those, but they still devastate wide areas from the Pacific Northwest to the Great Plains. About 2 million acres of Colorado have been eligible for treatment against grasshoppers in a single year, and it is not uncommon for several counties at a time to be affected by infestations. In an ordinary season, grasshoppers eat about 20 percent of the fodder on rangeland; in areas of infestation, the percentage can increase to 100, affecting millions of acres at $5 to $10 per acre. On cropland, the devastation is even greater.
Although the Rocky Mountain locust, the species that wreaked havoc here in the 19th century, does seem to have disappeared, its ecological niche may be only temporarily vacant.
At rest, the red-legged grasshopper, a close cousin of the Rocky Mountain locust, looks vaguely mechanical. Two immense eyes take up the sides of its head. Roughly between these are three smaller eyes and a pair of short antennae. The hard, yellow-green underside of its thorax is marked with deep indentations that resemble smiley faces doubled and distorted in mirrors. The abdomen is segmented. The rear of it ends in four blunt appendages closed together like pinching fingers. When the insect takes flight, its vitality is revealed. The camouflaged forewings open to show the vivid hind wings, which flutter loudly and too fast to be seen distinctly. The specialized hind leg is a marvel of complexity. It possesses sensory equipment as well as a comblike projection used by the male to coax music from his wings, and its muscles are powerful enough to accomplish some of the most impressive leaps, proportionally speaking, in the animal kingdom.
Much of the grasshopper's insides are taken up by reproductive equipment. The female's ovaries produce rows and rows of eggs, which are attached by stemlike parts to each other. The effect is something like an orderly bunch of grapes, lined up mostly in neat rows, and glistening with moisture. However, the naked eye is impressed mainly by the fat black strand of digestive tract. Narrower in spots and girdled with knotty, fibrous projections near the middle, it is essentially a tube. The dark color is that of chewed vegetation. At any given time, a substantial proportion of the grasshopper's body weight is its unconverted food. When a grasshopper is eaten by a mantis, the mantis typically eats around this unattractive vegetation. It is left holding the digestive tract, which resembles the stick at the center of a corn dog.
That summer in Oklahoma, I caught some grasshoppers in jars and fed them on weeds and grasses. They ate avidly, leaving nothing of my offerings. Their mouths had toothed jaws with multiple points of articulation, a complex arrangement that appeared to constitute two or three mouths working at once. In fact, this equipment allowed them to chew both vertically and horizontally. Typically, they chewed along a blade of grass, creeping up the blade several inches before going deeper. Or they chewed a hole through the blade, then expanded the hole until the blade was cut in two and it collapsed.
The jars didn't suit them: They were heavy, humid creatures, and in a day or two the glass was fogged with condensation, the bottom of the jar laden with their conical black feces and their spit. They died off quickly, even when placed in more commodious accommodations. I tossed a grasshopper into the web of a black widow spider, assuming the spider would dispatch it quickly. Instead, the grasshopper's energetic struggles wrenched it loose from the web, though at the cost of a hind leg. I repeated this experiment with a different individual. This time the grasshopper's leaps freed it easily, and further leaping knocked the spider from its web. The spider lay on its back, kicking frantically, as the grasshopper chewed its front leg.
It's such hunger that drives a locust swarm. A single desert locust can eat its own weight in a day. Multiplied by a billion, this hunger may be the most demanding our planet has known. The mechanisms behind the behavioral change from solitary grasshopper to swarming locust are not well understood. In the laboratory, scientists have been able to provoke grasshoppers into a phase shift by pelting them with wads of paper for hours on end. This result suggests that the jostling the grasshoppers experience in a group prompts the phase shift. Stephen Simpson of the University of Oxford has located the hardwiring for this mechanism more precisely on the insects' hind legs. A spot there (which Simpson calls "the G-spot—G for gregarization") is the trigger that somehow provokes morphological changes. (Other research, now largely disproved, pointed toward pheromones found in the feces, most likely produced by gut-dwelling bacteria, as the stimulus to change.) It may be that multiple cues are involved, and perhaps different locust species use different cues.
Population explosions occur in various animal species, from rabbits to mayflies. Vast migrations occur in creatures as diverse as monarch butterflies and wildebeests. But the phase shift and swarming of grasshoppers appears to be unique. It may provide a way to survive and reproduce when food supplies dwindle. In Africa, the desert locust's eggs can lie dormant in arid soil for several years until rain triggers their hatching. The hatchling nymphs thrive on the brief lushness that desert rains bring. When they've devoured everything in an oasis, they swarm to reach green regions beyond the desert. In the case of the Rocky Mountain locust, however, the swarming is more mysterious. These locusts never seemed to establish permanent populations beyond their home base in the Rockies.
When the young of swarming locusts hatch, they, too, are swarming locusts. The ability of grasshoppers to inherit traits their parents acquired was once a puzzle, for it seems to bypass the gradual genetic change that the theory of evolution predicts. Some entomologists describe this phenomenon as "cultural": The mother locust supplies her eggs with a heavier dose of nutrients and a chemical—called a maternal gregarizing agent—that encourage her offspring to develop toward the gregarious end of its potential. The environment into which it hatches (the jostling swarm itself) also constitutes a cultural influence. Such nongenetic inheritance is not unique; it has been observed in human populations when, for example, consistently good nutrition over several generations prompts an increase in average stature. The offspring of locusts born in a subsequent season may or may not develop into locusts; crowding is the deciding factor.