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Bt corn and Monarchs: The last word?

Dr. Ric Bessin, University of Kentucky, department of Entomology

      It is unusual to see such a controversy in agriculture brought about by a paper such as the one in the journal Nature in 1999. This paper alerted us to the potential threat to monarch larvae caused by pollen drifting from Bt corn. Prior to this, many had thought of Bt corn as a highly selective, precisely targeted tactic. It would only affect larvae of Lepidoptera and only those that fed on corn. What we learned from this study was that Bt corn does share one unwanted characteristic of some types of pesticide applications: pesticide drift.The protein that kills caterpillars is in the pollen and can drift onto other plants. Insects that donít feed on corn may also be exposed to the Bt toxin. But the laboratory study never addressed the risk posed by Bt corn to monarch populations in the field. Since then, environmentalists and government regulators called for more detailed studies to evaluate the environmental impact of Bt corn. Entomologists in several states and Canada responded by conducting extensive lab and field studies to study its impact on monarch and black swallowtail larvae.

      The Proceedings of the National Academy of Sciences recently published a set of six scientific papers in their October 2001 issue that address the Bt corn pollen and monarch caterpillar controversy. These papers critically evaluate the potential for impact of Bt corn pollen on monarch populations, and they summarize much of the work that has been conducted on the issue.

      The studies concluded that Bt protein expression in corn pollen in commercial lines greatly varies among events, but is generally low. Only pollen from Event 176 consistently affected monarch larvae at levels that are encountered in the field. [Event 176 is commercialized under the trade names of NatureGard and Knock Out.]Bt pollen from the other commercialized events, Bt11 and Mon810, should have no acute effects on the monarch larvae under field conditions. [Events Bt 11 and Mon810 are sold as YieldGard.] Currently, corn hybrids derived from Event 176 make up only a small fraction of the Bt corn acreage and will likely be phased out by 2003.

      These studies also noted that for monarch larvae to be exposed to the Bt toxin, the larvae must be feeding on the milkweed in or next to cornfields during or immediately following pollen shed in corn. But in Kentucky, monarch feeding is not synchronized with pollen shed in corn. Only a small fraction of the larvae are feeding during pollen shed and the overlap between pollen shed and monarchs is greater in the North than in the South. In southern corn-belt states there will be little overlap.

      Other factors that limit the effect of Bt corn on monarch populations are 1) only a fraction of the larvae feed on milkweed in or adjacent to corn fields and 2) a Bt corn field grown on only 19% of the acreage in the corn belt.Overall, the probability of monarch butterfly larvae being exposed to doses of Bt pollen that would cause a noticeable effect would be only 8 out of 1000.These papers conclude those potential effects of current Bt corn hybrids to monarch populations to be slight.

      The five-year EPA conditional registration for Bt corn expired in September 2001.Prior to the expiration, the EPA had requested a number of additional studies to be conducted to evaluate environmental and food safety concerns about Bt corn.The EPA conducted an extensive review of all Bt plant incorporated protectants (PIPís) and has granted a new conditional 7-year registration for Bt corn.They have also granted a new 5-year conditional registration for Bt cotton.Based on the information available, the EPA has concluded that these Bt-protected crops are safe to the environment and consumers.