The antibiotic resistance markers used to determine if new genes are successfully spliced into the host could be incorporated into disease-causing bacteria and cause an increase in public health problems. Other antibiotics used in the medical world to combat disease-causing bacteria are not used in GM crops. Polen from Bt corn poses a serious threat to Monarch butterflies. Many died compared to larvae that had eaten leaves without pollen. It is unlikely that many Monarch larvae would eat milkweed with Bt pollen in a natural setting.
Monarchs prefer short milkweed plants to lay their eggs and usually fly in open areas.
Corn pollen is heavy and is not blown far from the field. Also, Monarch larvae do not feed at the same time of year that corn produces pollen. Their largest threat is winter habitat destruction of Mexican forests. Other threats to Monarch butterflies include the clearing of milkweed plants in the U.S., the spraying of insecticides including Bt pesticides, and collision with cars and trucks.