How much Bt corn pollen does it take to have a toxic impact on monarchs?

There actually are a number of different types of Bt corn, each expressing a slightly different Bt protein. For the three most common types of Bt corn – Bt11, MON810 and TC1507 – doses of more than 1,000 Bt corn pollen grains/cm² of milkweed leaf surface were required – sometimes much more – to see significant negative effects on caterpillar development (Hellmich et al., 2001). Caterpillars were exposed to pollen on milkweed leaves for up to five days at doses ranging from 100 to more than 1,000 pollen grains/cm² without any observed effects in terms of weight gain or mortality.

Pollen levels measured on milkweed leaves in cornfields during pollination do not commonly exceed 1,000 grains/cm². In fact, the amount of pollen was significantly less than 1,000 grains/cm², with means ranging from 10 to 425 grains/cm² (Pleasants et al., 2001).

An adverse effect on caterpillars was seen at a lower dose with one rarely planted Bt corn: Event 176. It took 10 grains/cm² to affect the larvae. But event 176 was the earliest developed Bt corn and was quickly supplanted by other types. It has never been planted on more than 2 percent of all the acres planted with corn, and the biotech company marketing event 176 hybrids did not seek EPA re-registration in 2001.