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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 don’t 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 (PIP’s) 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.
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