Careless handling of seeds may be the key reason for the unintended spread of genetically modified (GM) crops, a study has found.

The discovery challenges the widespread belief that the main source of GM contamination is the transfer of pollen by bees from GM crops to non-GM counterparts in neighbouring fields. Human error during seed production and handling is the more likely culprit, say the researchers.

Stands of non-GM crop plants are currently planted near or within fields of modified crops to provide refuges for pests. This technique helps prevent the pests developing resistance to the pesticides used on GM crops. But human error could undermine this widely used strategy, the paper says.

Shannon Heuberger, an entomologist at the University of Arizona, United States, and her colleagues measured the gene flow — the movement of genes between different populations that occurs when a plant from one population fertilises a plant from the other — in Bt (Bacillus thuringiensis) cotton, the widely planted GM crop, in 15 fields in Arizona.

They found that gene flow via the transmission of pollen by bees was rare. Fewer than one per cent of seeds produced by ordinary cotton plants contained genes from Bt cotton that had been transmitted in this way.

But poor seed-sorting resulted in some seed bags intended for planting in non-GM fields containing as much as 20 per cent GM seed. One non-GM field was found to have a large number of GM plants due to human error in planting.

"Our most important result is that growers can minimise gene flow by screening the seed before planting it in seed-production fields and by being more cautious in their planting process," Heuberger told SciDev.Net.

"In comparison, designing strategies to minimise bee pollination between fields can be quite difficult because insect behaviour is hard to predict," she added.

The study concludes that seed producers and decision makers should consider screening seeds to monitor the presence of GM seeds in the supply, and that they also need to communicate "the importance of segregating seed types at planting to reduce human error".

María Isabel Manzur, head of biodiversity at the Sustainable Societies Foundation (FSS), a Chilean environmental non-governmental organisation, said: "This is a very interesting study because it helps elucidate at a greater depth how transgenic contamination takes place".

"It corroborates once more that transgenic crops can contaminate surrounding crops, which is something that biotech companies frequently deny despite all the evidence to the contrary."

The study was published in PLoS ONE last month (30 November).