Trouble On the Horizon for
Genetically Modified Crops?
ScienceDaily (June 20, 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 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
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
"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
"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.
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
"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 field."
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 refuge
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