A University of Arizona survey of 77 studies, released in 2013, found “well-documented cases of field-evolved resistance to Bt crops in five major pests as of 2010, compared with only one case in 2005. Three of the five cases were in the United States, where farmers have planted about half of the world’s Bt crop acreage.” In some cases, effectiveness of Bt crops was preserved for more than 15 years, but in others, insects developed resistance to the biological toxins in the plants within two or three years.
The authors of the report said that abundant “refuges,” areas planted with non-Bt crops, were a key factor. The other was whether the gene conferring resistance is rate among the pest population. If both factors are present when a Bt crop is introduced into a region, the chances of insect resistance are low and likely to stay there.