February 7, 2008 -
A University of Arizona researcher has declared that
populations of cotton bollworms in Arkansas and Mississippi are now
resistant to first-generation Bt cotton.
"What we're seeing is evolution in action," said
Bruce Tabashnik, a University of Arizona entomologist and the lead
researcher in a study about potential bollworm resistance worldwide.
"This is the first documented case of field-evolved resistance to a
Bt crop.”
But an Arkansas entomologists whose data was the
basis of the conclusion said today that the data does not support
that kind of sweeping conclusion.
The Arkansas entomologist, Randy Luttrell, said he
“respectfully disagrees” with Tabashnik’s interpretation.
Questions about Bt resistance were raised late last
year when Tabashnik and Luttrell appeared in a symposium on the
subject at the Entomological Society of American national meeting in
California. Today, a
press release about the Arizona study was issued ahead of an
article that will appear in the February issue of the journal
Nature Biotechnology.
Tabashnik is one of the article’s authors and is
listed as a lead researcher in the study.
In the study, Arizona entomologists analyzed
published data from monitoring studies of six major caterpillar
pests of Bt crops in Australia, China, Spain and the U.S., including
Luttrell’s data.
Luttrell said today that the study compared bollworm
populations from 1992 and 2002. The 1992 data were based on work
Luttrell did while at Mississippi State University, while the 2002
data came from work he and Ibrahim Ali did at the University of
Arkansas, where he now is a faculty member.
“The response (survival rate of bollworms) was
greater in 2002, which at face value would make it appear that there
might be resistance,” Luttrell said today. “But when you look
closer, there are other important factors that have to be taken into
account.”
Among them, he said, are:
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Bt cotton’s overall
weakness on bollworms. “From the start, Bt cotton
(Monsanto’s Bollgard varieties) really didn’t provide control of
bollworms, and it was clear from the beginning that there were
insects that survived,” he said. “We saw that effect several
years before Bt cotton was commercially introduced.”
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Highly variable
populations. Bollworm populations in some seasons
and/or geographic areas seem to be more able to “slip through”
Bt cotton, Luttrell said. Even in 1992, a population of
bollworms identified in Texas had the ability to escape the
effects of Bt cotton at a rate comparable to some “problem”
populations found since 2000.
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The role of
“avoidance” behavior or mechanisms. Soon after the
commercialization of Bt cotton, entomologists and consultants
found larger, surviving worms in blooms and under dried bloom
tags where less Bt toxin tended to be expressed. Researchers
have debated whether this is simply an indication of where some
moths lay eggs or if some worms after initial feeding move
from an area with concentrated toxin to a part of the plant with
lower Bt levels.
“Roger Leonard (Louisiana State University
entomologist) finds that some worms will take a bite in the upper
part of the plant, don’t like what they’ve found, and then spin down
with their silking gland into an open flower,” said Gus Lorenz,
Arkansas Extension Entomologist. “We know there’s less expression
down there. That’s well documented. So, how much survivability is
due to increased tolerance and how much is due to some type of
behavioral modification? That question has yet to be answered. I do
know that we’ve had bollworms going through Bt cotton ever since it
came on the market.”
Luttrell today referred to Tabashnik “as probably the
world’s leading authority on Bt resistance, so when he writes a
paper like this, it has impact.” And, he added, “An honest
interpretation of the data cannot eliminate the possibility (of
resistance). In other words, you can’t look at the data and say,
‘This hasn’t happened.’”
But, Luttrell continued, questions about population
variability and behavior traits make it difficult for him to
conclude that bollworms are resistant to the Bt toxin.
Luttrell said that, in fact, some inherited genetic
trait may be at play. But it can't be determined with the data on
hand whether this trait has to do with the insect’s tolerance to Bt,
it’s avoidance behavior or other survival traits.
The subject gets cloudier when trying to compare
lab-based analysis to real-world conditions, he added.
“Resistance in the laboratory may or may not
translate to field resistance,” he said, referring to comparisons
made of collected populations. “With bollworm and Bt, one should
consider the effects of fitness costs and large populations from
non-cotton hosts that may work to dilute field resistance, even if
the insect has the genetic capacity for resistance.”
In the report, Tabashnik and his colleagues said that
they believe Bt resistance is limited worldwide. The researchers
noted that Bt cotton and Bt corn have been grown on more than 400
million acres worldwide since 1996, “generating one of the largest
selections for insect resistance ever known."
Even so, the researchers found that most caterpillar
pests of cotton and corn remained susceptible to Bt crops.
"The resistance occurred in one particular pest in
one part of the U.S.," Tabashnik said. "The other major pests
attacking Bt crops have not evolved resistance. And even most
bollworm populations have not evolved resistance."
The field outcomes refute some experts' worst-case
scenarios that predicted pests would become resistant to Bt crops in
as few as three years, he said.
“The only other case of field-evolved resistance to
Bt toxins involves resistance to Bt sprays," Tabashnik said. He
added that such sprays have been used for decades, but now represent
a small proportion of the Bt used against crop pests.