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Monarchs safe from Bt

GM plant pollen may be off the hook, but regulators are still feeling the heat.

Anti-GM protesters in Luxembourg used the butterflies for their cause. Credit: © Greenpeace/Ernst

Pollen from genetically modified (GM) corn plants that contain the insecticidal Bt toxin is not a significant danger to North American monarch butterflies (Danaus plexippus), a suite of new research suggests.

Previous studies pointing to Bt toxin as the most tangible ecological threat so far identified from transgenic crops made it the cause célèbre of the anti-GM movement.

The corn contains a bacterial gene encoding a substance that poisons insects eating the plant's tissues. Although GM corn pollen may be off the hook, critics feel that the debate has highlighted weaknesses in the regulation of transgenic crops in the United States.

"The impact of this technology on monarchs is negligible," says entomologist Mark Sears of the University of Guelph in Ontario, Canada. Sears has contributed to all six of the field studies, which are due to be published later this month1-6.

GM corn's impact on monarch caterpillars "negligible". Credit: © SPL

The monarch's potential plight came to light in 1999. A preliminary laboratory-based study found that milkweed leaves - the monarch's food plant, which often grows beside corn fields - contaminated with pollen from Bt corn plants could kill monarch caterpillars that fed on them7. The finding caused immediate protest from GM-sceptics.

Now researchers, including the authors of the original study, have completed large-scale field and lab studies of the toxicity of Bt pollen, its release into the environment, and the risk it poses to monarchs. The investigation was funded by the US Environmental Protection Agency (EPA) and the agricultural biotechnology industry.

The research finds that pollen from the varieties of Bt corn most commonly grown in the United States do not contain enough toxin to harm monarchs, and that the crop is not grown widely enough to pose a significant risk. According to the EPA, about 19 per cent of corn in North America contains the Bt gene.

One variety of Bt corn that contains a gene called 'event 176' was found to be harmful to another American butterfly, the black swallowtail (Papilio polyxenes)1. Fortunately this variety, marketed under the trade name 'KnockOut', proved so unpopular with growers it constituted only 2 per cent of the annual US crop and is now being withdrawn by its manufacturer, Syngenta.

We have a regulatory system based on luck Jane Rissler, Union of Concerned Scientists

"We are pleased [that] it looks like transgenic corn pollen is harmless," says Jane Rissler of the Washington-based Union of Concerned Scientists, which is calling for more caution over the introduction of transgenic crops. In the case of event 176 corn, "the monarchs were lucky that is was unpopular with growers," she says.

That KnockOut was approved points to weaknesses in how the introduction of GM crops in the United States is regulated, says Rissler. "We have a regulatory system based on luck," she argues.

"The EPA has learned a lot from this process," agrees Sears who feels that the monarch scare holds lessons for scientists too. "We've had to learn how to say what the hazards are and how we go about providing the information to regulatory bodies," he says.

The EPA may be extending the public comment period for re-approval licenses, which is required for growing transgenic crops, in light of the new findings.


  1. Zangerl, A.R. et al. Effects of exposure to event 176 Bacillus thuringiensis corn pollen on monarch and black swallowtail caterpillars under field conditions. Proceedings of the National Academy of Sciences, in press (2001).

  2. Hellmich, R. L. et al. Monarch larvae sensitivity to Bacillus thuringiensis purified proteins and pollen. Proceedings of the National Academy of Sciences, in press (2001).

  3. Pleasants, J. M. et al. Corn pollen distribution on milkweeds in and near cornfields. Proceedings of the National Academy of Sciences, in press (2001).

  4. Oberhauser, K. S. et al. Temporal and spatial overalap between monarch larvae and corn pollen. Proceedings of the National Academy of Sciences, in press (2001).

  5. Sears, M. K. et al. Impact of Bt corn pollen on monarch butterfly populations: A risk assessment. Proceedings of the National Academy of Sciences, in press (2001).

  6. Stanley-Horn, D. E. Assessing the impact of Cry1Ab-expressing corn pollen on monarch butterfly larvae in field studies. Proceedings of the National Academy of Sciences, in press (2001).

  7. Losey, J. E., Rayor, L. S. & Carter, M. E. Transgenic pollen harms monarch larvae. Nature 399, 214, (1999).

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Clarke, T. Monarchs safe from Bt. Nature (2001).

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