Mar. 29, 2013 — The popular
new strategy of planting genetically engineered crops that make two or
more toxins to fend off insect pests rests on assumptions that don't
always apply, UA researchers have discovered. Their study helps explain
why one major pest is evolving resistance much faster than predicted and
offers ideas for more sustainable pest control.
Mar. 29, 2013 — The popular
new strategy of planting genetically engineered crops that make two or
more toxins to fend off insect pests rests on assumptions that don't
always apply, UA researchers have discovered. Their study helps explain
why one major pest is evolving resistance much faster than predicted and
offers ideas for more sustainable pest control.
A strategy widely used to prevent pests from quickly adapting to
crop-protecting toxins may fail in some cases unless better preventive
actions are taken, suggests new research by University of Arizona
entomologists published in the
Proceedings of the National Academy of Sciences.
Corn and cotton have been genetically modified to produce pest-killing proteins from the bacterium
Bacillus thuringiensis,
or Bt for short. Compared with typical insecticide sprays, the Bt
toxins produced by genetically engineered crops are much safer for
people and the environment, explained Yves Carrière, a professor of
entomology in the UA College of Agriculture and Life Sciences who led
the study.
Although Bt crops have helped to reduce insecticide sprays, boost
crop yields and increase farmer profits, their benefits will be
short-lived if pests adapt rapidly, said Bruce Tabashnik, a co-author of
the study and head of the UA department of entomology. "Our goal is to
understand how insects evolve resistance so we can develop and implement
more sustainable, environmentally friendly pest management," he said.
Tabashnik and Carrière are both members of the UA's BIO5 Institute.
Bt crops were first grown widely in 1996, and several pests have
already become resistant to plants that produce a single Bt toxin. To
thwart further evolution of pest resistance to Bt crops, farmers have
recently shifted to the "pyramid" strategy: each plant produces two or
more toxins that kill the same pest. As reported in the study, the
pyramid strategy has been adopted extensively, with two-toxin Bt cotton
completely replacing one-toxin Bt cotton since 2011 in the U.S.
Most scientists agree that two-toxin plants will be more durable than
one-toxin plants. The extent of the advantage of the pyramid strategy,
however, rests on assumptions that are not always met, the study
reports. Using lab experiments, computer simulations and analysis of
published experimental data, the new results help explain why one major
pest has started to become resistant faster than anticipated.
"The pyramid strategy has been touted mostly on the basis of
simulation models," said Carrière. "We tested the underlying assumptions
of the models in lab experiments with a major pest of corn and cotton.
The results provide empirical data that can help to improve the models
and make the crops more durable."
One critical assumption of the pyramid strategy is that the crops
provide redundant killing, Carrière explained. "Redundant killing can be
achieved by plants producing two toxins that act in different ways to
kill the same pest," he said, "so, if an individual pest has resistance
to one toxin, the other toxin will kill it."
In the real world, things are a bit more complicated, Carrière's team
found out. Thierry Brévault, a visiting scientist from France, led the
lab experiments at the UA. His home institution, the
Center for Agricultural Research for Development, or CIRAD, is keenly interested in factors that could affect pest resistance to Bt crops in Africa.
"We obviously can't release resistant insects into the field, so we
breed them in the lab and bring in the crop plants to do feeding
experiments," Carrière said. For their experiments, the group collected
cotton bollworm -- also known as corn earworm or
Helicoverpa zea -, a species of moth that is a major agricultural pest, and selected it for resistance against one of the Bt toxins, Cry1Ac.
As expected, the resistant caterpillars survived after munching on
cotton plants producing only that toxin. The surprise came when
Carrière's team put them on pyramided Bt cotton containing Cry2Ab in
addition to Cry1Ac.
If the assumption of redundant killing is correct, caterpillars
resistant to the first toxin should survive on one-toxin plants, but not
on two-toxin plants, because the second toxin should kill them,
Carrière explained.
"But on the two-toxin plants, the caterpillars selected for
resistance to one toxin survived significantly better than caterpillars
from a susceptible strain."
These findings show that the crucial assumption of redundant killing
does not apply in this case and may also explain the reports indicating
some field populations of cotton bollworm rapidly evolved resistance to
both toxins.
Moreover, the team's analysis of published data from eight species of
pests reveals that some degree of cross-resistance between Cry1 and
Cry2 toxins occurred in 19 of 21 experiments. Contradicting the concept
of redundant killing, cross-resistance means that selection with one
toxin increases resistance to the other toxin.
According to the study's authors, even low levels of cross-resistance
can reduce redundant killing and undermine the pyramid strategy.
Carrière explained that this is especially problematic with cotton
bollworm and some other pests that are not highly susceptible to Bt
toxins to begin with.
The team found violations of other assumptions required for optimal
success of the pyramid strategy. In particular, inheritance of
resistance to plants producing only Bt toxin Cry1Ac was dominant, which
is expected to reduce the ability of refuges to delay resistance.
Refuges consist of standard plants that do not make Bt toxins and
thus allow survival of susceptible pests. Under ideal conditions,
inheritance of resistance is not dominant and the susceptible pests
emerging from refuges greatly outnumber the resistant pests. If so, the
matings between two resistant pests needed to produce resistant
offspring are unlikely. But if inheritance of resistance is dominant, as
seen with cotton bollworm, matings between a resistant moth and a
susceptible moth can produce resistant offspring, which hastens
resistance.
According to Tabashnik, overly optimistic assumptions have led the
EPA to greatly reduce requirements for planting refuges to slow
evolution of pest resistance to two-toxin Bt crops.
The new results should come as a wakeup call to consider larger
refuges to push resistance further into the future, Carrière pointed
out. "Our simulations tell us that with 10 percent of acreage set aside
for refuges, resistance evolves quite fast, but if you put 30 or 40
percent aside, you can substantially delay it."
"Our main message is to be more cautious, especially with a pest like
the cotton bollworm," Carrière said. "We need more empirical data to
refine our simulation models, optimize our strategies and really know
how much refuge area is required. Meanwhile, let's not assume that the
pyramid strategy is a silver bullet."
1:20 PM
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