Purdue Scientists Finding Ways To Outsmart Crop-damaging Bugs

The process is designed to identify chemical compounds that could be added to current pesticides to overcome resistance insects have developed to them. In a recent issue of the journal Pesticide Biochemistry & Physiology, the scientists report that the method will be applicable to a variety of insects and chemicals.

"It's becoming more and more difficult to find new, effective pesticides," said Barry Pittendrigh, assistant professor of entomology and senior author of the study. "If we can kill these pesticide-resistant insects in the field, then we have the potential to increase the functional life of the insecticides currently in use."

Crop-damaging insects mutate over time so they are able to overcome the effects of chemicals developed to kill them. A toxin that protected a crop for more than a decade or two eventually may lose its lethality due to resistance in the insect population.

According to the U.S. Department of Agriculture, more than $7.5 billion is spent annually on agricultural pesticides. This is about 30 percent to 50 percent of the variable costs involved in managing harmful insects.

Pittendrigh and his research team studied common research fruit flies, Drosophila melanogaster, in which the molecular mechanism that provides the insect with chemical resistance was known. They applied that knowledge to test known chemicals' toxicity to the resistant insects.

A pesticide's toxic effect occurs when a molecule on an insect's cells, called a receptor, acts as a loading dock for molecules in the pesticide. When a toxic chemical is used, its docking molecule, called a ligand, joins the receptor and kills the bug.

But nature allows pests to challenge control methods by altering their own receptors. These biochemical changes prevent binding of the chemical to the receptor and its entry into the bugs' system. Once this occurs, the chemical becomes ineffective and a new way to stop the insects is needed.

Discovery of other toxins to attack insects that have the altered receptor offers a new way of minimizing resistance in the insect population, Pittendrigh said. The newly introduced insecticide provides negative cross-resistance, meaning the chemicals react with the mutated molecule.

"Insects have a tremendous capacity to adapt to chemicals that we use to control them," Pittendrigh said. "That's just evolution in motion. With negative cross-resistance, we're buying time for the commercial life of another pesticide. Using resistance-breaking compounds is a way to potentially double or triple the time that the original compound is effective."

In this study, the researchers tested nine related insecticides in order to identify a negative cross-resistance toxin. They found that the resistant flies were highly susceptible to one compound called deltamethrin. Use of deltamethrin dramatically reduced the numbers of pesticide-resistant insects in a fruit fly population.

The researchers used DDT (dichlorodiphenyltrichloroethane) as their base chemical because they know the insect molecule with which it reacts. This gave them insight into how other chemicals would behave.

After finding that deltamethrin was the most effective, they added the DDT. Then they tested the combined toxicity.

Though it's banned in developed countries, DDT is commonly used for mosquito control in Third World countries where malaria is still the No. 1 killer.

"In the fly line, we have a known mechanism of resistance, and we understand how DDT works at the molecular level," Pittendrigh said. "So then we can describe and understand molecularly how negative cross-resistance occurs. DDT was used simply because it allowed us to test a model system."

One argument against negative cross-resistance has been that it will be difficult, if not impossible, to find compounds toxic to mutated insects, Pittendrigh said. However, this study shows it may not be as difficult to identify negative cross-resistance compounds as once assumed.

The screening process will speed up and simplify identifying effective compounds and add another weapon in the arsenal to fight crop-destroying insects.

"If we can extend the commercial lifetime of a current pesticide with a negative cross-resistance compound, that's the best we can hope for," Pittendrigh said.

The screening system for identifying negative cross-resistance compounds has the potential to be applicable to other insects and to be produced and used at a commercial level, he said. But first, the molecular evolution of pesticide resistance in each targeted insect must be known.

For the negative cross-resistance toxin to be beneficial and financially viable, it would have to be used in cases where the evolutionary change in the target insect is seen in more than one line of the bug, which is found across a wide geographical area, Pittendrigh said. The chance of successful use of a chemical is even better if this resistance mechanism is the same across a wide variety of pest insects.

The other researchers involved in this study were: Joao Pedra and Andrew Hostetler, a doctoral student and a researcher assistant, respectively, in Purdue's Department of Entomology; Patrick Gaffney, University of Wisconsin, Madison, Department of Statistics; and Robert Reenan, associate professor, University of Connecticut Department of Genetics and Developmental Biology. Pedra and Pittendrigh also are part of the Purdue Molecular Plant Resistance and Nematode Team.

The Purdue Department of Entomology provided funding for this research.

Related Web sites:

Purdue Department of Entomology: http://www.entm.purdue.edu/

Environmental Protection Agency, DDT History: http://www.epa.gov/history/topics/ddt/01.htm

Pesticide Biochemistry & Physiology: http://authors.elsevier.com/JournalDetail.html?PubID=622930&Precis=DESC