TROUBLE ON THE HORIZON FOR GM CROPS?
University of Arizona, USA
Press Release, by Daniel Stolte
19 June 2012
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
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.
“Dominant resistance is more difficult to manage and cannot be
readily slowed with refuges, which are especially useful when resistance
is recessive,” Tabashnik said.
Refuges consist of plants that do not have a Bt toxin gene and thus
allow survival of insects that are susceptible to the toxin. Refuges are
planted near Bt crops with the goal of producing enough susceptible
insects to dilute the population of resistant insects, by making it
unlikely two resistant insects will mate and produce resistant
According to Tabashnik, the refuge strategy worked brilliantly
against the pink bollworm in Arizona, where this pest had plagued cotton
farmers for a century, but is now scarce.
The dominant mutations discovered in China throw a wrench in the
refuge strategy because resistant offspring arise from matings between
susceptible and resistant insects.
He added that the study will enable regulators and growers to better manage emerging resistance to Bt crops.
“We have been speculating and using indirect methods to try and
predict what would happen in the field. Only now that resistance is
starting to pop up in many places is it possible to actually examine
resistance in the field. I think the techniques from this study will be
applied to many other situations around the world, and we‚ll begin to
develop a general understanding of the genetic basis of resistance in
The current study is part of a collaboration funded by the Chinese
government, involving a dozen scientists at four institutions in China
and the U.S. Yidong Wu at Nanjing Agricultural University designed the
study and led the Chinese effort. He emphasized the importance of the
ongoing collaboration for addressing resistance to Bt crops, which is a
major issue in China. He also pointed out that the discovery of dominant
resistance will encourage the scientific community to rethink the
Tabashnik said China is the world‚s top cotton producer, with about
16 billion pounds of cotton per year. India is number two, followed by
the U.S., which produces about half as much cotton as China.
In 2011, farmers worldwide planted 160 million acres of Bt cotton and
Bt corn. The percentage of cotton planted with Bt cotton reached 75 per
cent in the U.S. in 2011, but has exceeded 90 per cent since 2004 in
northern China, where most of China‚s cotton is grown.
The researchers report that resistance-conferring mutations in cotton
bollworm were three times more common in northern China than in areas
of northwestern China where less Bt cotton has been grown.
Even in northern China, however, growers haven’t noticed the emerging
resistance yet, Tabashnik said, because only about 2 percent of the
cotton bollworms there are resistant.
“As a grower, if you’re killing 98 percent of pests with Bt cotton,
you wouldn‚t notice anything. But this study tells us there is trouble
on the horizon.”
Third World Network
131 Jalan Macalister,
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