Bt Corn Risk to Monarchs Is ‘Negligible’

A much-anticipated report from a consortium of U.S. and Canadian scientists states that the most commonly planted forms of genetically engineered Bt corn pose only a “negligible” risk to monarch butterfly populations.

Monarch butterflies in their wintering ground in Mexico. L. S. Rayor

These types of corn make their own pesticides, nicknamed Bt in honor of the Bacillus thuringiensis bacterium that supplied the toxin’s gene. The pesticide rides within corn pollen as it drifts on the wind, sometimes settling on the milkweed plants that monarch caterpillars eat.

The vast majority of the 19 percent of cornfields in North America planted with Bt corn relies on two genetically engineered strains, known as events Mon810 and Bt111. Neither has been proven to kill young monarch caterpillars under field conditions, report Mark K. Sears of the University of Guelph in Ontario and seven of his colleagues.

Another Bt corn, event 176, pumps out toxin concentrations in corn pollen that are 50 to 100 times those in Mon810 and Bt111, the researchers report. Event 176 corn is slated to be withdrawn from the market.

The assessment of Bt corn’s threat to monarchs appears in the Oct. 9 Proceedings of the National Academy of Sciences (PNAS). Five other papers by various combinations of the eight scientists and their colleagues appear in the same issue.

“The take-home messages from these papers are that not all Bt corn events are alike and that not all laboratory studies predict what happens in the field,” says May R. Berenbaum at University of Illinois, at Urbana-Champaign. Berenbaum is a coauthor on one PNAS paper that focuses on event 176 corn. As a member of the National Academy of Sciences, she sponsored the papers’ publication.

Her comment evokes the origin of the consortium of researchers—in the wake of a 1999 report that up to half of monarch caterpillars in the lab died after eating pollen-dusted leaves (SN: 5/22/99, p. 324: http://www.sciencenews.org/sn_arc99/5_22_99/fob1.htm). The highly polarized atmosphere surrounding genetically modified crops was hampering attempts to rationally plan and evaluate research. Fueled by public alarm, the Agricultural Research Service, which is part of the U.S. Department of Agriculture, organized a research effort to investigate Bt corn’s effects out in the world. Funding came from both government and industry.

These studies included toxicity tests in the lab and field. Researchers monitoring fields found that in Ontario, 62 percent of monarchs were in their plant-eating stages at a time when Bt corn was shedding pollen; the number was only 15 percent in Iowa. Using surveys of plants on farmland, the researchers found that cornfields make up about 28 percent of monarch breeding grounds. Butterflies were nearly twice as likely to lay eggs on a milkweed in a cornfield than in some other environment.

“I was surprised,” says Karen S. Oberhauser of University of Minnesota, St. Paul, coauthor of that portion of the study. “I’ve been studying monarchs for years and never knew that.”

Putting the pieces together, the research consortium concludes that “the impact of Bt corn pollen from current commercial hybrids on monarch butterfly populations is negligible.”

Despite the large amount of work invested in them, the studies leave gaps, grumbles longtime monarch biologist Lincoln P. Brower of Sweet Briar College in Virginia. “There’s nothing on long-term effects,” he says.

That’s one of the points that Oberhauser raised in a document that she’s filed with the Environmental Protection Agency.

She adds that there’s little overall perspective on monarch population trends. Monitoring efforts have been so scant that she can’t say whether the species is thriving or dwindling. That makes it hard to assess how much mortality is too much.

Berenbaum concurs. For example, she says that one of her students has completed a survey indicating that Illinois highway traffic each year kills some 27 million moths and butterflies. Yet, she says, there’s no way to know whether the monarch portion of this mortality significantly affects the overall population.

The concern over Bt corn’s effects on monarch butterflies has obscured other important concerns, comments entomologist Tom Turpin of Purdue University in West Lafayette, Ind. For example, researchers don’t know what long-term effects Bt toxins might have on soil organisms, he says.

A foreign observer, longtime butterfly biologist Myron Zalucki of the University of Queensland in Brisbane, Australia, comments on the strong polarities that cloud research on monarchs and Bt. “If you think it’s just scientists getting together to sort out a problem, that’s codswallop,” he says.


Susan Milius

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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