Soaring demand for corn, spurred by federal requirements that gasoline be laced with corn-based ethanol, has tripled prices in a decade and encouraged farmers to plant even in places once deemed worthless. Since 2007, farmers nationwide have taken more than 17,500 square miles of land out of federal conservation reserves, an Agriculture Department venture that pays growers modest sums to leave land fallow for wildlife. Iowa has lost a quarter of its reserve land; Kansas, nearly 30 percent; South Dakota, half.

A study published in February in the Proceedings of the National Academy of Sciences analyzed land use in five states — Iowa, Minnesota, the Dakotas and Nebraska — in the broad arc of farmland where corn and soybeans are intensively planted. Over the five years from 2006 to 2011, the study concluded, 5 percent to 30 percent of the grasslands were converted to corn and soybean fields, a rate it said was “comparable to deforestation rates in Brazil, Malaysia and Indonesia.”

At the same time, farmers have switched in droves to new varieties of crops that are genetically engineered to tolerate the most widely used weed killer in the United States. The resulting use of weed killers has wiped out much of the milkweed that once grew between crop rows and on buffer strips separating fields and roads.

Roughly half of all Mexico-bound monarchs are hatched in the Midwest and depend as caterpillars on milkweed for food, according to a 2012 study that concluded that the region lost 58 percent of its milkweed and 81 percent of its monarchs between 1999 and 2010.

Said Dr. Jackson, “I can drive five hours east, five hours north, five hours south, five hours west and see nothing — nothing — but corn and soybeans.”

Beleaguered as they are, the butterflies do have one advantage: their seemingly unmatched popularity. Scientists allow that neither the butterfly nor its migration is crucial to the balance of nature. But as rallying points for conservationists and early warning signals of environmental problems, they are invaluable, backers say.