Submitted to: Crop Protection Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 26, 2000
Publication Date: N/A
Interpretive Summary: The role of native plants in the life cycle of other organisms is not well understood. Common milkweed is a plant that is native to the northeastern and north central United States and adjacent areas of Canada and is an important component of the life cycle of the monarch butterfly. Common milkweed and other closely related plant species are the sole food source of monarch butterfly larvae, whereas the adults feed on a wide range of flowers. The importance to the monarch of common milkweed populations in the central United States, coupled with concerns about toxicity to monarch larvae of the pollen of Bt corn deposited on common milkweed leaves, have generated interest in the distribution of common milkweed in crop lands and adjacent areas. The objective of our research was to determine the relative distribution of common milkweed in different habitats in Iowa. Common milkweed was found in 71% of the roadsides and approximately 50% of the corn and soybean fields. Corn and soybean fields had 85% fewer patches of common milkweed than roadsides. While common milkweed was frequently found in corn and soybean fields, average frequency and patch sizes were much greater in roadside and other undisturbed areas. These results are of direct benefit to those attempting to assess the proportion of the milkweed population that may be affected by changing herbicide use patterns in crop fields and the potential impact of Bt pollen on monarch populations.
Interest in the population dynamics and geographic distribution of common milkweed has recently increased due to the importance of common milkweed in the life cycle of the monarch butterfly. A survey of common milkweed occurrence in various habitats was conducted in Iowa in June and July of 1999. Common milkweed was found in 71% of the roadsides and approximately 50% of the corn and soybean fields. Corn and soybean fields had 85% fewer patches than roadsides. Conservation reserve program fields had the greatest average area infested. While common milkweed was frequently found in corn and soybean fields, average frequency and patch sizes were much greater in non crop areas.