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West Lafayette, Indiana
August 21, 2001
For years, farmers and
agribusinesses have talked about being on the "pesticide
treadmill": A few years after a pesticide is introduced, insects
develop resistance to it. So another chemical is used
- at least until the bugs overwhelm that one.
Then another chemical is used. Then another. Then another.
But Barry Pittendrigh, assistant professor of entomology at
Purdue University, says it's possible to stop the treadmill, or
at least slow it to a crawl.
Pittendrigh and Patrick Gaffney, of the University of
Wisconsin-Madison, have developed a method to use pesticides so
that genetic resistance doesn't arise.
The technique is called negative cross-resistance, and it
involves using multiple pesticides in a precise way to stop the
pests.
With the technique, scientists would identify a second biocide ‹
pesticide, antibiotic, herbicide or fungicide ‹ that
specifically kills the resistant pest. Then the two biocides
would be used together, either concurrently or alternated, to
prevent resistance.
Previous attempts to find compounds that would have a negative
cross-resistance effect haven't worked because they focused on
fewer than several dozen compounds, Pittendrigh says.
However, Pittendrigh says it is necessary to screen upwards of
100,000 compounds to develop a negative cross-resistance system.
Pittendrigh and Gaffney have invented a method to conduct these
screens.
"Specifically, in our paper, we outline how companies or
individuals can search for and develop NCR compounds to a
commercially applicable level," Pittendrigh says. "This paper
provides part of the theoretical framework for research
currently in progress here at Purdue for the development of
negative cross-resistant toxins and their use in field
applications."
The researchers say their model shows that using negative
cross-resistant biocides could delay resistance for decades, or
even more than 100 years in some situations.
"Although negative cross-resistance is not 'the' answer to
dealing with resistance to pesticides, it certainly has the
potential to play a significant role in dramatically slowing the
rate at which resistance enters insect populations," Pittendrigh
says.
The result, the researchers say, would be reduced costs, both
financial and social.
"Nature will always find a way to get around whatever we do to
control organisms," Pittendrigh says. "But in some cases, this
method may buy us years of usefulness for compounds that are on
the market. It costs a large amount of money to bring a
pesticide to market. If it's a highly important biocide, such as
an insecticide for a major pest or an important antibiotic, this
method could have great value."
The method is described in a paper appearing Tuesday (8/21) in
the Journal of Theoretical Biology. The research was funded by
the Purdue Department of Entomology.
Pittendrigh says, in theory, the method also should work to
prevent antibiotic resistance in bacteria.
"Although this paper is primarily focused on issues of
insecticide resistance, we don't rule out the possibility that
this approach may also be useful in combating antibiotic
resistance," he says. "But, we will leave the applicability of
NCR in bacteria to those that work in antibiotic resistance."
The method also could be used with herbicides or fungicides.
No pesticide is 100 percent effective against its target, and
that's where the problem of chemical resistance comes in.
If a pesticide kills 98 out of 100 bugs, the only two left are
both resistant to the chemical. If those two mate, then all of
their offspring also will be resistant.
If the same thing happens in field after field, soon entire
populations of the pest are immune to the effects of the
pesticide.
The situation is worse with genetically modified crops, such as
Bt corn. Because these plants deliver pesticide in such a direct
and effective manner, they are even more susceptible to the rise
of resistant insects.
Although resistance can vary, some examples of insect resistance
can be dramatic.
Dieldrin is a compound no longer used commercially, but still
commonly used in laboratories. Scientists often use fruit flies,
called Drosophila, in their experiments, and certain strains of
Drosophila are so immune to Dieldrin that they can walk unharmed
on pure crystals of the pesticide.
Scientists are able to create resistant insects in the
laboratory by using a process known as EMS
(ethylmethylsulfanate) mutagenesis. Using the compound,
scientists can produce insects with great genetic variability,
and screen for those that are resistant to the insecticide being
tested.
"With EMS mutagenesis you can actually create resistance in the
laboratory that is similar to that in the field," Pittendrigh
says. "As a general rule, this mimics nature, but at a much
faster rate."
Once a new compound has been identified as being effective on
resistant pests, it can either be alternated with the original
biocide, or they can be paired together.
"My own bias is to use two compounds at once, because, at the
end of the day, it's the simplest method," Pittendrigh says.
"Farmers could spray with the original pesticide for five years,
and then in the sixth year everybody would have to use both
pesticides. But if somebody tried to cut corners and didn't use
both compounds, the method wouldn't work. That's why my bias is
to use two compounds concurrently because it's the easiest to
manage."
Although using two pesticides is obviously more expensive than
using just one, Pittendrigh says genetically modified crops
lower this hurdle.
"With traditional agriculture, there are concerns about the
costs of delivering two different pesticides at once,"
Pittendrigh says. "But with genetically modified crops, it's
much easier and much more cost effective to deliver two
pesticides."
For additional background, see
Pittendrigh, B., Gaffney, P., and Murdock L. 2000.
"Deterministic modeling of negative cross-resistance for use in
transgenic host-plant resistance," Journal of
Theoretical Biology, 204:135150.
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Source: Barry Pittendrigh, (765)
494-7730;
barry_pittendrigh@entm.purdue.edu
Writer: Steve Tally, (765) 494-9809;
tally@aes.purdue.edu
Jeanne Norberg, Director, Purdue News Service
(765) 494-2084;
jnorberg@purdue.edu
Pager: 423-8662; Home: 449-4986
Fax: (765) 494-0401
http://news.uns.purdue.edu
--
Beth Forbes, Ag News Coordinator
Ag Communications Service
(765) 494-2722
bforbes@aes.purdue.edu
Fax: (765) 496-1117
http://persephone.agcom.purdue.edu/AgCom/news/
Purdue University news release
N3735
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