More pest species are becoming resistant to crops genetically modified to repel insects, but not in areas where farmers plant sufficient "refuges" of land with unmodified crops.
The study, published in the journal Nature Biotechnology, looks at so-called Bt corn and cotton -- plants modified to exude Bacillus thuringiensis, a bacterium toxic to insects.
U.S. and French researchers analyzed a total of 77 studies of 13 major pests covering eight countries on five continents. Five species were considered resistant by 2011, compared with only one in 2005, the study found.
To qualify, more than 50 percent of insects in a local population had to be resistant. Of the five species, three were cotton pests and two were corn pests. Three of the five cases were in the U.S, and the others were in South Africa and India.
Another U.S. cotton pest was found to be showing early resistance, with less than 50 percent of the population affected. Four other pests in China, the Philippines and the U.S. showed "early warning" signs with one percent or less resistance in the population.
The authors also found that farmers who set aside sufficient "refuge" land for planting non-Bt crops slowed the growth of resistance in pests.
Insects can only survive on Bt plants if they carry two copies of the recessive gene. Keeping non-Bt crops nearby reduced the likelihood of two parents passing on the resistance gene.
“Computer models showed that refuges should be good for delaying resistance,” study co-author Yves Carriere, an entomologist at the University of Arizona at Tucson, said in a press release.
Study co-author Bruce Tabashnik said there is practical evidence of the refuges slowing down resistance. A cotton pest called the pink bollworm became resistant within six years in India, but in the southwestern U.S. farmers used a vigorous refuge strategy and don't yet have any resistance problems.
Although refuge planting can slow down the pests' adaptations, the authors warn that resistance is only a matter of time.
More than a billion acres of land have been planted with Bt crops since the mid-1990s. In 2011, Bt corn accounted for 67 percent of corn planted in the United States and Bt cotton for between 79-95 percent of cotton planted in the US, Australia, China, and India.
Though transgenic crops have been opposed in Europe and some other places, The U.S. Environmental Protection Agency has relaxed its refuge-planting requirements in recent years.