What is in this article?:
- GM crop trouble as pests adapt
- Refuge limitations
- Rare genetic mutations that confer resistance to Bt toxins are becoming more common as a growing number of pest populations adapt to Bt crops.
Pests are adapting to genetically modified crops in unexpected ways, researchers have discovered. The findings underscore the importance of closely monitoring and countering pest resistance to biotech crops.
Resistance of cotton bollworm to insect-killing cotton plants involves more diverse genetic changes than expected, an international research team reports in the journal Proceedings of the National Academy of Sciences.
To decrease sprays of broad-spectrum insecticides, which can harm animals other than the target pests, cotton and corn have been genetically engineered to produce toxins derived from the bacterium Bacillus thuringiensis, or Bt.
Bt toxins kill certain insect pests but are harmless to most other creatures including people. These environmentally friendly toxins have been used for decades in sprays by organic growers and since 1996 in engineered Bt crops by mainstream farmers.
Over time, scientists have learned, initially rare genetic mutations that confer resistance to Bt toxins are becoming more common as a growing number of pest populations adapt to Bt crops.
In the first study to compare how pests evolve resistance to Bt crops in the laboratory vs. the field, researchers discovered that while some the of the lab-selected mutations do occur in the wild populations, some mutations that differ markedly from those seen in the lab are important in the field.
Caterpillars of the cotton bollworm, Helicoverpa armigera, can munch on a wide array of plants before emerging as moths. This species is the major cotton pest in China, where the study was carried out.
Bruce Tabashnik, head of the department of entomology at the University of Arizona College of Agriculture and Life Sciences, who co-authored the study, considers the findings an early warning to farmers, regulatory agencies and the biotech industry.
"Scientists expected the insects to adapt, but we're just finding out now how they're becoming resistant in the field," Tabashnik said.
To avoid surprises, researchers have exposed cotton bollworm populations to Bt toxins in controlled lab experiments and studied the genetic mechanisms by which the insects adapt.
"We try to stay ahead of the game," he said. "We want to anticipate what genes are involved, so we can proactively develop strategies to sustain the efficacy of Bt crops and reduce reliance on insecticide sprays. The implicit assumption is what we learn from lab-selected resistance will apply in the field."
That assumption, according to Tabashnik, had never been tested before for resistance to Bt crops.
Now for the first time, the international team gathered genetic evidence from pests in the field, enabling them to directly compare the genes involved in the resistance of wild and lab-reared populations.
They found some resistance-conferring mutations in the field were the same as in lab-reared pests, but some others were strikingly different.
"We found exactly the same mutation in the field that was detected in the lab," Tabashnik said. "But we also found lots of other mutations, most of them in the same gene and one in a completely different gene."
A major surprise came when the team identified two unrelated, dominant mutations in the field populations. "Dominant" means that one copy of the genetic variant is enough to confer resistance to Bt toxin. In contrast, resistance mutations characterized before from lab selection are recessive meaning it takes two copies of the mutation, one provided by each parent, to make an insect resistant to Bt toxin.