Effects on Monarch Butterfly Larvae (Lepidoptera: Danaidae) After Continuous Exposure to Cry1Ab-Expressing Corn During Anthesis

Effects on monarch buttersy, Danaus plexippus L., after continuous exposure of larvae to natural deposits of Bacillus thuringiensis (Bt) and non-Bt pollen on milkweed, were measured in Þve studies.First instars were exposed at 3Ð 4 and 6 Ð7 d after initial anthesis, either directly on milkweed plants in commercial cornÞelds or in the laboratory on leaves collected from milkweeds in corn plots.Pollen exposure levels ranging from 122 to 188 grains/cm 2 /d were similar to within-Þeld levels that monarch buttersy populations might experience in the general population of cornÞelds. Results indicate that 23.7% fewer larvae exposed to these levels of Bt pollen during anthesis reached the adult stage.A risk assessment procedure used previously was updated with a simulation model estimating the proportion of second-generation monarch buttersies affected.When considered over the entire range of the Corn Belt, which represents only 50% of the breeding population, the risk to monarch buttersy larvae associated with long-term exposure to Bt corn pollen is 0.6% additional mortality.Exposure also prolonged the developmental time of larvae by 1.8 d and reduced the weights of both pupae and adults by 5. 5%.The sex ratio and wing length of adults were unaffected.The ecological signiÞcance of these sublethal effects is discussed relative to generation mortality and adult performance.