Plant genetics

Fighting cotton pests

Print edition : May 26, 2017

Seed packages containing various cotton seed mixes for sale in China. Photo: Pen Wang

UNIVERSITY of Arizona scientists, together with Chinese researchers, have discovered a strategy that can delay and even reverse evolution of resistance by pests to genetically modified (GM) crops.

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 that hybridisation of GM cotton with conventional cotton can reduce the resistance in the pink bollworm, a voracious cotton pest.

Over 11 years the team tested more than 66,000 pink bollworm caterpillars from China’s Yangtze River Valley, a vast region of southeastern China that is home to millions of smallholder farmers. According to the study’s authors, this is the first reversal of substantial pest resistance to a Bt crop.

Cotton, corn and soyabean 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. Unfortunately, without adequate countermeasures, pests can quickly evolve resistance.

The primary strategy for delaying resistance is providing refuges of the pests’ host plants that do not make Bt proteins. 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 such 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, pink bollworm rapidly evolved resistance because of non-compliance by farmers.

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 per cent Bt cotton plants side by side with 25 per cent non-Bt cotton plants. The success, however, seems to depend critically on the hybridisation mix.

“Because cotton can self-pollinate, the first-generation hybrids must be created by tedious and costly hand pollination of each flower,” said senior author Bruce Tabashnik of the UA’s College of Agriculture and Life Sciences.. “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.”

This strategy emerged from the farming community of the Yangtze River Valley. While most previous attention has focused on the drawbacks of interbreeding between GM and conventional plants, the authors point out that the new results demonstrate gains from such hybridisation.

“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,” said Tabashnik. “Our results show 96 per cent pest suppression and 69 per cent fewer insecticide sprays. Our study provides the first evidence that planting mixtures of Bt and non-Bt seeds within fields has a resistance-delaying or, in this case, resistance-reversing effect,” he added.

“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 co-author Kongming Wu of the Institute of Plant Protection, Beijing, who led the work in China. “We know it works for millions of farmers in the Yangtze River Valley. Whether it works elsewhere remains to be determined,” Tabashnik said.

Compiled by R. Ramachandran