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United States Department of Agriculture

Agricultural Research Service

Title: CORN POLLEN DEPOSITION ON MILKWEEDS IN AND NEAR CORNFIELDS

Authors
item Pleasants, John - IOWA STATE UNIV
item HELLMICH, RICHARD
item Dively, Galen - UNIV OF MARYLAND
item Sears, M - UNIV OF GUELPH
item Stanley-Horn, D - UNIV OF GUELPH
item Mattila, H - UNIV OF GUELPH
item Foster, John - UNIV OF NEBRASKA
item Clark, Thomas - UNIV OF NEBRASKA
item Jones, Gretchen

Submitted to: Proceedings of the National Academy of Sciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 17, 2001
Publication Date: N/A

Interpretive Summary: A consortium of scientists has studied whether pollen from transgenic Bacillus thuringiensis (Bt) corn could be hazardous to the caterpillars of the monarch butterfly. These studies assess ecological risk by considering toxicological effect and environmental exposure. In this paper pollen deposition on milkweed leaves, an important component of exposure, is considered. The density of corn pollen on leaves of milkweed plants inside and outside of cornfields was measured in several studies from different localities. Pollen density (grains/cm2) was highest (avg. 171) inside the cornfield and was progressively lower from the field edge outward, reaching low levels (~14) at 2 m and decreasing beyond. The average pollen density varied among studies due to several factors, including leaf position on a plant and rainfall. Leaves on the upper portion of milkweed plants had 30-50% of the pollen density levels of leaves in the middle. This is significant because the majority of young monarch larvae feed on upper leaves. A single rain event was observed to remove 86% of the pollen on leaves. At each distance from the field edge and for the inside of the field, a frequency distribution is presented showing the proportion of leaf samples with different pollen densities. This information will be useful for all stakeholders interested in the potential nontarget effects of transgenic plants.

Technical Abstract: The density of corn pollen on leaves of milkweed plants inside and outside of cornfields was measured in several studies from different localities. Pollen density was highest (avg. 171 grains/cm2) inside the cornfield and was progressively lower from the field edge outward, reaching low levels at 2 m (14 grains/cm2) and decreasing beyond. The average pollen density varied among studies due to several factors, including leaf position on a plant and rainfall. Leaves on the upper portion of milkweed plants had 30-50% of the pollen density levels of leaves in the middle. This is significant because the majority of young monarch larvae feed on upper leaves. Rainfall reduced the amount of pollen on leaves; a single rain event was observed to remove 86% of the pollen on leaves. The highest pollen density observed was 1400 grains/cm2, which was at the end of a rainless anthesis period. All other studies had rainfall events during the anthesis period and had mean pollen densities well below this level. At each distance from the field edge and for the inside of the field, a frequency distribution is presented showing the proportion of leaf samples with different pollen densities. The frequency distributions can be used in conjunction with laboratory and field bioassay information on the pollen density threshold of toxicity to determine the probability of a monarch larva encountering a toxic pollen dose while feeding on milkweed leaves in and near a Bt cornfield.

Last Modified: 8/27/2014