Science in Society Archive

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.

Article first published 30/10/07


  1. 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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