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Pesticide use is surging among U.S. corn farmers who are worried that some insects have become resistant to genetically modified versions of the crop.

That’s an unexpected reversal since one of the promises of engineered corn when it was introduced 17 years ago was its ability to kill pests. The use of soil insecticides for the crop plunged 90 percent through 2010, according to the U.S. Department of Agriculture.

Whether the return to pesticide use makes sense, or is simply spurred by a chemical industry marketing campaign, is at the center of one of the biggest debates in the corn belt this spring. At the heart of the controversy is whether snuffing out pests in the short term with chemicals may create a worse problem down the road.

Farmers say they need to do whatever it takes now to control the western corn rootworm, the most damaging U.S. corn pest. Although Monsanto Co. (MON) designed its corn to kill the worms, resistant bugs have been found in four states and growers say pesticides are needed again to protect their crops.

It would be “financial suicide” to plant rootworm-killing corn without a soil insecticide as a secondary way to control the larvae, said Illinois farmer Mike Jenks, echoing the views of growers across the Midwest.