Assessing the impact of Cry1Ab-expressing corn pollen on monarch butterflylarvae in field studies

Survival and growth of monarch larvae, Danaus plexippus (L.), after exposur e to either Cry1Ab-expressing pollen from three Bacillus thuringiensis (Bt) corn (Zea mays L.) events differing in toxin expression or to the insectic ide, lambda -cyhalothrin, were examined in field studies. First instars exp osed to low doses (approximate to 22 grains per cm(2)) of event-176 pollen gained 18% less weight than those exposed to Bt11 or Mon810 pollen after a 5-day exposure period. Larvae exposed to 67 pollen grains per cm(2) on milk weed leaves from within an event-176 field exhibited 60% lower survivorship and 42% less weight gain compared with those exposed to leaves from outsid e the field. In contrast, Bt11 pollen had no effect on growth to adulthood or survival of first or third instars exposed for 5 days to approximate to 55 and 97 pollen grains per cm(2), respectively. Similarly, no differences in larval survivorship were observed after a 4-day exposure period to leave s with 504-586 (within fields) or 18-22 (outside the field) pollen grains p er cm(2) collected from Bt11 and non-Bt sweet-corn fields. However, survivo rship and weight gain were drastically reduced in non-Bt fields treated wit h lambda -cyhalothrin. The effects of Bt11 and Mon810 pollen on the survivo rship of larvae feeding 14 to 22 days on milkweeds in fields were negligibl e. Further studies should examine the lifetime and reproductive impact of B t11 and Mon810 pollen on monarchs after long-term exposure to naturally dep osited pollen.