Graphic. AgriculturesAgriculturesGraphic. Purdue University.

Winter 2002

Jumping off the pesticide treadmill
By Steve Tally

Image: Larry Murdock
Entomology professor Larry Murdock says that a pesticide resistance method developed to prevent chemical resistance in insects can also be applied to other organisms, such as weeds, bacteria and fungal diseases. (Photo by Tom Campbell)

Treadmills aren't known for their entertainment value, and even metaphorical treadmills can be an aggravation.

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--until the bugs overwhelm that one.

Then another chemical is used. Then another. And another.

But Barry Pittendrigh, assistant professor of entomology at Purdue, says it's possible to stop the treadmill, or at least slow it to a barely perceptible crawl.

Pittendrigh, together with Larry Murdock, professor of entomology, and Patrick Gaffney, of the University of Wisconsin-Madison, have developed a method to use pesticides so that genetic resistance is severely limited.

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

How insects develop resistance

No pesticide is 100 percent effective against its target, and that's where the problem of chemical resistance arises.

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.

"Basically insecticides are chemicals of some kind," Murdock says. "They all act to kill insects by interrupting some physiological or biochemical process. It may be the nervous system, it may be the digestive system or it may be the hormonal system. That chemical interacts with a specific membrane component or protein--we call those "target sites"--to interrupt its function."

However, all of biology is variability, and even individual insects of the same species can vary quite a bit.

"Take, for example, the lower southwestern part of Indiana. If there is an insect infestation in that area, you could have tens of millions--or even billions-- of that species of insect in that area at the same time," Murdock says. "In this huge population of insects, there are, by chance, a few individuals that are not vulnerable to the particular insecticide that you use to stop the insect problem. Their proteins don't bind it, or the site where the insecticide attaches to the membrane or target enzyme is a little bit different biochemically. So the chemical doesn't have the normal effect, and that insect can survive."

Although just one in a million insects may be resistant to the insecticide, that would mean that in a population of a billion insects, there would be 999 other insects in the area that are also resistant.

"If they are lucky enough to meet up with another one that is resistant, they'll mate and produce dozens or hundreds or even thousands of offspring. All of these offspring will be resistant to the insecticide," Murdock says. "And these insects will have a huge advantage, because the insecticide will have eliminated much of their competition for food, so that population can grow quickly."

Once this happens, the population of insects in an area rapidly comes to be dominated by the new resistant strain of the insects. The result is that the crop damage reappears because the insecticide is no longer effective, and the insecticide is pulled from that market.

"The classic example was in Scandinavia in 1946," Murdock says. "Right after World War II, DDT was used to control mosquitoes, flies and lice. Soon people began noticing the number of houseflies was increasing. Eventually, houseflies from that area could walk on the DDT itself, and they were completely resistant to it."

Since insecticides were first used in the mid-1800s, there have been more than 500 reported cases of insects becoming resistant to the insecticide meant to control them.


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