Iowa Census Shows Where Milkweed
Grows By Kim
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 states 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 Iowas landmass. Of the various other habitats
surveyed--pasture, roadsides and undisturbed areas--lands in USDAs
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 Services
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 USDAs chief
scientific research agency.
Scientific contact: Douglas D. Buhler, Michigan State
University, East Lansing, Mich, phone (517) 355-0217, fax (517 353-5174),