URBANA, Ill. (AP) -- Instead of planting test crops, a University of Illinois professor used a computer model to study how to extend the life of corn genetically engineered to kill a major pest.

The corn crops, called "Bt corn," carry a gene from bacteria called Bacillus thuringiensis that produces a protein toxic to European corn borers. Those caterpillars can seriously damage the plants.

This is just the third year Bt corn has been available commercially. Researchers and the companies that developed it are worried that if farmers plant too many acres, corn borers will develop resistance to the Bt toxin.

A computer model that takes into consideration the genetics, ecology and population dynamics of the corn borer confirms that fear, according to David Onstad, a professor in the university's Department of Natural Resources and Environmental Sciences.

Onstad's model found that if every farmer used Bt corn seed for all his corn acreage -- an unlikely scenario -- resistance would develop in just one year.

"We'll lose this technology if we allow resistance to develop," Onstad said.

Onstad also tested less drastic scenarios, exploring what would happen if a farmer planted, for instance, a mixture of seed containing 10 percent non-Bt corn and 90 percent Bt corn, or blocks or strips of non-Bt corn next to Bt corn.

The outcomes of those scenarios are based on principles of population genetics.

Although none has been found yet, scientists assume some corn borers are just born resistant to the Bt toxin. They could become a major problem if allowed to mate with other tolerant corn borers -- producing more resistant corn borers.

And if a farmer were to plant all Bt corn, all the susceptible corn borers would be killed off -- leaving only resistant corn borers to mate.

But if farmers plant some non-Bt corn, that would provide a refuge for susceptible corn borers to survive and ultimately mate with potentially resistant corn borers -- producing offspring that are susceptible to the Bt toxin.

Onstad's model found the best way to delay resistance to the Bt toxin is for farmers to plant 20 percent of non-Bt corn in strips or blocks adjacent to Bt corn. He estimates that would delay resistance for about 20 years.

The issue of resistance is of great interest not only to farmers and researchers but to industries that spent millions of dollars developing Bt corn.

In fact, the Environmental Protection Agency is requiring all seed companies that sell Bt corn to implement after 2000 a plan aimed at slowing the spread of corn borer resistance to the Bt toxin.

St. Louis-based Monsanto Co. already requires farmers who buy its Bt seed to choose between two management programs. One is somewhat similar to Onstad's recommendation.

"Commercially, it's good for all the growers and it's good for the environment that this technology be viable for a long time," Monsanto spokesman Gary Barton said. His company is also working on technological advancements in gene enhancement to head off resistance.