Successes and failures of transgenic bt crops: Global patterns of field-evolved resistance

Farmers planted genetically engineered crops that produce insecticidal proteins from the bacterium Bacillus thuringiensis (Bt) on a cumulative total of 570 million ha worldwide from 1996 to 2013. These Bt crops kill some key insect pests, yet they are not toxic to most other organisms, including people. Bt crops can suppress pests, reduce the use of insecticide sprays and increase farmer profits, but their benefits are diminished or lost when pests evolve resistance. Here we review data monitoring resistance to seven Bt proteins in 13 major pest species targeting Bt maize and Bt cotton on six continents. Of the 27 sets of monitoring data analysed, seven show severe field-evolved resistance in 2 to 8 years with practical consequences for pest control (i.e. practical resistance), eight show statistically significant but less severe fieldevolved resistance and 12 show no evidence of decreased susceptibility after 2 to 15 years. The surge in cases of practical resistance since 2005 is associated with increased planting of Bt crops, increased cumulative exposure of pests to Bt crops and increased monitoring. In addition, practical resistance to Bt crops is associated with a scarcity of refuges, which consist of host plants that do not produce Bt proteins. To maximize the benefits of Bt crops, we encourage collaboration between growers and scientists in industry, academia and government to implement large refuges of non-Bt host plants, particularly when the inheritance of resistance is not recessive and alleles conferring resistance are not rare.