The Rocky Mountain Locust

by Katie Bowell, Curator of Cultural Interpretation

During our recent grasshopper emergence, I’ve heard several people refer to our abundance of insects as a “plague.” Well, they’re sort of right. The correct term for a group of generic grasshoppers is a “cloud,” not a “plague,” but the term for a group of locusts, the swarming phase of grasshoppers in the family Acrididae, is a “plague.” Locusts are differentiated from other grasshoppers by their destructive, migratory behavior. With huge and rapid population increases, locusts consume all the vegetation within a region and then, en masse, migrate to the next abundant supply of food.

The most famous locust to plague this part of North America? Melanoplus spretus, the Rocky Mountain locust.

Rocky Mountain locust

As European settlers from the east began moving further west during the mid 1800s, locusts from the west were moving east and the two groups met in the middle. Normally, M. spretus maintained its range in the Rocky Mountains, but when the species’ population became too large, that range expanded out onto the prairie. Accounts of grasshoppers in numbers so great they blocked out the sun began to come out of the Great Plains.

The Cloud was hailing grasshoppers. The cloud was grasshoppers.Their bodies hid the sun and made darkness. Their thin, large wings gleamed and glittered. The rasping whirring of their wings filled the whole air and they hit the ground and the house with the noise of a hailstorm – Laura Ingles Wilder, On the Banks of Plum Creek

From July 20 to July 30 of 1874, a plague of locusts was recorded over the prairie that covered 198,000 square miles (approximately twice the size of Colorado!) and contained at least 12.5 trillion individuals weighing approximately 27.5 million tons.* The locusts ate everything: crops, grass, trees, clothing, leather, dead animals, and even each other. The number of insect bodies on the ground became so great that they literally stopped railroad traffic: the tracks around Colorado Springs became so slick with locusts that the train wheels couldn’t roll over them.

The largest recorded swarms of the Rocky Mountain locust peaked from 1873-1877, the same time people were actively settling the west. People tried everything to get rid of the insects: burning fields, blasting land with gunpowder, bringing in locust predators and parasites, and building mechanical contraptions to collect and destroy the insects.

However, as quickly as the insect and reports of its destruction had spread across the prairie, the Rocky Mountain locust disappeared just as quickly. The last confirmed sighting of a living specimen was in 1902.

There’s debate over what factors contributed to the extinction of the Rocky Mountain locust. The most accepted theory is that the very settlers the locusts wrecked havoc on ended up, inadvertently, destroying the species by destroying the insects’ breeding ground to plant crops.

Because locusts are a behavioral form of grasshoppers that emerge when populations reach high enough densities, there is a theory that other grasshopper population can become “locust-ized” and behave like M. spretus if conditions are right. However, DNA testing from museum specimens of the Rocky Mountain locust suggests that M. spretus was a distinct, and now extinct, species and the days of the locust on the scale of 12.5 trillion individuals are gone.

If you do still want to find Rocky Mountain locusts, the best place to look (other than in a museum) is in a glacier. Throughout the west there are glaciers that have preserved the frozen bodies of locusts that once flew over them.

For an excellent exploration of the natural and cultural history of the locust, I recommend reading Jeffrey A. Lockwood’s Locust: The Devastating Rise and Mysterious Disappearance of the Insect that Shaped the American Frontier.

*Because of this plague, M. spretus holds a place in The Guinness Book of World Records as “the greatest concentration of animals.”