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October 11, 2005
By Tom Simonite,
Nature.com via
Checkbiotech
Cotton that has been genetically
engineered to be toxic to pests remains effective after nearly a
decade in the field, scientists have announced, defying
predictions that insects would evolve to tolerate them.
Widespread planting of genetically modified (GM) cotton across
the southern United States has not increased the incidence of
resistance in the major insect pest, pink bollworm.
Cotton varieties genetically
modified to produce a natural insecticide, the Bt toxin,
borrowed from the bacterium Bacillus thuringiensis, have
been planted commercially in the United States since 1996. Many
experts predicted that pests would evolve to resist the toxin
within a few years.
But research published in
Proceedings of the National Academy of Sciences today
shows that this has not yet happened1. A survey of
pink bollworm in Arizona, where Bt cotton makes up more than
half of the cotton grown, shows that genes for Bt resistance
have not become more common since 1997.
"Ten years ago many experts were predicting resistance within
three years," says Bruce Tabashnik, lead author of the new
research, which was partially funded by the company Monsanto.
"If I had made a prediction, I would have said it would be maybe
four to eight years until resistance evolved." Tabashnik and
colleagues were surprised in 2003 when they found little
evidence of resistance in pests2 (see 'Resistance
to Bt toxin surprisingly absent from pests'). Two years on,
their genetic study confirms those earlier results.
But there is no question of Bt cotton being indefinitely
effective, Tabashnik adds. "I would say we might see resistance
in maybe five additional years," he says.
Refugees
The researchers used a mathematical model in an attempt to
explain how and why the insects have not developed resistance.
Farming practices are one reason, they conclude. The US
Department of Agriculture requires that farmers growing Bt
cotton plant at least 5% of their crop interspersed with
'refuge' zones of non-Bt cotton. Mixing of insects from within
the refuges, which do not gain an advantage from the resistance
genes, with insects from the GM crops, is intended to dilute the
concentration of resistance genes. The latest research shows
that this strategy seems to work.
But there are additional factors at play. Bt-resistant insects
do not survive as well in refuges as non-resistant ones, the
researchers note. And the genes for resistance are recessive, so
that only insects carrying two copies have the trait.
Not over yet
Despite the findings, researchers continue to be concerned that
resistance will one day be a problem. "Compared to conventional
insecticides, Bt crops have done extremely well, but it's
inevitable that resistance will evolve," says Graham Moores, a
senior research biochemist at Rothamsted Research in Harpenden,
UK.
Moores recently contributed to work that found a novel kind of
Bt resistance in cotton bollworm bugs from GM fields in
Australia3. If these insects have a different kind of
resistance mechanism, then others could too, he says; so there
could be genes for resistance out there that behave differently
from the one studied in Arizona.
"The existing management strategies have been successful, but we
cannot rest on our laurels," says Moores. Bt cotton is grown on
a large scale in China, South Africa and Australia.
One hope that the evolution of pests resistant to Bt cotton can
be delayed further lies with the second generation of the GM
crop. These plants carry another bacterial gene and so produce
two Bt toxins. In Australia, the government allows only this
variety to be grown, and it is also becoming more popular in the
United States. These crops are safer because it is less likely
that pests could evolve to be resistant to both toxins.
"Two-toxin cotton should last longer, at the very least twice as
long, and it could be much more," says Tabashnik.
References
1. Tabashnik B.E., et al. Proceedings of the National Academy
of Sciences of the United States of America,
http://www.pnas.org/cgi/doi/10.1073/pnas.0507857102
2. Carrière Y., et al. Proceedings of the National Academy
of Sciences of the United States of America, 100. 1519 -
1523 (2003).
3. Gunning R.V., et al. Applied and Environmental
Microbiology, 71. 2558 - 2563 (2005).
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