For the most part, researchers have looked to large-scale environmental changes. Some have blamed the disappearance of buffalo, suggesting that bison wallows may have been a critical habitat for the locusts. Others suggest the reduction in American Indian populations and their use of controlled fires may have led to habitat changes that brought on the locusts' decline. But these theories do not hold up under scrutiny, said Dr. Jeffrey A. Lockwood, entomologist at the University of Wyoming, and others.

Instead Dr. Lockwood suggests the locust was more likely done in on a much smaller scale by the very farmers whom the locust caused so much misery.

''As far as I know, this is the only example of a pest insect driven extinct anywhere'' in the history of agriculture, he said. And, he says, the settlers appear to have carried out this extermination entirely inadvertently.

Rather than eliminating the locusts as they intended with fires and hopperdozers, the pioneers, Dr. Lockwood theorizes, killed the locusts by transforming the land to their own tastes, land that now appears to have been the heart of the species' breeding ground and ultimately, its Achilles' heel.

When the locusts swarmed every 7 to 12 years or so, they covered much of the continent. Eventually, however, after each outbreak, they would retreat to a limited number of fertile river valleys scattered around the West.

Unfortunately for the locust, those same valleys were favored by farmers as well. By the 1880's, the farming of corn, hay and wheat in the Western states showed a nearly complete overlap with areas identified as the cradles of the Rocky Mountain locust.

The settlers brought along insect-eating birds, planted new, habitat-altering plants and let their cattle stomp the ground. Pioneers also nearly eradicated beavers, a change that brought flooding that probably killed off eggs and young locusts. But perhaps most damaging was the farming itself.