Twenty million acres of Bt corn were planted in the United States last year, a quarter of the total crop. One-third of Ontario cornfields are now Bt corn. Use of these genetically altered crops is expected to increase in the future. But Bt can be expressed in other areas of the plant, including pollen. And corn, like other grasses, uses wind for fertilization.
Airborne pollen is free to land on any plants in the area, including milkweed.
Monarch larvae live, and feed, on milkweed. In the Cornell study, adding Bt pollen to their diet proved dangerous, even fatal. That said, it doesn't seem to pose any threat to butterflies in other stages of development. Bt pollen capable of killing monarch larvae might not have been news to Monsanto, but it made the front page of the New York Times. They're beautiful and elusive, with secret lives.