tato cultivars are protected against the Colorado potato beetle by a Cry3 Bt protein.
There is a tendency to consider Bt toxins as all biochemically similar, but the DNA sequence similarity among toxins can be less than 25% (Feitelson et al. 1992) and the biochemical properties of the more than 100 different Bt toxins vary widely.
Transgenic potato was the first Bt crop variety approved for commercial use (EPA 1995a). The target pest for transgenic Bt potatoes is the Colorado potato beetle. This pest is not a major problem in all areas of potato production, but the need for an alternative to conventional insecticides for controlling it by conventional farming techniques was apparent in the years before approval because the beetle had become resistant to all available classes of conventional insecticides. Just as Bt potatoes reached the market, a novel insecticide, imidicloprid, also reached the market. The new insecticide was so effective on a number of potato pests that it competed effectively with Bt potatoes that controlled only the beetle pest. In 1998, Bt potatoes in the United States were planted in 50,000 acres, which is 3.5% of the total US potato acreage (Idaho Statesman 1998). Strains of Colorado potato beetle resistant to imidicloprid are already evolving in a number of locations (for example, Suffolk County, NY), so Bt potatoes may soon be planted on a much larger scale. However, low rates of adoption of Bt potato may ultimately be due to the need for potato growers to use chemicals to control insect pests other than the Colorado potato beetle. In those cases, protection from the Colorado potato beetle may not offset the cost of the chemicals and the transgenic seed (Gianessi and Carpenter 1999).
Some of the Cry3 protein produced in Bt potatoes is coded from the full length bacterial gene. However, a significant fraction of the Cry3 toxin produced in Bt potato is a smaller, truncated form of molecule (Perlak et al. 1993). EPA documentation indicates that the potential for the smaller Cry3 molecule to induce a food allergy is similar to that of the larger Cry3 molecule. EPA (1995c) indicates that “despite decades of widespread use of Bacillus thuringiensis as a pesticide there have been no confirmed reports of immediate or delayed allergic reactions from exposure”. Bt toxin's history of use (microbial Bt sprays have been registered since 1961) and rapid digestion in simulated gastric fluids (in less than 30 seconds; EPA 1995c) are considered evidence of safety by the EPA. In addition, in acute toxicity studies, no adverse effects were exhibited (Lavrik et al. 1995) (see section 3.1.3). However, it must be recognized that the microbial Bt toxins that have been widely used for decades to