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Iowa Census Shows Where Milkweed Grows

By Kim Kaplan
November 30, 2000

A census of the common milkweed in Iowa by U.S. Department of Agriculture scientists indicates the plant can be found along almost three-quarters of the roadsides and in about half of the state’s corn and soybean fields.

Corn and soybean fields had the lowest average percent area infested with common milkweed-- less than 0.03 percent--but these crops cover 78 percent of Iowa’s landmass. Of the various other habitats surveyed--pasture, roadsides and undisturbed areas--lands in USDA’s Natural Resources Conservation Service Conservation Reserve Program (CPR) had the highest percentage of area infested with milkweed (2 percent).

Common milkweed, Asclepias syriaca, is a native of the northeastern and north central United States and adjacent areas of Canada. It is the primary plant on which monarch butterfly eggs are laid and the main food source for monarch caterpillars.

The census was conducted by Douglas D. Buhler, formerly an agronomist at the Agricultural Research Service’s National Soil Tilth Research Laboratory in Ames, Iowa, now chair of the Department of Crop and Soil Sciences at Michigan State University-East Lansing. The study is part of the research response to concerns raised by a note published in Nature in June 1999 about a small, preliminary laboratory study on the potential for the pollen of Bt corn to cause harm to monarch caterpillars. Bt corn is corn that has been genetically modified to express proteins found in the bacteria Bacillus thuringiensis, to reduce the need for applying chemical insecticide to the crop.

The distribution of common milkweed is simply one piece of the picture helping to clarify the true risk, if any, that Bt corn might pose to monarch butterflies. Other pieces needed to complete the risk assessment include identifying monarch preferences for milkweed in particular habitats, the distribution and extent of Bt corn pollen during pollen shed, and the actual toxicity of Bt corn pollen to monarchs.

ARS is USDA’s chief scientific research agency.

Scientific contact: Douglas D. Buhler, Michigan State University, East Lansing, Mich, phone (517) 355-0217, fax (517 353-5174),