Impact of Bt corn pollen on monarch butterfly populations: A risk assessment

Authors

Mark K. Sears, University of Guelph, Guelph, ON, Canada
Richard L. Hellmich, Iowa State University, Ames, IA
Diane L. Stanley-Horn, University of Guelph, Guelph, ON, Canada
Karen S. Oberhauser, University of Minnesota, St. Paul, MN
John M. Pleasants, Iowa State University, Ames, IA
Heather R. Mattila, University of Guelph, Guelph, ON, Canada
Blair D. Siegfried, University of Nebraska-LincolnFollow
Galen P. Dively, University of Maryland, College Park, MD

Document Type

Article

Date of this Version

October 2001

Comments

Published in PNAS - October 9, 2001 - vol. 98 - no. 21 - 11937–11942. www.pnas.orgycgiydoiy10.1073ypnas.211329998

Abstract

A collaborative research effort by scientists in several states and in Canada has produced information to develop a formal risk assessment of the impact of Bt corn on monarch butterfly (Danaus plexippus) populations. Information was sought on the acute toxic effects of Bt corn pollen and the degree to which monarch larvae would be exposed to toxic amounts of Bt pollen on its host plant, the common milkweed, Asclepias syriaca, found in and around cornfields. Expression of Cry proteins, the active toxicant found in Bt corn tissues, differed among hybrids, and especially so in the concentrations found in pollen of different events. In most commercial hybrids, Bt expression in pollen is low, and laboratory and field studies show no acute toxic effects at any pollen density that would be encountered in the field. Other factors mitigating exposure of larvae include the variable and limited overlap between pollen shed and larval activity periods, the fact that only a portion of the monarch population utilizes milkweed stands in and near cornfields, and the current adoption rate of Bt corn at 19% of North American corn-growing areas. This 2-year study suggests that the impact of Bt corn pollen from current commercial hybrids on monarch butterfly populations is negligible.