Bt-Corn: The Biggest GE Crop

Bacillus thuringiensis, or Bt, is a common soil bacterium whose genome contains genes for several proteins toxic to insects. For decades, Bt has been sprayed on fields as an organic pesticide; several major pests of corn that are difficult and expensive to control with chemical insecticides are susceptible to Bt. When sprayed on the surface of crops, however, Bt toxins break down quickly when exposed to ultraviolet light, and they also wash off in a strong rain.

To address these problems, several varieties of corn have been genetically engineered to incorporate Bt genes encoding proteins called “delta-endotoxins” and “vegetative insecticidal proteins” (VIPs), which are specific to various insect pests. Some strains of Bt produce proteins that are selectively toxic to caterpillars, such as the southwestern corn borer, while others target mosquitoes, root worms, or beetles. To create a Bt crop variety, plant scientists select the gene for a particular Bt toxin and insert it into the cells of corn or cotton plant at the embryo stage. The resulting mature plant has the Bt gene in all its cells and expresses the insecticidal protein in its leaves. Caterpillars ingest the toxin, which fatally damages the lining of the gut.

Because Bt-corn produces an insecticide within its tissues, the toxic proteins are protected from the sun and thus persist longer. Moreover, Bt-corn makes the toxin continually over a season, extending its protective effects. Since Bt-corn offers an alternative to spraying chemical insecticides, it offers environmental and economic benefits to farmers. Most Bt toxins are selective for specific caterpillars and closely related species. There are no known effects to mammals, fish, or birds, and they appear safe for consumers. Nevertheless, future varieties that entail changes in plant metabolism could possibly be associated with toxicity.¹