Transgenic crops may hurt aquatic ecosystems

The U.S EPA declared in 1996 that transgenic Bt corn is unlikely to harm aquatic organisms. The corn contains a gene from the bacterium Bacillus thuringiensis (Bt), which makes a toxin that kills pests, and the agency had tested the corn's impact on water fleas—organisms not closely related to the insects that Bt targets. Now, a new study published October 9 in Proceedings of the National Academy of Sciences U.S.A. shows that the Bt toxin enters streams adjacent to cornfields and impedes the growth of caddis flies—a group of aquatic insects very closely related to the crop's pests. The study sheds new light on the potential environmental impacts of transgenic crops.

Bt corn targets the European corn borer and other closely related pest species and is a popular choice among farmers throughout the U.S. To evaluate its impacts on aquatic ecosystems, Emma Rosi-Marshall of Loyola University and colleagues examined the transport of crop litter—leaves, pollen, and cobs—in 12 streams near a heavily farmed region of Indiana. They found litter washing into streams, suggesting a pathway for Bt toxin to reach the water.

The streams were full of leaf-shredding and filter-feeding caddis flies that thrive on such agricultural wastes. When fed crop litter in the lab, these insects grew only half as much as those on a toxin-free diet. In addition, very high doses of pollen in the water were shown to kill as many as 43% of the insects.

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