PARIS – More pest species are becoming resistant to the most popular type of genetically modified, insect-repellent crops, but not in areas where farmers follow expert advice, a study showed Monday.
The paper delves into a key aspect of so-called Bt corn and cotton — plants that carry a gene derived from a soil bacterium called Bacillus thuringiensis to make them produce an insecticidal protein.
Publishing in the journal Nature Biotechnology, U.S. and French researchers analyzed the findings of 77 studies from eight countries on five continents that reported on data from field monitors.
Of 13 major pest species examined, five were resistant by 2011, compared with only one in 2005, they found. The benchmark was resistance among more than 50 percent of insects in a location.
Of the five species, three were cotton pests and two were corn pests.
Three of the five cases of resistance were in the United States, which accounts for roughly half of Bt crop plantings, while the others were in South Africa and India.
The authors said they picked up a case of early resistance, with less than 50 percent of insects, in yet another U.S. cotton pest.
And there were “early warning” signs (1 percent resistance or less) from four other cotton or corn pests in China, the U.S. and the Philippines.
The scientists found big differences in the speed at which Bt resistance developed.
In one case, it took just two years for the first signs to emerge; in others, the Bt crops remained as effective in 2011 as they were 15 years earlier.
What made the difference was whether farmers set aside sufficient “refuges” of land for non-Bt crops, the study’s authors said.
The idea behind such refuges comes from evolutionary biology.
The genes that confer resistance are recessive, meaning that insects can survive on Bt plants only if they have two copies of a resistance gene — one from each parent.
Planting refuges near Bt crops reduces the chances of two resistant insects mating and conferring the double gene to their offspring.
“Computer models showed that refuges should be good for delaying resistance,” said study co-author Yves Carriere, an entomologist at the University of Arizona.