in the insect gut and forming pores (Gill et al. 1992). The pores cause the gut contents to leak into the blood, and this eventually leads to insect death. About 60 proteins from more than 50 subspecies of Bt have been identified in the last 20 years (Federici 1998). Which insects Bt toxins affect depends on the class of Bt protein; they include moths and butterflies (lepidopterans), flies and mosquitoes (dipterans), and beetles (coleopterans).
Mixtures of Bt have been used to spray crops for over 50 years. However, the Bt toxin generally loses its effectiveness in the environment within a few days. Sometimes spraying needs to be done frequently. In transgenic crops, Bt toxin is continuously produced and is protected from the elements. It therefore retains its ability to kill pests during the entire growing season. Moreover, the toxin is generally expressed in every part of the plant, including internal tissues that are difficult to protect with topically applied pesticides. This internal production provides protection against pests that are internal feeders such as the pink bollworm in cotton and the European corn borer in corn. On the other hand, the constant presence of Bt toxin in transgenic pest-protected plants during the growing season has led to concerns about its persistence in the environment and increased probability of pest evolution to overcome the protection mechanism.
The first permit to field-test transgenic crops that contained Bt genes was issued to Monsanto in 1988 for tomato. Initial attempts to make crops that would resist pests in the field were not successful because of problems with the expression of the bacterial genes. In 1990, the first successful Bt crop, cotton (Gossypium hirsutum), was produced by overcoming translational and transcriptional barriers to bacterial-gene expression in plants (Perlak et al. 1990). Transgenic methods for introducing Bt are often followed by conventional breeding with varieties that express other useful agronomic traits.
Before transgenic crops were commercialized (from 1987 to 1994), the USDA approved field trials of nine nematode-resistant transgenic pest-protected plant varieties, 45 fungus-resistant varieties, 17 bacteria-resistant varieties, 322 insect-resistant varieties, and 194 virus-resistant varieties.
In 1992, four years after the first field trials began, USDA proposed a regulation that described a petition process for determining that particular plants would no longer be regulated and therefore could be commer-