Maybe not eradication, but quite likely the strong incentives/requirements for ethanol use (and the ensuing clearing of land to raise more corn and soybeans) have seriously reduced the population of monarchs.
The number of monarchs that completed the largest and most arduous migration this fall, from the northern United States and Canada to a mountainside forest in Mexico, dropped precipitously, apparently to the lowest level yet recorded. In 2010 at the University of Northern Iowa, a summertime count in some 100 acres of prairie grasses and flowers turned up 176 monarchs; this year, there were 11. ...
[T]he greatest threat to the butterfly, most experts agree, is its dwindling habitat in the Midwest and the Great Plains, the vast expanse over which monarchs fly, breed new generations and die during migrations every spring and autumn. Simply put, they say, the flyway’s milkweed may no longer be abundant enough to support the clouds of monarchs of years past.
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.
Citing the above quoted piece that appeared in the NYTimes, Bjorn Lomborg writes on Facebook,
That's what you get for burning food in cars. [emphasis added]
In the US midwest, conversion from grasslands to corn and soybean fields have been so fast -- about 5-30% in just 5 years -- that the rates are “comparable to deforestation rates in Brazil, Malaysia and Indonesia.”
Dr. Jackson, a University of Northern Iowa biologist: “I can drive five hours east, five hours north, five hours south, five hours west and see nothing — nothing — but corn and soybeans.”
The ethanol programme is a shining example of the failure of gubmnt planning as policy makers capitulate to special interests [farmers and the owners of farm land] to the detriment of everyone else.