Risk assessment for the dispersal of Bt-corn pollen to non-target lepidopterous insects

Genetically modified (GM) corn with the gene for the Bacillus thuringiensis (Bt) toxin (abbreviated Bt-corn) is resistant to the European corn borer, Ostrinia nubilalis, and offers reduced pest control costs, increased yields, and improved product quality. Consequently, the area of cultivation of Bt-corn continues to expand in the USA. However, in Japan, GM crops must be confirmed as safe for consumers and the environment before import into Japan. The safety of GM crops to the environment is assessed by the Ministry of Agriculture, Forestry and Fisheries (MAFF) in experiments examining cultural and reproductive characteristics, overwintering, weediness, production of harmful substances and crossing with wild relatives. Losey et al. (1999) recently reported that the larvae of the monarch butterfly, Danaus plexippus, fed on Bt-corn pollen dusted on the leaves of the, milkweed, Asclepias curassavica, showed lower survival rate, slower growth rate and reduced consumption of the leaves. This report had an impact around the world. But the authors did not study the importance of pollen density. So the extent of any environmental impacts by Bt-corn pollen dispersal remains unknown. Consequently, we started an urgent research project in 1999 to quantitatively assess the environmental impact of Bt-corn pollen on non-target organisms. We collected data for a risk assessment. Risk is evaluated as follows :Risk = Hazard * Exposure. Exposure means the probability of larval encounter with Bt-corn pollen deposited on leaves of host plants of a given distance from the edge of a cornfield. Hazard means the toxicity of Bt-corn pollen to the larvae of non-target lepidopterous species. Risk is thus the relationship between the density of Bt-corn pollen and the response of larvae to the pollen.