The Rocky Mountain locust (Melanoplus spretus) was the major form of locust that ranged through almost the entire western half of the United States (and some western portions of Canada) until the end of the 19th century. The insect may have produced larger swarms than any other type of locust. One 1874 sighting famous to entomologists recorded a swarm 198,000 square miles (513,000 km²) in estimated size—greater than the area of California. According to The Guinness Book of Records under the heading 'greatest concentration of animals', the swarm must have contained at least 12.5 trillion insects with a total weight of 27.5 million tons. But less than 30 years later, the species was extinct. The last recorded sighting of a live specimen was in 1902 in southern Canada. And because no one expected such a ubiquitous creature to become extinct, very few samples were ever collected. However, preserved remains have been found in Grasshopper Glacier in Montana.
The locust largely afflicted prairie areas, but the insects existed on both sides of the Rocky Mountains. They liked to breed in sandy areas, and thrived in hot, dry conditions. Droughts caused prairie plants to concentrate sugars in their stalks, which gave the locusts a good food supply. The heat influenced the insects to grow more quickly. Movement of the locusts was probably assisted by a low-level jet stream that persists through much of central North America.
The last major swarms of rocky mountain locust were at their peak between 1873 and 1877, when the locust caused $200 million in crop damage in Colorado, Nebraska and other states. The cause of their extinction is disputed. Maybe the plowing and irrigation by settlers disrupted the natural life cycle of the insects. Some say that the beginning of the end for the Rocky Mountain locust can be traced to its pattern of swarming for a period, then retreating to sandy river beds to breed. It was at that moment, when its population had collapsed back into its habitat, that farmers began digging up that same ground to plant crops. There are stories of plows bringing up thousands of eggs. Through one of the most spectacular coincidences in agricultural history, western agriculture basically destroyed the permanent breeding ground of the locusts. Had the Rocky Mountain locust not died out, North American agriculture would have had to adapt to its presence. Thus, at least initially, it would likely have developed quite differently as it did in absence of this insect.
Because locusts are actually a form of grasshopper that appears when grasshopper populations appear in high densities, it was theorized that M. spretus might not be extinct; "solitary phase" individuals of the Migratory grasshopper might be able to turn into the Rocky Mountain locust given the right conditions. Breeding experiments using many grasshopper species in high-density environments attempt to invoke the famous insect. However, these experiments have not been successful. Also, analysis of mitochondrial DNA from museum specimens and related species suggests that the Rocky Mountain locust was a distinct and now extinct species, possibly closely related to the Bruner spurthroat grasshopper (Chapco & Litzenberger, 2004).
Grasshoppers still cause significant levels of crop damage in North America, but they do not approach the densities of true locusts. This leaves North America as the only populated continent without a major locust.