Many Americans are distrustful when scientists tinker with the food supply by altering crops or cloning animals. But the practice of planting biotech crops is widespread.

Arizona's main foray into biotech agriculture has been the use of a modified cottonseed designed to wipe out a pest that some say once endangered the state's cotton crop. The genetically modified seed, known as Bt Cotton, has improved cotton yields and nearly eradicated the pink bollworm.

The National Cotton Council of America estimates that pink bollworm cost cotton producers as much as $32 million in lost yields and related costs. Such losses have been all but eliminated under a federally funded program targeting the bollworm for eradication in Arizona, California, New Mexico, Texas and parts of northern Mexico.

"In my opinion, it probably saved the cotton industry in the state of Arizona," said Bruce Heiden, who owns a farm in Buckeye, Ariz. "In our farming operation, genetically engineered plants and seeds have removed tons and tons of pesticides. I have been fighting all my life to make pesticides available. Now we're going to learn to live without them, and that's safer for the environment."

That's the argument farmers across the United States use as they switch to biotech crops, touting them as a way to use fewer pesticides and improve yields. Farmers increasingly are turning to biotech cotton, corn, soybeans, alfalfa and other crops.

U.S. farmers have led a worldwide charge for the use of such crops. U.S. farms account for 135 million of the 252 million acres of biotech crops planted worldwide in 2006, according to the International Service for Acquisition of Agri-Biotech Applications.

Even as more farmers plant such crops, a recent survey shows that most Americans aren't aware that some corn, alfalfa, processed foods and other items on supermarket shelves have been altered at the genetic level.

Michael Fernandez, executive director of the Pew Initiative on Food and Biotechnology, said public opinion on the use of such farming methods is up for grabs.

"There's a relatively small number of people who are strongly opposed and a small amount in favor," Fernandez said. "Most people fall somewhere in the middle. Their opinions are not firmly held."

Vicki Chandler, director of the Bio5 Institute in Tucson, Ariz., said each biotech product must be evaluated on a case-by-case basis.

"You need to take a look at the product and evaluate the safety from there," said Chandler, who cited cotton as productive use of biotechnology.

Heiden said he used to be skeptical about biotech seeds. At the height of the pink bollworm invasion, Heiden said, he would spray insecticides as often as 15 times a year. He worried that the modified seed would not curb the need for pesticides and would end up costing a bundle.

"We're always skeptical of something until we have a chance to try it," said Heiden, a second-generation farmer who has been working his family's H-Four Farm since the 1950s.

He has become a convert. Crop sprayings have dwindled to just a handful each year to ward off other pests and weeds.

Environmental groups abroad, particularly in the European Union, have assailed the use of Bt Cotton over fears that the modified cotton could develop resistance to antibiotics and prove harmful to humans. Those fears have not materialized, according to Bruce Tabashnik, head of the University of Arizona's entomology department.

"It is probably the most closely watched (eradication) program in the world. The resistance has not increased," Tabashnik said.