"One of the most important conclusions of this study is that agricultural habitat is important to monarchs," said Oberhauser. "If an event that hurts monarchs or their host plant, milkweeds, should occur in cornfields, it could have population-wide impacts."
Oberhauser cautioned, however, that studies by other researchers have suggested that Bt toxin is not harmful to monarchs at levels found in cornfields. But many of those studies occurred in laboratories and looked at pollen only. In cornfields, plants shed part of the anthers (the structures that produce pollen), and anthers tend to have higher concentrations of Bt than pollen does. Even the studies done in the field may have underestimated the amount of time that monarchs were exposed to toxic corn tissue. While monarch larvae don't avoid pollen on milkweed leaves, it is not known whether they also eat anther material, Oberhauser said.
"I think we still don't know the effects of long-term exposure to Bt," she said. "Some lab studies may not have exposed them long enough to tell." And, even if Bt doesn't kill monarchs at levels found in cornfields, "We don't know how Bt affects factors like monarch reproduction, flight ability and size," Oberhauser said.
Oberhauser's colleagues in the study were from the University of Guelph, Ontario; the Univers
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Contact: Deane Morrison
morri029@umn.edu
612-624-2346
University of Minnesota
21-Sep-2001