For decades, Midwestern farmers have relied on the simple process of crop rotation to fight one of their worst enemies, the corn rootworm, without excessive use of pesticides.

But researchers say the insects have evolved, and the time-honored practice is no longer as effective. They predict higher costs and a greater use of chemicals to combat the beetle larva.''This is one of the most amazing curve balls that Mother Nature has thrown us in a long, long time,'' said Mike Gray, associate professor of crop sciences at the University of Illinois.

The Department of Agriculture says rootworms can cost $1 billion annually in crop protection expenses and lost yields. The larvae destroy the roots, cutting off sources of water and nutrition. Debilitated plants fall over, making harvest difficult.

Under crop rotation, farmers alternate planting corn with soybeans or another crop.

Until recently, rootworm larvae could not survive in fields planted with soybeans. But the insects have evolved into what one scientist called a ''new race'' of pest. In some cases, the larvae have even added soybean foliage to their diet.

''They are circumventing crop rotation,'' said Eli Levine, an entomologist with the Illinois Natural History Survey. ''It's very strange behavior.''

University of Illinois researchers first detected a big change in corn rootworms' behavior in 1993 in an area near Piper City.

There is now ample evidence the pests are defeating the crop rotation strategy in parts of 10 counties in east-central Illinois. They also are showing up in droves in northwestern Indiana, and they have been spotted in southern Michigan and parts of Ohio.

University of Illinois scientists say that if current trends continue, corn growers who used to rely on crop rotation could spend a total of $100 million a year on chemicals.

Scientists also fear that farmers might apply corn pest chemicals to soybean fields where the insects have migrated. If pesticides are overused, they say, insects could develop resistance more quickly.

''We're saying that if you rotate, don't jump to chemicals unless you are in one of the problem areas,'' Mr. Levine said.

Farmers also must beware of killing beneficial insects, Mr. Gray said, and be careful about applying chemicals in fields subject to flooding and erosion.

''If you get heavy rains in areas near streams and rivers, the insecticides could move into the water,'' Mr. Gray warned.

The specific pest in Illinois cornfields is the western corn rootworm, which was first detected in the state 33 years ago. Its cousins, the northern and southern corn rootworms, have staked out other parts of the United States, as well as Brazil and Mexico.