by Glen Martin
Chronicle Staff Writer
May 20, 1999
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Scientists have discovered that pollen from genetically manipulated corn could be killing native butterfly caterpillars in the Midwest, casting a shadow over the emerging industry of bioengineered crops.
Although the discovery, reported today in the journal Nature, must be confirmed by more field studies, the authors of the study said it is a warning that agricultural biotechnology may have unintended environmental consequences.
In the laboratory study, a group of Cornell University entomologists found that pollen from corn infused with genes from the bacterium Bacillus thuringiensis (Bt) is toxic to monarch butterfly larvae when lightly sprinkled on milkweed, the natural food source for the caterpillars.
Bt is a natural bacterium that is fatal to caterpillars and other larvae. When its genes are spliced into those of corn, a plant results that produces Bt toxins in its tissues, killing corn borers and other worm-like pests. Bt corn is considered safe for human consumption.
Under laboratory conditions, almost half of the monarch caterpillars that ate milkweed dusted with Bt corn pollen died after four days, the Cornell team found.
That compares with a 100 percent survival rate for caterpillars that ate milkweed that either was free of pollen or was sprinkled with pollen originating from corn that did not have Bt genes.
The implications of the study are ominous. Half of North America's eastern monarch butterfly population is concentrated in the "corn belt." And about 20 percent of the U.S. corn crop, or about 8 million acres, is planted in bioengineered Bt corn, a figure that grows every year.
While the plant strains have been credited with greatly reducing pesticide use, any "drift" of pollen could prove devastating for native butterflies and moths. Bt is harmless to most insects, but it obliterates caterpillars and other larvae.
Linda Rayor, one of the authors of the study, observes that corn is wind pollinated -- meaning its pollen is carried by the wind rather than by bees and other insects.
"When you have a toxin freely dispersed by the wind, it could affect any moth or butterfly larvae feeding near a cornfield," Rayor said. "Our article isn't meant to be alarmist -- but it is a warning, a flag of concern."
The study could mean that monarch butterflies are at particular risk. Populations of these big, orange and black migratory butterflies have been in steep decline because of pesticide use and habitat destruction.
Some environmentalists said the study is an indication agribusiness should go slowly in promoting transgenic crops.
"I think this clearly shows transgenic corn could be a serious threat to monarchs," said Rebecca Goldburg, a senior scientist with the New York-based Environmental Defense Fund. "I doubt if it would push them over the edge by itself, but it adds substantially to the other risks they face."
But Sarah Hake, a plant geneticist specializing in corn, said she is unsure if the study is valid for field conditions.
"There is a lot of wind outdoors, and corn pollen doesn't usually stick to plants in heavy concentrations, particularly if any distance is involved," said Hake, who directs the Plant Gene Expression Center in Albany, California, for the U.S. Department of Agriculture. "I think it would be difficult to mimic the actual concentrations you'd see under natural conditions. We need to see field research on this now before we can draw any firm conclusions."
Rayor said Cornell researchers plan to conduct field research this summer, but added that her group took pains to reproduce pollen concentrations expected for milkweed located about 60 yards from a cornfield. Corn pollen typically drifts 60 to 100 yards from the plant under windy conditions.
Even if Bt corn could be adversely affecting certain butterflies, say some scientists, those effects must be weighed against the dire environmental effects of heavy pesticide use.
The Bt bacteria, which occurs naturally and breaks down rapidly in the environment, was developed as an alternative to chemical pesticides. Anecdotal reports indicate that Bt cornfields are much richer in insect and avian life than Bt-free corn that is sprayed regularly with poisons.
"I essentially believe those reports," said Arthur Shapiro, a professor of entomology, ecology and evolution at the University of California at Davis. "There is no such thing as a risk-free technology. Everything involves trade-offs, and we need to make the best decisions we can based on the best information we have."
Jay Byrne, a spokesman for Monsanto Co., a St. Louis chemical and agricultural products firm that produces Bt corn, said his company supports further research on the matter.
"We need to objectively assess the risk," Byrne said, "and we have to do everything possible to reduce any risk that exists. But along with many scientists, we are not convinced that laboratory conditions are the same as field conditions. And we should also remember that Bt crops are reducing pesticide use by hundreds of thousands of gallons annually. That is a very substantial environmental benefit."
John E. Losey, the primary investigator of the Cornell report, agreed with Byrne.
"We need to look at the big picture here," he said. "Pollen from Bt corn could represent a serious risk to monarchs and other butterflies, but we can't predict how serious that risk is until we have a lot more data. And we can't forget that Bt corn and other transgenic crops have a huge potential for reducing pesticide use and increasing yields.
NEW THREAT TO THE MONARCH
Increasingly, farmers are planting a genetically altered form of corn that protects itself from pests by producing a toxin. The plants were thought to be harmless to nonpest insects. But researchers report that the plants produce a wind-borne pollen that can kill monarch butterflies. The pollen is blown onto milkweed, which is food for the monarch butterfly (top right) and the similar queen butterfly (larvae at right). The crop could prove to be a serious threat because the nation's corn belt is the heart of the monarch's breeding range.