One of America'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 United States food supply is not in any immediate danger because the problem remains isolated. But scientists believe potentially risky farming practices could be blunting the hybrid's sophisticated weaponry.
When it was introduced in 2003, "Bt corn" seemed like the answer to farmers' dreams. It would enable growers to bring in bountiful harvests using fewer chemicals because the corn naturally produced a toxin to poison a major pest, the western corn rootworm.
The hybrid was such a swift success that it and similar varieties now account for 65 per cent of all US corn hectares - 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 have planted Bt corn year after year in the same fields.
Most farmers rotate corn with other crops in a method long used to curb the spread of pests, but some have abandoned rotation because they need extra grain for livestock or have grain contracts with ethanol producers.
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