Reversing pest resistance to GM cotton

Insect pests that are rapidly adapting to genetically engineered crops threaten agriculture worldwide. A new study published in the Proceedings of the National Academy of Sciences reveals the success of a surprising strategy for countering this problem: Hybridizing genetically engineered cotton with conventional cotton reduced resistance in the pink bollworm, a voracious global pest.

The study is the result of a long-standing collaboration between researchers at the University of Arizona and in China. Over 11 years, they tested more than 66,000 pink bollworm caterpillars from 17 field sites in six provinces of China.

Cotton, corn and soybean have been genetically engineered to produce pest-killing proteins from the widespread soil bacterium Bacillus thuringiensis, or Bt. These Bt proteins are considered environmentally friendly because they are not toxic to people and wildlife. They have been used in sprays by organic growers for more than 50 years, and in engineered Bt crops planted by millions of farmers worldwide on more than 1 billion acres since 1996. Unfortunately, without adequate countermeasures, pests can quickly evolve resistance.

The main strategy for delaying pest resistance to Bt crops aims to increase the survival of susceptible insects with “refuges” of host plants that do not produce Bt toxins. Although refuges can delay insect adaptation to Bt crops, the optimal spatial scale for planting refuges remains unresolved. Also, because refuges are often perceived to cause short-term economic sacrifices for growers, they are usually imposed by regulations.

Planting such non-Bt cotton refuges is credited with preventing evolution of resistance to Bt cotton by pink bollworm in USA for more than a decade. By contrast, despite a similar requirement for planting refuges in India, farmers there did not comply and pink bollworm rapidly evolved resistance.

The ingenious strategy used in China entails interbreeding Bt cotton with non-Bt cotton, then crossing the resulting first-generation hybrid offspring and planting the second-generation hybrid seeds. This generates a random mixture within fields of 75 percent Bt cotton plants side-by-side with 25 percent non-Bt cotton plants. The frequency of resistant individuals in the field increased before applying this strategy and then declined after its widespread adoption boosted the percentage of non-Bt cotton plants in the region. Despite the increased percentage of non-Bt cotton, the results showed 96% suppression of pink bollworm and 69 % fewer insecticide sprays.

Unlike other resistance management tactics that require regulatory intervention, growers adopted this strategy voluntarily, apparently because of advantages that may include better performance as well as lower costs for seeds and insecticides.

Peng Wan el al., “Hybridizing transgenic Bt cotton with non-Bt cotton counters resistance in pink bollworm,” PNAS (2017). doi: 10.1073/pnas.1700396114



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