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 last recorded sighting of a live specimen was in 1902. The insect may have produced larger swarms than any other type of locust. One sighting famous to entomologists recorded a swarm 198,000 square miles (513,000 km²) in estimated size—greater than the area of California, the third-largest U.S. state.

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 is persistent through much of central North America.

It remains somewhat of a mystery why the locusts died out. Many theorize that the plowing and irrigation of settlers disrupted the natural life cycle of the insects. Since the locust crossed the Rocky Mountains, bodies of some specimens have been found in western glaciers. Certainly, if the Rocky Mountain locust had not died out, the economics of North American cropland would be much more questionable than it is today. The last major swarms were at their peak in the mid-1870s.

Because locusts are actually a form of grasshopper that appears when grasshopper populations appear in high densities, it is difficult to say with absolute certainty that the locust is completely extinct. “Solitary phase” grasshoppers that could turn into the Rocky Mountain locust under the right conditions may still exist. Experiments have been undertaken with many grasshopper species to breed them in high-density environments to attempt to recreate this famous insect. However, those experiments have not been successful, and the Rocky Mountain locust has not been seen in more than a century.

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.

References

• Lisa Levitt Ryckman (June 22, 1999). The Great Locust Mystery. Colorado Millennium 2000. Denver Rocky Mountain News.