An increasing number of pest species are developing resistance to crops genetically engineered to be toxic to insects, according to new research. In an analysis of 77 studies conducted in eight countries, a team of U.S. and French scientists found that five of 13 major pest species had become resistant to so-called Bt cotton or corn plants, which are genetically modified to exude a bacterium, Bacillus thuringiensis, that is toxic to insects. While researchers say all insects inevitably adapt to threats such as pesticides, the study found that farmers who planted non-Bt crops in nearby “refuges” were more likely to slow that resistance. “Either take more stringent measures to delay resistance, such as requiring larger refuges, or this pest will probably evolve resistance quickly,” said Bruce Tabashnik, a professor at the University of Arizona and lead author of the study published in the journal Nature Biotechnology. The total land area planted annually with Bt crops has increased from 1.1 million hectares in 1996 to more than 66 million hectares in 2011.