Bollworms Can Overcome Insecticidal Proteins in GM Cotton, Scientists Say

To inhibit pest resistance, some transgenic crops produce two different Bt toxins targeting the same pest. Bt cotton, for instance, produces the insecticidal proteins Cry1Ac and Cry2Ab. These toxins have very different amino-acid sequences and bind to different target sites. But results of a study conducted by scientists at the University of Arizona showed that it might be possible for some insect pests to develop resistance to two different Bt toxins produced by genetically modified cotton.

Bruce Tabashnik and colleagues were able to generate laboratory strains of pink bollworms (Pectinophora gossypiella) resistant to 420-times higher levels of Cry1Ac than normal. These strains, which were raised on a diet that contained Cry2Ab, were also resistant to the particular toxin (resistant to 240-times higher levels of Cry2Ab than normal). The scientists hypothesize that the resistance may be due to changes in an enzyme that activates the toxins.

The results of the study, published in PNAS, indicate that cross-resistance occurs between Cry1Ac and Cry2Ab in some key cotton pests. Tabashnik and colleagues however noted that "this does not pose a threat for control by the current pyramided Bt cotton of this insect." The scientists showed that larvae from the lab strains resistant to Cry1Ac and Cry2Ab survived on cotton bolls producing only Cry1Ac, but not on cotton bolls producing both toxins.

Nature magazine has quoted Tabashnik as saying: "Pyramids are not a panacea and evolution by insects is not something that scientists are going to stop."

The open access paper published by PNAS is available at http://dx.doi.org/10.1073/pnas.0901351106 An article published by Nature summarizes the findings. It is available to subscribers at http://www.nature.com/news/2009/090706/full/news.2009.629.html#B1#B1