Insect infestations can cause a significant decrease in crop productivity. Infestations have been historically difficult to control without the use of highly toxic synthetic chemicals. The most well known biopesticides for insect control are the Bacillus thuringiensis (Bt) formulations for the control of lepidopterous pests. This bacterium produces a protein that by itself is harmless to most insects, but is converted to a potent toxin in the gut of specific target insects following ingestion. The mode of action of Bt is very specific. Different strains of Bt are specific to different receptors in the gut wall of insects. Bt toxicity depends on recognizing receptors; damage to the gut by the toxin occurs upon binding to the receptor. Each insect species possesses different types of receptors that will match only certain toxin proteins. Application of particular Bt toxin proteins has to be carefully matched to the target pest species. This is also an advantage to beneficial insects since they will not be adversely affected by select strains of Bt.
BOX 17.1 History of Discovery: Bacillus thuringiensis (Bt)
In 1901, a Japanese biologist, Shigetane Ishiwatari, studying the cause of sotto disease that was devastating silkworm populations first isolated the bacterium B. thuringiensis (Bt) as the cause of the disease, but at that time named it Bacillus sotto. In 1911, Ernst Berliner isolated a bacterium that had killed a Mediterranean flour moth and named it B. thuringiensis. In 1915, Berliner reported the existence of a crystal within Bt, but the function of the crystal was not determined until 1956. In 1956, Hannay, Fitz-James, and Angus found that the primary insecticidal activity against lepidoteran insects was due to the parasporal crystal. Following this discovery research was begun to determine the crystal structure, biochemistry, and general mode of action of Bt. In the United States, Bt was used commercially starting in 1958. By 1961, Bt was registered as a pesticide by the Environmental Protection Agency.
By 1977, 13 Bt strains had been described. All were toxic to specific species of lepidopteran larvae. In 1977 the first subspecies toxic to dipteran species was described, and the first discovery of strains toxic to species of coleopteran were discovered in 1983. In the 1980s, use of Bt increased when insects became increasingly resistant to synthetic insecticides.
With advancements in molecular biology, it became possible to move the gene that encodes the toxin into plants. The first genetically engineered plant, corn, was registered by the EPA in 1995. Since 1996 plants have been modified with short sequences of Bt genes to express the crystal protein. With this method, transgenic plants produce the proteins and protect themselves from insects. Genetically modified crops, including potato and cotton, are planted throughout the world. In 1999, 29 million acres of Bt corn, potato, and cotton were grown globally. Today, there are thousands of strains of Bt. Many of them have genes that encode unique toxic crystals in their DNA.
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