Researchers with the University of Arizona and China discovered a surprising strategy to reverse pink bollworm resistance to genetically engineered cotton.

Cotton growers have been able to use genetically engineered cotton to fight the pink bollworm. This has happened as scientists have been able to produce pest-killing proteins from the widespread soil bacterium Bacillus thuringiensis, or Bt. Without adequate countermeasures, scientists have discovered that pests can quickly evolve resistance.

According Bruce Tabashnik, one of the authors of the study and a regents’ professor with the University of Arizona, 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.

Scientists from the U.S. and China discovered that by hybridizing genetically-engineered cotton with conventional cotton, it reduced the resistance of the pink bollworm. Details of the 11-year study that tested more than 66,000 pink bollworm caterpillars from China’s Yangtze River Valley are published in the Proceedings of the National Academy of Sciences.

Tabashnik says this is the first-ever case where scientists have seen substantial reversal to resistance developed by a pest.

The primary strategy for delaying resistance is providing refuges of the pests’ host plants that do not make Bt proteins, according to Tabashnik. This allows survival of insects that are susceptible to Bt proteins and reduces the chances that two resistant insects will mate and produce resistant offspring.

Before 2010, the U.S. Environmental Protection Agency required refuges in separate fields or large blocks within fields. Planting non-Bt cotton refuges is credited with preventing evolution of resistance to Bt cotton by pink bollworm in Arizona for more than a decade.

By contrast, despite a similar requirement for planting refuges in India, farmers there did not comply and the 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, according to the University of Arizona. 

“Because cotton can self-pollinate, the first-generation hybrids must be created by tedious and costly hand pollination of each flower,” said Tabashnik, “However, hybrids of the second generation and all subsequent generations can be obtained readily via self-pollination. So, the hybrid mix and its benefits can be maintained in perpetuity.”

Tabashnik calls this strategy revolutionary because it was not designed to fight resistance and arose without mandates by government agencies. Rather, it emerged from the farming community of the Yangtze River Valley. While most previous attention has focused on the drawbacks of interbreeding between genetically engineered and conventional plants, the authors point out that the new results demonstrate gains from such hybridization. 

“For the growers in China, this practice provides short-term benefits,” Tabashnik added. “It’s not a short-term sacrifice imposed on them for potential long-term gains. The hybrid plants tend to have higher yield than the parent plants, and the second-generation hybrids cost less, so it’s a market-driven choice for immediate advantages, and it promotes sustainability. Our results show 96 percent pest suppression and 69 percent fewer insecticide sprays.”

There are implications in transgenic corn and soybeans as well.

“This study gives a new option for managing resistance that is very convenient for small-scale farmers and could be broadly helpful in developing countries like China and India,” explained coauthor Kongming Wu, who led the work conducted in China and is a professor in the Institute of Plant Protection in Beijing.

“A great thing about this hybrid seed mix strategy is that we don’t have to worry about growers’ compliance or regulatory issues,” Tabashnik said. “We know it works for millions of farmers in the Yangtze River Valley. Whether it works elsewhere remains to be determined.”