As recently as a decade ago, farms in the Midwest were commonly marred — at least as a farmer would view it — by unruly patches of milkweed amid the neat rows of emerging corn or soybeans.

Not anymore. Fields are now planted with genetically modified corn and soybeans resistant to the herbicide Roundup, allowing farmers to spray the chemical to eradicate weeds, including milkweed.

And while that sounds like good news for the farmers, a growing number of scientists fear it is imperiling the monarch butterfly, whose spectacular migrations make it one of the most beloved of insects — “the Bambi of the insect world,” as an entomologist once put it.

Monarchs lay their eggs on milkweed, and their larvae eat it. While the evidence is still preliminary and disputed, experts like Chip Taylor say the growing use of genetically modified crops is threatening the orange-and-black butterfly by depriving it of habitat.