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Bt corn May Harm Stream Ecosystems

October 10, 2007

A new study indicates that a popular type of genetically engineered corn--called Bt corn--may damage the ecology of streams draining Bt corn fields in ways that have not been previously considered by regulators.

Genetically modified corn, commonly called Bt corn, is engineered to kill pests such as the European corn borer. However, a new study shows that Bt corn may also harm the caddisfly, which serves as food for fish and amphibians. The new study also shows that parts of Bt corn, such as leaves, cobs and pollen, can travel as far as 2000 meters away from source areas--a phenomenon that was not considered when Bt corn was licensed.

This study provides the first evidence that toxins from Bt corn may travel long distances in streams and may harm stream insects that serve as food for fish. These results compound concerns about the ecological impacts of Bt corn raised by previous studies showing that corn-grown toxins harm beneficial insects living in the soil.

Licensed for use in 1996, Bt corn is engineered to produce a toxin that protects against pests, particularly the European corn borer. Bt corn now accounts for approximately 35 percent of corn acreage in the U.S., and its use is increasing.

"As part of the licensing process for genetically modified crops, the U.S. Environmental Protection Agency (EPA) was responsible for testing and identifying potential environmental consequences from the planting of Bt corn," says Jennifer Tank, who is from the University of Notre Dame and is a member of the team studying Bt corn.

Typical accumulations of debris from corn plants found in headwater streams during floods in the U.S. corn belt. If such debris comes from corn that has been genetically modified to contain insecticidal toxins (known as Bt corn), it may harm stream organisms that consume it.


To fulfill this requirement, EPA completed studies that assumed that plant parts would remain in fields without being carried away by streams draining agricultural lands, says Tank. In addition, EPA only tested the impacts of Bt corn on small lake organisms that are typically used to test the impacts of chemicals on aquatic ecosystems.

The agency did not evaluate the impacts of Bt corn on organisms that live in streams--even though Midwest agricultural lands where Bt corn is grown are heavily intersected by streams draining the landscape. But despite the limitations of its tests, EPA concluded that Bt corn "is not likely to have any measurable effects on aquatic invertebrates."

To more comprehensively evaluate the ecological impacts of Bt corn than did the EPA, the research team did the following:

  1. Measured the entry of Bt plant parts--including pollen, leaves and cobs--in 12 streams in a heavily farmed Indiana region. The research team's results demonstrate that these plant parts are washing into local steams. Moreover, during storms, these plant parts are carried long distances and therefore could have ecological impacts on downstream water bodies, such as lakes and large rivers.
  2. Collected field data indicating that Bt corn pollen is being eaten by caddisflies, which are close genetic relatives of the targeted Bt pests. Todd V. Royer, a member of the research team from Indiana University, says that caddisflies "provide a food resource for higher organisms like fish and amphibians."
  3. Conducted laboratory tests showing that consumption of Bt corn byproducts increased the mortality and reduced the growth of caddisflies. Together with field data indicating that the caddisflies are eating Bt corn pollen, these results "suggest that the toxin in Bt corn pollen and detritus can affect species of insects other than the targeted pest," Tank said.

Royer says that "if our goal is to have healthy, functioning ecosystems, we need to protect all the parts. Water resources are something we depend on greatly."

"Overall, our study points to the potential for unintended and unexpected consequences from the widespread planting of genetically engineered crops," Tank said. "The exact extent to which aquatic ecosystems are, or will be, impacted is still unknown and likely will depend on a variety of factors, such as current ecological conditions, agricultural practices and climate/weather patterns."

A typical headwater stream running through a corn field shown during pollen shed.

James Raich, a National Science Foundation program director, adds that "increased use of corn for ethanol is leading to increased demand for corn and increased acreage in corn production. Previous concerns about the nutrient
enrichment of streams that accompany mechanized row-crop agriculture are now compounded by toxic corn byproducts that enter our streams and fisheries, and do additional harm."

The Bt corn researchers stress that their study should not be viewed as an indictment of farmers."We do not imply that farmers are somehow to blame for planting Bt corn, nor are they responsible for any unintended ecological consequences from Bt corn byproducts," Tank said. "Farmers are, to a large extent, required to use the latest technological advances in order to stay competitive and profitable in the current agro-industrial system."

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