In practice it became impossible to keep BT corn out of the food supply. With corn as a staple crop and the international trade in corn everyone on the planet became a test case.
There was controversy on the impact of corn pollen on non-target species, including monarchs and lady-birds/lady bugs and bees. Follow-up studies didn’t show significant impact in quantities likely to be encountered in the field.
All GMs have risk of pollen drift and the risk of gene spread. Farmers growing non-BT corn for export may have their crops rejected in some markets if contaminated. Corn cross pollinates with a genetic ancestor of corn in South America which is considered a weed because it can cause domestic corn kernels to not develop properly. There is some risk of gene transfer and creating a “super” weed.
https://ejbpc.springeropen.com/articles/10.1186/s41938-018-0...