In the formal risk assessment of Bt corn on monarch populations, scientists carefully considered all of the questions of toxicity and exposure. Their conclusion: Bt corn pollen does not pose a risk to monarch populations.
To reach this conclusion, scientists integrated the following results:
The density of Bt11, MON810 and TC1507 types of Bt corn pollen that overlay milkweed leaves in the environment rarely comes close to the levels needed to harm monarch butterflies. Both laboratory and field studies confirmed this.
There is limited overlap between the period that Bt corn sheds pollen and when caterpillars are present.
Only a portion of the monarch caterpillar population feeds on milkweeds in and near cornfields.
Toxicity is negligible and exposure under field conditions is low.