countries of LAC. The economic benefits have been accompanied by social changes such as the displacement of small-scale producers and the consequent migration to the cities (Pengue, 2000), the concentration of lands and agribusinesses (Verner, 2005; Altieri and Pengue 2006) and the loss of food sovereignty (Jordan, 2001; Souza, 2004; Altieri and Pengue, 2005; Verner, 2005). Moreover, environmental benefits have been reported related to the increase in area planted with reduced or zero tillage and to reduced pesticide use associated with Bt crops. For example, in Argentina, where more than half of the transgenic soybean in the region is grown, 80% of the area requires zero tillage, contributing to a reduction in the rate of soil erosion (Trigo and Cap, 2003; Qaim and Traxler, 2005). In the state of Coahuila, Mexico, where 96% of the area in cotton is planted with Bt cotton, an 80% reduction was reported in the number of applications of insecticides, although the authors recognize that not all of the reduction could be attributed to the transgenic cotton because the region also has a strong program to eradicate the boll weevil and an effective integrated pest management program (Traxler and Godoy-Avila, 2004). In general, adopting transgenic cotton appears to be highly determined by the presence of a particular pest and in many regions producers have opted to continue using the conventional seed (Traxler and Godoy-Avila, 2004; Qaim et al., 2003).

          These environmental benefits of transgenics are overshadowed by other negative environmental impacts. Many scientists have expressed concern over the use of transgenic crops on a large scale considering the environmental risks, which may threaten the sustainability of agriculture (Goldberg, 1992; Paoletti and Pimentel, 1996; Rissler and Mellon, 1996; Kendall et al., 1997; Snow and Moran, 1997; Royal Society, 1998; Altieri and Rosset, 1999). For example, the widespread adoption of homogeneous transgenic varieties inevitably leads to genetic erosion and the loss of local varieties developed and used traditionally by thousands of small-scale producers (Robinson, 1996). In the case of transgenic soybean, a dramatic increase has been reported in the use of herbicides, especially glyphosate (Trigo et al., 2002; Qaim and Traxler, 2005); the evolution of resistance

    to glyphosate has already been reported in some weeds, limiting the possible benefit of the technology (Holt and Le Baron, 1990; Papa, 2000). The massive use of Bt crops affects other organisms and some ecological processes and can lead to resistance. For example, it has been shown that the Bt toxin may affect beneficial insects that feed on pests that eat the Bt crop (Hilbeck et al., 1998). There is also evidence that the pollen from Bt crops that is deposited on the leaves of wild plants around the areas planted in Bt crops may kill other lepidopterans that are not pests, such as the Monarch butterfly (Losey et al., 1999). There is also evidence that the Bt toxin adheres to soil colloids and lasts up to three months, having a negative impact on the populations of invertebrates that help in the decomposition of organic matter (Donnegan et al., 1995). In addition, the intensive use of Bt varieties increases the pressure of selection and generates resistance, threatening not only the future utility of these crops, but also annulling one of the most useful tools available to the organic producers for fighting pests (Pimentel et al., 1989; Mallet and Porter, 1992; Gould, 1994; Alstad and Andow, 1995).

          Transgenic crops have also had a negative impact on biodiversity due to the conversion of forest areas and natural savannahs to transgenic plantations, in particular soybean. In Brazil and Argentina the expansion of transgenic soybean has affected directly and indirectly on the deforestation of unique ecosystems such as the tropical forest of the Amazon region and the Cerrado in Brazil and the Yungas forest in Argentina (Fearnside, 2001b; Montenegro et al., 2003; Pengue, 2005).

          As LAC is important as a center of origin of crops of global importance, such as maize, potato and tomato, there is concern over genetic contamination should transgenic crops be introduced in the centers of origin, for example transgenic potato in Bolivia, or transgenic maize in Mexico. Indeed, there is already evidence of genetic contamination of local varieties of maize in Mexico (Chapela and Quist, 2001), although it is argued that this contamination may have been temporary (Ortiz-García et al., 2005). Also worrisome is the possible contamination by transgenics of edible crops that are given non-food uses, for example the produc-