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Byproducts from Bt maize enter streams, and in feeding experiments affected stream insects.

Manipulated Organism: Maize (Zea mays L.).

Inserted Transgene and Intended Effect: crylAb gene derived from Bacillus thuringiensis (Bt) and fused to the cauliflower mosaic virus (CaMV-35S) promoter so that the crylAb gene would be expressed in all parts of the plant. This gene gives transgenic Bt maize (also called Bt corn) the ability to produce insecticidal delta-endotoxin that can kill the larvae of the European corn borer and other insect pests feeding on the corn.

Goal of This Study: The authors investigated whether byproducts from Bt maize (pollen and leaf, stem, and cob litter) enter streams via wind and runoff and, if so, whether they might affect nontarget stream insects. The researchers took samples from 12 typical headwater streams in a strongly agricultural part of Indiana to determine the amount of maize byproducts in the water. Since about 35% of the maize grown in the U.S. in the study years was Bt maize, they estimated that the same proportion of Bt maize byproducts was present in their samples. They also carried out laboratory feeding experiments with caddisflies, common stream insects closely related to moths. Moth larvae are the primary target insects for Bt maize.

Results of This Study:
  • The researchers found pieces of maize leaves and cobs, as well as pollen, in all the headwater streams in varying amounts and concentrations. In fast moving streams pollen was transported up to 2 km downstream. Their findings suggest that "transgenic material is retained during base flow and thus is available for microbial processing, consumption by aquatic insects, or export during storms" (p. 16204).
  • 50% of those collected caddisflies that filter small particles for food were found to have pollen grains in their guts. Litter-feeding caddisflies were "located in accumulations of decomposing corn litter in the streams after harvest" (p. 16205).
  • In laboratory feeding trials they found that specimens of a caddisfly species fed Bt maize litter at a rate comparable to their typical feeding habits had greater than 50% slower growth rates compared to the controls that were fed non-Bt maize. In streams, therefore, feeding on Bt maize litter may "negatively impact fitness, because adult size of aquatic insects is directly related to fecundity" (p. 16206).
  • They also fed an algal-scraping caddisfly species algae and maize pollen in amounts comparable to the daily stream input rates. They found no significant difference in mortality between those specimens fed Bt maize and non-Bt maize pollen at this concentration. At two to three times higher concentrations, mortality was higher (43% of the animals died) in the animals fed Bt maize pollen compared to those fed non-Bt maize (18% mortality).

The authors conclude: "Lower growth rates and higher mortality of stream caddisflies, as measured in our laboratory feeding studies, could potentially reduce secondary production and consequently the prey biomass available to stream and riparian predators, such as fishes, amphibians and birds. . . . We suggest that the assessment of potential nontarget effects from transgenic crops should be expanded to include relevant aquatic organisms, such as stream insects" (p. 16206).

Source: Rosi-Marshall, E. J., J. L. Tank, T. V. Royer, M. R. Whiles et al. (2007). "Toxins in Transgenic Crop Byproducts May Affect Headwater Stream Ecosystems," PNAS vol. 104, pp. 16204-8.

Author Affiliations: Loyola University, University of Notre Dame, Indiana University, and Southern Illinois University (all USA).

Funding: Grant from the U.S. National Science Foundation.

Product Status: Commercialized crop; Bt, maize has been grown commercially in the U.S. since 1996.

Copyright 2008 The Nature Institute.

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