http://www.cabi.org/cabreviews/default.aspx?LoadModule=Review&ReviewID=
99418&site=167&page=1486

The ubiquitous nature of Bacillus thuringiensis (Bt), a Gram-positive bacterium capable of producing crystal proteins with insecticidal activity during sporulation, is now being mirrored in major crops plants that have been engineered through recombinant DNA to carry genes responsible for producing these crystal proteins and providing host plant resistance to major lepidopteran and coleopteran pests. In 2007, the 11th year of commercial production, Bt maize and Bt cotton were commercially produced on a total of ~42 million hectares in 20 countries.

Assessment of environmental safety has been and continues to be a key element of transgenic crop technology. This review focuses on two environmental elements, effects on non-target invertebrates and changes in insecticide use patterns since the adoption of Bt maize and cotton. Meta-analyses of the extant literature on invertebrate non-target effects reveals that the pattern and extent of impact varies in relation to taxonomy, ecological or anthropomorphic guild, route of exposure and the non-Bt control against which effects are gauged.

Hazards identified in the laboratory may not always manifest in the field and the minor negative effects of Bt crops demonstrated in the field pale in comparison with alternative pest suppression measures based on insecticides. The efficacy of Bt maize and cotton against major pest species has been associated with an estimated 136.6 million kg global reduction in insecticide active ingredient used between 1996 and 2006 (29.9% reduction). Benefits vary by country and region and are heavily weighted towards cotton production, which has historically been one of the largest users of insecticides in the world.