The finding by researchers at the University of Arizona challenges the long-held belief that the main source of GM contamination is the transfer of pollen by bees, Britain's The Guardian reported Wednesday.
Entomologist Shannon Heuberger and her colleagues measured the movement of genes between different populations of Bacillus thuringiensis cotton, a widely-planted GM crop, in 15 fields in Arizona.
Gene flow via the transmission of pollen by bees was rare, they found. Fewer than 1 percent of seeds produced by ordinary cotton plants contained genes from Bt cotton that had been transmitted in this way, they said.
But poor seed sorting resulted in some seed bags intended for planting in non-GM fields containing as much as 20 percent GM seed, they discovered.
And one non-GM field was found to have a large number of GM plants due to human error in planting, they said.
"Our most important result is that growers can minimize gene flow by screening the seed before planting it in seed-production fields and by being more cautious in their planting process," Heuberger said.
"In comparison, designing strategies to minimize bee pollination between fields can be quite difficult because insect behavior is hard to predict," she said.