Maize crops engineered to produce insecticidal toxins derived from the bacterium Bacillus thuringiensis (Bt) are planted on millions of hectares annually, reducing the use of conventional insecticides and suppressing pests, but a U.S. study funded by the Biotechnology Risk Assessment Grant program of the U.S. Department of Agriculture, and the College of Agriculture and Life Sciences at Iowa State University calculates that the evolution of resistance could cut the benefits.

The research team of Aaron Gassmann at Iowa State warned that the western corn rootworm is spreading in genetically modified crops and showing cross-resistance to two of the three types of toxins the plants produce. The researchers said the number of affected fields has been on the rise since 2009.

Researchers team said they found that fields identified by farmers as having severe rootworm feeding injury to Bt maize produced from Monsanto seeds contained populations of western corn rootworm displaying significantly higher survival in laboratory bioassays than those from fields not associated with such feeding injury.

Independent experts speculate that the development - said to result from farmers ignoring scientists' warnings that rootworms would evolve to overcome the poison - could cause billions of dollars in damage to U.S. crops.  

Bt corn makes up 75% of the US. crop. A product made by DuPont and the seed treatment company Pioneer is likely to be approved soon by the European Commission for planting in the 28 member states that choose not to enact restrictive legislation.