A U.S. study finds that insects quickly develop resistance to genetically engineered crops when single-gene plants are grown near double-gened ones.

Researchers at Cornell University looked at the history of diamondback moths kept in a greenhouse with genetically engineered broccoli. The broccoli had been modified using strains of a common soil bacterium that produces toxins.

The team found that when moths were kept with plants engineered to produce two types of toxin, all the insects died within 26 generations or about two years. But when they were kept with a mix of plants producing one toxin and plants producing two the insects eventually developed resistance to both toxins.

Single-gene plants really function as a steppingstone in resistance of two-gene plants if the single gene plants contain one of the same Bt proteins as in the two-gene plant, said Anthony Shelton, a Cornell entomology professor.

Maize and cotton are the only commercial crops now engineered to contain the Bt proteins. Both single- and double-gened strains are sold in the United States, but Australia stopped importing single-gened seed after finding problems with resistance.

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