BT

Bt Crystals as Insecticides

Bt's appeal as a pesticide is the specificity with which it works. Most Bt strains produce different combinations of insecticidal crystal proteins (ICPs), and different ICPs prove toxic to different types of insects. This is so because the toxins bind only to particular receptors in midgut epithelial cells.

Basically, insects must ingest Bt crystals and spores to be affected. Once ingested, the crystal toxins dissolve if the conditions in the gut are suitable, and then the toxins bind to receptors and erode the gut lining, which allows the Bt spores and gut bacteria to spread out of the gut and eventually kill the host. Although this entire process may take days, the insect will stop feeding within hours of initial ingestion.

Because not all insects possess the same gut receptors, different ICPs—from different strains of Bt—may be developed to target specific insect species. Bt is considered nontoxic or even harmless to vertebrates, including humans, and many other arthropods, including a wide range of “beneficial” insects, which do not possess receptors as described.

Bt in Commercial Products

In 1901, Japanese bacteriologist Shigetane Ishiwatari first isolated what is now known as Bt as the cause of sotto, or sudden-collapse disease, which was decimating silkworm populations and threatening Japan's silk industry. Given the negative effects of the bacterium, there was little desire by Ishiwatari to expand its use. But in 1911, German scientist Ernst Berliner rediscovered it as he examined dead flour moth caterpillars. Because of the bacterium's effectiveness in killing a “pest” species, Berliner detailed the bacterium as a potentially valuable pesticide. He published a scientific description and assigned the name Bacillus thuringiensis in 1915. Efforts to cultivate Bt as a pesticide then began in earnest.

French insecticide industries began to commercially produce Bt-based formulas, such as Sporeine in 1938, as powders containing sporulated cells and their toxic crystals. Over the decades, additional formulations of Bt-based products targeting a variety of insects were developed and marketed in Europe and in the United States. By 1995, the U.S. Environmental Protection Agency listed 182 registered Bt products, but they accounted for only 2 percent of the money spent globally on insecticides.

Bt insecticides are mixed as sprays and applied to leaves or other surfaces where the targeted insect larvae feed. Bt toxins demonstrate great specificity, degrade when exposed to ultraviolet light, and are easily washed away by rain. These qualities attracted environmentalists to Bt as an appealing alternative to dangerous and bioaccumulating organochlorides, such as DDT. In fact, Bt even received an endorsement from Rachel Carson in her 1962 best seller Silent Spring. However, those same qualities led many large-scale farmers to dismiss Bt spray applications: They complained that Bt had a narrow spectrum of activity and was less toxic compared with broader-range and more potent pesticides that killed multiple insect species (e.g., organophosphate pesticides), and Bt had to be sprayed more often and at higher prices than readily available synthetic pesticides. So, although external applications of Bt eventually became approved for use on certified organic farms, Bt did not become a pesticide of serious renown until the mid-1990s, when the biotech industry began to genetically engineer Bt crops.

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