Byproducts from Bt maize enter streams, and in feeding experiments
affected stream insects.
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).
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.
Loyola University, University of Notre Dame, Indiana University, and
Southern Illinois University (all USA).
Grant from the U.S. National Science Foundation.
Commercialized crop; Bt, maize has been grown commercially in the
U.S. since 1996.
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