Sustaining the Efficacy of Bt Toxins

In 1997 corn, cotton and potato cultivars that produce insecticidal proteins were grown on more than five million acres of US farmland. The acreage dedicated to these cultivars is likely to increase dramatically in the next twoto-five years. The use of these cultivars decreases the use of broad spectrum insecticides, especially in cotton and potato production. The toxin gene is derived from the bacterium, Bacillus thuringiensis (Bt), which has been naturally fermented and used as an organic pest control tool for decades. The toxin breaks down rapidly in the environment and is harmless to humans, vertebrates, and even most beneficial insects. In almost all ways this is the natural insecticide that you might expect environmentalists to dream about. So why have Bt toxin-producing crops been met with so much concern from the environmental and academic community? The issue is sustainability. Until recently, all formulations of fermented Bt had incredibly short insecticidal half lives in the field. The toxic action of the bacteria all but disappeared within two days after exposure to sunlight. Organic and conventional farmers who relied on Bt had to carefully time their spraying of the spore/crystal formulations to make sure that the bacteria were in the right place at the right time. This was difficult, but the positive side to this was that the pest population was typically exposed to the toxin only at times of peak pest densities. (Farmers would be wasting money if they sprayed pests when densities were low.) From an evolutionary perspective, this meant that the majority of insects in the pest population were never exposed to the toxin. These unexposed insects served as an “evolutionary buffer” to the development of resistance in the pest population. Sustaining the Efficacy of Bt Toxins