By Richard Black
BBC science correspondent
A US study reveals new evidence to show how genes from biotech crops can
spread to nearby non-GM plant relatives.
Maize (corn) production dominates US agriculture
The data comes from research on maize engineered to produce powerful toxins in its leaves and stems.
These substances, normally produced by bacteria, and are lethal to insect pests that try to eat the maize plant.
But an Arizona-Texas team says the way the crop is grown in some countries may lead to insects becoming resistant to the GM plant and pesticides.
The research is reported in the journal Proceedings of the National Academy of Sciences.
In the study paper, the scientists say guidelines on how to cultivate some GM crops should now be revised.
The work concerns Bt maize (corn). This crop has been modified to incorporate genetic material from the bacterium Bacillus thuringiensis (Bt).
This makes the plant tissues toxic to insects such as the European corn borer, a significant pest that hides in the stalks of the plant, making it difficult to control with chemical sprays.
In the US and some other nations, Bt maize has to be grown alongside so-called "refuges" of conventional varieties - a strategy aimed at preventing the insects from becoming resistant to Bt.
The logic is simple: if rare, resistant insects do emerge from the GM fields, their success will be restricted if they are breeding with non-resistant pests on nearby fields. Their progeny are likely to die if they attack the modified maize.
But the new work shows that the Bt gene is finding its way into those refuge plants through pollen that is spreading tens of metres.
"[The refuge] is supposed to be toxin-free but in fact the seeds, that is the next generation - some produce the Bt toxin," Professor Bruce Tabashnik, from the University of Arizona Department of Entomology, told BBC News Online.
"This may increase the potential for some insects to become resistant." And this tolerance could also extend to Bt sprays as well.
Professor Tabashnik was involved in drawing up the current US guidelines on GM-free refuges, and his latest research is likely to lead to a major review within the US.
A number of other countries have similar regulations; and the finding could be relevant to other crops such as cotton in which the Bt modification has also been introduced.
"[Contamination of non-GM plants] occurs at high levels near the Bt maize and it declines as you move away from the Bt maize.
"And that allows us to infer that what's happening is there's pollen moving from the Bt plants into the nearby refuge, and this is producing Bt toxin in the kernels of the plants in the refuge.
"This is the first time that gene flow has been documented in the context of a refuge," Professor Tabashnik said.
"The guidelines need to be re-examined in the face of this new insight.
"The current thinking is that the refuges should be as close as possible to the Bt maize; and the reason for that is to encourage gene flow in the insects; in other words, mating between the non-resistant insects that come out of the refuge and the ones in the Bt fields which may have become
Commenting on the research, the UK campaign group GeneWatch said it demonstrated how little was still known about the environmental impact of GM crops.
"Bruce Tabashnik's work in this area is seminal, and so this study will I think be highly regarded and influential," said GeneWatch's Sue Mayer.
"It exposes how little we do know about the environmental impact of GM crops, and how much monitoring needs to be done. We keep hearing these statements about how it's all completely safe but really we need proper monitoring to find out."