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A Twist on Pest-Proofing Crops

Scientists develop a model to slow insect resistance to pesticides

TUESDAY, Dec. 25, 2001 (HealthDayNews) -- Two scientists say they've come up with a theory of pesticide use that could prevent or delay insects from developing resistance to insecticides.

The technique is called "negative cross-resistance" and involves using multiple pesticides in a precise way to stop agricultural pests.

It's meant to put a halt to the "pesticide treadmill," where insects develop resistance to a chemical a few years after it has been introduced to the crop fields. The farmer then has to use another pesticide until the insects develop resistance to it. And so on, and so on.

With negative cross-resistance, scientists identify a second pesticide that specifically kills any resistant insects. The two pesticides are then used together, either concurrently or alternatively, to prevent insect resistance.

"Our paper actually lays down the foundation of how to go about effectively screening for these compounds," says researcher Barry Pittendrigh, assistant professor of entomology at Purdue University.

He developed his negative cross-resistance model with Patrick Gaffney of the University of Wisconsin-Madison. They did it using a combination of computer modeling and lab tests on fruit flies.

Pittendrigh says there were previous attempts to develop this kind of model but they didn't work because the researchers looked at only about several dozen chemicals. Pittendrigh and Gaffney's model provides a method to screen about 100,000 different chemicals.

They say their model shows using negative cross-resistant pesticides could delay insect resistance for decades and, in some cases, for more than a century.

Their paper was published in a recent issue of the Journal of Theoretical Biology.

Pittendrigh agrees many people may be wary of a process that seems to introduce more chemicals into the environment. But he says the negative cross-resistance method may actually reduce the overall amount of pesticides needed by a farmer.

That's because the total amount of two chemicals required may be less than increasing amounts of a single pesticide needed to overcome insect resistance.

And although this research focused on insects, Pittendrigh says the screening method can also be used with herbicides and fungicides. It may even be useful in countering antibiotic resistance in farm animals and humans, he adds.

At least one agriculture and pest management expert is skeptical of Pittendrigh's claims, however.

"This makes zero sense to me," says Chuck Benbrook, of Benbrook Consulting Services of Idaho, which works on agricultural technology, food safety and regulatory issues for consumer and environmental groups, the government and the private sector.

Benbrook says the wisdom of mixing agricultural insecticides to prevent resistance has been widely researched and debated for years. The general consensus is that it's a bad idea, he says.

"I know of nothing that should or has changed this consensus view," Benbrook says.

He says scientists suspect, but haven't confirmed, that there is something genetic about the process or mechanism that allows insects to develop resistance to pesticides. And often, that mechanism will develop resistance to two pesticides used together more quickly than if they're used alone and rotated.

"My guess is these scientists have developed a model virtually no one has seen -- and probably maybe two people in the world understand -- that proves that their method or definition of mixing agricultural insecticides will work. Well, I doubt that to be the case," Benbrook says.

What to Do: Here's a Web site with plenty of information about insect resistance. And this U.S. Environmental Protection Agency site tells you what you and your family need to know about pesticides and food.

SOURCES: Interviews with Barry Pittendrigh, Ph.D., assistant professor of entomology, Purdue University, West Lafayette, Ind.; Chuck Benbrook, Ph.D., Benbrook Consulting Services, Sandpoint, Idaho; photo courtesy of the USDA
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