The 1875 swarm, described by Albert Child from Nebraska using telegraph reports of its passage, was estimated at hundreds of thousands of square kilometres, an area comparable to a large country. Estimates of the number of insects run to the trillions. Whatever the precision of those figures, they place the event among the largest animal aggregations ever documented.
The collapse followed quickly. The last confirmed live specimens were collected in 1902, and the species has not been recorded since. The leading explanation is habitat destruction at the permanent recession range: between plagues the locust persisted in relatively small areas of river valley in the Rocky Mountain foothills, and those valleys were exactly the ground that settlers ploughed, irrigated and grazed. Destroying eggs in that limited breeding refuge would have removed the species' base.
The explanation remains an inference rather than a demonstration. No one was monitoring the recession areas at the time, and the extinction was recognised only in retrospect, so the mechanism is reconstructed from land-use history rather than observation.