《自然》报告《美国科学院刊》论文：转基因不敌害虫，传统轮种控虫有效
Pests worm their way into genetically modified maize
Broadening of rootworm resistance to toxins highlights the
importance of crop rotation.
Brian Owens
17 March 2014
Article toolsRights & Permissions
Biosphoto/FLPA
Defending fields from the rapid adaptation of western corn rootworm
to transgenic toxins may require weapons beyond those of
biotechnology.
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Even with biotech crops, farmers still need to make use of age-old
practices such as crop rotation to fight insect pests.
That’s the lesson to be drawn from the latest
discovery of resistance to the pest-fighting toxins added to maize
— also known as corn.
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According to a team led by Aaron Gassmann, an entomologist at Iowa
State University in Ames, in some Iowa fields a type of beetle
called the western corn rootworm (Diabrotica virgifera virgifera
LeConte) has developed resistance to two of the three types of
Bacillus thurinigiensis (Bt) toxin produced by genetically modified
maize. Resistance to one type of Bt toxin has cropped up in the
worms in recent years, but now there is a twist —
the researchers have found that resistance to that type of Bt toxin
also confers protection against another, more recently introduced
type. Their work appears in this week's Proceedings of the National
Academy of Sciences1.
“That’s two of the three toxins
on the market now,” says Gassmann.
“It’s a substantial part of the
available technology.”
Genetically modified (GM) maize producing the Bt toxin Cry3Bb1,
which provided protection against pests such as rootworm, was first
approved for use in the United States in 2003. By 2009, farmers had
started to see rootworm damage in their GM crops. In 2011, that
damage had spread to GM maize containing a second toxin, mCry3A. In
lab tests, Gassmann showed that this was a case of cross-resistance
— worms that had become resistant to Cry3Bb1 were
also resistant to mCry3A, possibly because the toxins share
structural similarities and some binding sites in the
insect’s gut.
Part of the problem is that rootworms are tough, and the Bt maize
does not produce enough toxin to fully control them. The Bt toxins
used against pests such as the European corn borer (Ostrinia
nubilalis) kill more than 99.99% of their targets, whereas more
than 2% of rootworms can survive Bt maize. Resistance in the worms
can evolve rapidly in fields where the same kind of maize is grown
every year — in Iowa it showed up after an
average of 3.6 years.
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Nicholas Storer, a global science-policy leader for biotechnology
at Dow AgroSciences in Washington DC, says that the study
illustrates that if GM crops are not used as part of an integrated
pest-management policy, resistance can develop quickly in an
individual field. Agricultural biotechnology companies such Dow are
now ‘pyramiding’ their seeds so
that they produce two different Bt toxins to attack the rootworm.
For example, Dow has teamed up with Monsanto of St Louis, Missouri,
to sell seeds that combine Cry3Bb1 with Cry34/35Ab1, a toxin that
has so far not seen any resistance develop.
Gassmann says that the pyramiding of toxins is an important way to
delay the development of resistance, but that the combination is
less effective once resistance arises to one of the toxins. So
farmers should not rely exclusively on technology to fight pests,
and should instead periodically change the crop grown on a field to
help disrupt the pest’s life cycle.
“The rootworm can’t survive if
the corn’s not there,” Gassmann
says.
Storer agrees that even the best technologies will always need to
be combined with the old methods. “Crop rotation
was the primary tool to combat rootworm before Bt came
along,” he says. “We need to
keep it up.”