Temporal and spatial overlap between monarch larvae and corn pollen

To assess the likelihood that monarch larvae will be exposed to Bacillus th uringiensis (Bt) pollen, we studied milkweed and monarch densities in habit ats which comprise much of the land available to breeding monarchs, e.g., c ornfields, cornfield edges, other agricultural fields, and nonagricultural areas, in four regions of the monarch breeding range. We found that monarch s use milkweed in cornfields throughout their breeding season, and that per plant densities are as high or higher in agricultural habitats as in nonag ricultural habitats. As a result of the prevalence of agricultural land, mo st of the monarchs produced in the upper Midwest are likely to originate in cornfields or other agricultural habitats. There was a greater temporal ov erlap between susceptible monarchs and corn anthesis in the northern than t he southern part of the summer breeding range, because of earlier pollen sh ed in the south. The importance of agricultural habitats to monarch product ion suggests that, regardless of the impact of genetically modified crops, agricultural practices such as weed control and foliar insecticide use coul d have large impacts on monarch populations.