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Feeding on a weed seems like a good evolutionary bet. And for a long time, it worked well for the monarch butterfly.

The butterfly’s life cycle is exquisitely synchronized to the seasonal growth of milkweed, the only plant its larvae will eat. In a game of hopscotch, successive generations of monarchs follow the springtime emergence of milkweed from Mexico as far north as Canada. The hardy plant once flourished in grasslands, roadsides, abandoned lots, and cornfields across much of the continent. It fueled a mass migration that ended each winter with more than 60 million butterflies converging on pine forests in the Sierra Madres.

Then came Roundup.

The number of monarchs reaching Mexico has been falling for years, and it has now reached the lowest level on record. The World Wildlife Fund announced Wednesday that butterflies this winter were found in 1.7 acres across 11 sanctuaries, down from a high of 45 acres in 1996. If you want to know a main reason why, look no further than your corn chips and ethanol-spiked gasoline.

The monarch population sank while agriculture boomed. More than a million acres of Upper Midwest grassland have been plowed under in recent years for corn and soybean fields-a rate of loss comparable to deforestation in places like Brazil and Indonesia. Demand for these crops has surged with the rise of biofuels. At the same time, technology enabled farmers to squeeze ever more from each acre. For monarchs, the most important development was Roundup Ready corn and soybeans.

Since the turn of the century, these genetically modified crops have risen to dominance in the Midwest. Designed to withstand dousing from the Monsanto company’s Roundup weed killer, the plants enabled farmers to swiftly kill competing weeds, including milkweed, while leaving their crops untouched. In 2013, 83 percent of all corn and 93 percent of soybeans in the United States were herbicide tolerant, totaling nearly 155 million acres, much of it in the Midwest.