One of the nation's most widely planted crops - a genetically engineered corn plant that makes its own insecticide - may be losing its effectiveness because a major pest appears to be developing resistance more quickly than scientists expected.

The U.S. food supply is not in any immediate danger because the problem remains isolated. But scientists fear potentially risky farming practices could be blunting the hybrid's sophisticated weaponry.

When it was introduced in 2003, so-called Bt corn seemed like the answer to farmers' dreams: It would allow growers to bring in bountiful harvests using fewer chemicals because the corn naturally produces a toxin that poisons western corn rootworms. The hybrid was such a swift success that it and similar varieties now account for 65 percent of all U.S. corn acres - grain that ends up in thousands of everyday foods such as cereal, sweeteners and cooking oil.

But over the last few summers, rootworms have feasted on the roots of Bt corn in parts of four Midwestern states, suggesting that some of the insects are becoming resistant to the crop's pest-fighting powers.

Scientists say the problem could be partly the result of farmers who've planted Bt corn year after year in the same fields.

Most farmers rotate corn with other crops in a practice long used to curb the spread of pests, but some have abandoned rotation because they need extra grain for livestock or because they have grain contracts with ethanol producers. Other farmers have eschewed the practice to cash in on high corn prices, which hit a record in June.

"Right now, quite frankly, it's very profitable to grow corn," said Michael Gray, a University of Illinois crop sciences professor who's tracking Bt corn damage in that state.

A scientist recently sounded an alarm throughout the biotech industry when he published findings concluding that rootworms in a handful of Bt cornfields in Iowa had evolved an ability to survive the corn's formidable defenses.

Similar crop damage has been seen in parts of Illinois, Minnesota and Nebraska, but researchers are still investigating whether rootworms capable of surviving the Bt toxin were the cause.