Unintended effects of Bt crops.

Reinforcing a number of concerns voiced by entomologists, crop scientists, and environmentalists, several recent studies suggest that the risks posed by crops genetically engineered to produce their own pesticides may in fact be greater than the risks posed by spraying the same pesticides on crops.

The three studies - two from the Swiss Federal Research Station for Agroecology and Agriculture, and one from the Scottish Crop Research Institute - were conducted on potato and corn varieties that were genetically modified to produce the Bt toxin to ward off various crop pests. The Bt toxin - derived from the soft bacterium, Bacillus thuringiensis - has been used as a sprayable insecticide throughout the world since the early 1950s. In genetically modified crops the bacterial gene for producing the toxin is spliced into the crop's DNA so that the plant itself can produce the toxin. However, there are two important differences between the sprayable Bt insecticide and the form of Bt engineered into the crop genome - differences that have serious implications for public health and environmental safety.

First, while the naturally derived spray version of Bt is highly species-specific (its toxicity is activated only in the gut of certain insect species), the genetically modified version has been altered to work against an array of insects - harmful or not. The recent studies showed that beneficial insects - so named because they prey on crop pests - were also exposed to harmful quantities of Bt. The result was a two-fold increase in the adult mortality and reproductive failure in two very different beneficial species. The studies also showed dramatically reduced fitness and increased mortality in the beneficials' larvae and eggs, in addition to a significantly prolonged development rime for the survivors. These side effects of Bt crops have now been demonstrated for a wide variety of insects and soil organisms, and preliminary studies suggest that the adverse effects could even be felt by insect-caring bird populations, many levels up the farm foodweb - a food-web that includes plants and animals consumed by humans.

The second significant difference is that the Bt crops deliver extremely high levels of the toxin - roughly 10 to 20 times the lethal dose of the sprayable formulation. Mark Whalon, director of the Pesticide Research Center at Michigan State University, notes that, in contrast to the carefully timed applications of sprayable Bt and the "micrograms" of pest-fighting compounds that plant immune systems naturally secrete from specific tissues at specific times during the season, "these transgenic crops are now pumping out huge amounts of toxins from all tissues throughout the entire growing season, from germination to senescence." While many insecticides, such as neem and Bt, occur naturally and are often used sparingly by organic farmers, says Whalon, "this does not qualify them as non-toxic to non-target organisms, especially in the amounts produced by Bt crops."

Both the Swiss and Scottish scientists also warn that the more aggressive the measures for pest eradication, the greater the likelihood that successive generations of pests become more resistant. The long-term result would be a mutual arms race between farmers and pests, in which plants engineered to secrete increasingly toxic chemicals would be deployed against increasingly resistant strains of pests. This heightened threat of Bt-resistance, coupled with the devastation of beneficial insect populations that help keep pests in check, could lead to massive crop losses.

Although initial large-scale plantings of Bt crops may appear benign, it will likely take several seasons for toxicity and resistance problems to emerge. Both the Swiss and Scottish scientists argue that the safety of Bt crops "cannot simply be deduced from the safety record of Bt insecticides," which has been the basis for their approval in the United States and the European Union. The scientists advocate a moratorium on large-scale releases of Bt crops until the long-term ecological effects are better assessed.

For more information, contact: Jane Rissler, Union of Concerned Scientists, Washington, DC, WWW.ucsusa.org.