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Biology
BT Corn and Butterflies
Studies at several U.S. universities over the past two years have raised concern that certain pesticides genetically inserted into corn to kill the destructive European corn borer are harming the beloved monarch and black swallowtail butterflies. Recent field tests led by LAS entomologists confirm that the risk is real, but that its extent varies and may be manageable.
Field tests this past summer led by entomologists May Berenbaum and Arthur R. Zanger showed that a variety of Bt corn called event 176, which was sold under the name Knockout, dramatically reduced growth rates of black swallowtail caterpillars. The caterpillars fed on parsnip plants placed near cornfields planted with Bt 176 corn. Those munching on plants one-half meter away were three times smaller than those feeding on plants 7 meters away. The researchers had also tried to study the impact on the monarch butterflies, but their high death rates were attributed to predation.
"Bt" refers to Bacillus thuringiensis, the soil bacterium that produces the toxin that kills the borers. The use of Bt corn became controversial in 1999 after the journal Nature published a Cornell University study linking the bioengineered product with the death of monarch caterpillars feeding on milkweed dusted with wind-blown pollen from Bt corn in nearby fields. At the time, millions of acres of Bt corn were planted annually in the United States to control against the European corn borer, which was responsible for crop losses averaging $1.2 billion a year.
The researchers chose Bt 176 for study from the nearly two dozen varieties on the market because it is known to produce pollen with higher concentrations of the pesticide than other varieties. It also had been linked in earlier studies to a reduced survival of monarchs at concentrations naturally occurring in and near cornfields.
Berenbaum believes the risk posed by Bt is manageable because Bt 176 has been withdrawn from the market (due to poor sales), and the other varieties of Bt corn are more benign. Another study last summer by Berenbaum showed that black swallowtail caterpillars munching on parsnip dusted with Bt 810 were unharmed.
Says Berenbaum: "Our findings suggest that different Bt events, or strains, have different biological properties, and nontarget impacts are difficult to predict."
Fall 2001