Pests are adapting to genetically modified crops in unexpected ways, U.S. researchers say, showing the importance of closely monitoring biotech crops. To decrease the need for insecticides that can harm animals other than the target pests, cotton and corn have been genetically engineered to produce toxins derived from the bacterium Bacillus thuringiensis, or Bt, which kill certain insect pests but are harmless to most other creatures including people. However, scientists say, initially rare genetic mutations that confer resistance to Bt toxins are becoming more common as a growing number of pest populations adapt to Bt crops. That discovery should be an early warning to farmers, regulatory agencies and the biotech industry, researchers writing in the journal Proceedings of the National Academy of Sciences said. \"Scientists expected the insects to adapt, but we\'re just finding out now how they\'re becoming resistant in the field,\" University of Arizona entomologist Bruce Tabashnik said. \"We try to stay ahead of the game,\" he said. \"We want to anticipate what genes are involved, so we can proactively develop strategies to sustain the efficacy of Bt crops and reduce reliance on insecticide sprays.\" The growing resistance of some pests to modified crops is a wakeup call, he said. \"As a grower, if you\'re killing 98 percent of pests with Bt cotton, you wouldn\'t notice anything. But this study tells us there is trouble on the horizon.\"