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ISIS Report 30/10/07
Bt Crops Threaten Aquatic Ecosystems
Scientists find wastes from transgenic Bt corn impair growth of common aquatic
insect and call on future risk assessment to include aquatic ecosystems previously
overlooked. Dr. Mae-Wan
Ho
In 2006, 35 percent of the 33.1 million hectares of the corn planted in
the United States was transgenic, modified to express the Bt
toxin Cry1Ab from Bacillus thuringiensis.
Bt corn is widely planted in the Midwestern US, often next to headwater streams.
Yet, no environmental impact studies have been made of Bt crop by-products
on stream insects such as caddisflies (trichopterans), which are common in
streams, and closely related to the lepidopterans (butterflies and moths)
targeted by the Cry1Ab protein in Bt corn.
As a group, the caddisflies
have diverse feeding habbits. Some are filter- feeders, others scrape bioflims
off submerged surfaces, and still others feed on detritus. All these caddisflies
may consume Bt corn by-products.
A team of scientists led by Emma J. Rosi-Marshall at:Loyola
University Chicago in Illinois have now carried out the first study on the
fate of transgenic Bt corn wastes in headwater streams next to the fields
and their impact on the caddisflies [1]
In laboratory trials, leaf-shredding trichopteran Lepidostoma liba fed Bt corn litter had less
than half the growth rate of controls fed non Bt corn litter; while 43 percent
of Helicopsyche borealis, an
algal-scraping trichopteran, died when fed high concentrations of Bt corn
pollen (2 to 3 times the maximum input expected during Bt corn pollen-shed)
compared with 18 percent mortality in controls fed non Bt corn pollen.
In the field, 50 percent
of filtering caddisflies collected during pollen-shed had pollen grains in
their gut and detritus-feeding trichopterans were found in the accumulations
of decomposing corn litter in the streams after harvest.
Bt crop by-products fall into the streams as pollen and detritus,
they are stored in the sediment, eaten, and transported downstream, and hence
their impacts could spread widely within the aquatic ecosystem.
The researchers conclude that “widespread planting of Bt crops
has unexpected ecosystem-scale consequences”, and call on future assessment
of potential non-target effects to be expanded to include relevant aquatic
organisms such as stream insects.
They fell short of calling
a halt to planting Bt corn next to streams, which would be in keeping with
the evidence they have provided.
References
- Rosi-Marxhall EJ, Tank JL, Royer TV, Whiles MR,
Evans-White M, Chamgers C, Griffiths NA, Pokelsek J and Stephen ML. Toxins
in transgenic crop byproducts may affect headwater stream ecosystems. PNAS 2007, 104, 16204-8.
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