Yep, the supposition was that a GM strain of corn that produced a chemical to protect it from a corn pest would release pollen that would accumulate on milkweed leaves that grown in turn rows. Since monarch butterfly larvae feed voraciously on milkweed, the fear was that the larvae would accumulate enough of the corn pollen incidental to feeding on milkweeds that it would poison the monrach caterpillars.

It's a reasonable hypothesis and even better, it is scientifically falsifiable. I recall two separate series of experiments that demonstrated that the pollen did indeed accumulate the chemical and that there was sufficient concentration that a lethal does could be ingested. They did controlled lab experiments and were able to get poisoned caterpillars. Next they ran the experiemnt in filds by placing virgin caterpillars on milkweed deep in cornfields, in turnrows and some distance removed from the cornfields. No fatalities were found among the experimental population.

However, there are lots of species out there. Discounting one species among thousands does not mean that no species is affected.

B