Weeds have become stronger and fitter by cross-breeding with genetically modified crops, leading to fears that superweeds which are difficult or impossible to control may invade farms growing standard crops.

Two separate teams, one working on sunflowers in the US and the other on sugar beet in France, have shown weeds and GM food crops readily swapping genes.

In the case of wild sunflowers, classed as "weed" varieties in America, specimens became hardier and produced 50% more seeds if they were crossed with GM sunflowers which had been programmed to be resistant to seed-nibbling moth larvae.

Allison Snow, who headed the team at Ohio State University, confessed in New Scientist that she was "shocked" by the results. "It does not prove all GM crops are dangerous," she said. "I just think we need to be careful because genes can be very valuable for a weed and persist for ever once they are out there."

Pioneer Hi-Bred, which developed the GM sunflower, has abandoned the idea of selling the strain commercially.

The sugar beet results show that wild and GM varieties swapped genes, sometimes to the advantage of the wild varieties and the detriment of the GM plants, which produced lower yields. Writing in the Journal of Applied Ecology, the University of Lille team said they had underestimated the likelihood of GM beets swapping genes with the beet weeds that grow among them.

The two sets of results add to the fears of environmental groups and organic farmers that normal crops could be contaminated by GM varieties - and make weeds impossible to control. This is less of a problem in countries where crops have been introduced, for instance soya grown the US, because no native weed varieties exist. But in Europe, particularly in Britain, where weed species of both beet and oil seed rape exist, the risk is potentially serious.

Adrian Bebb, GM campaigner at the environmental group Friends of the Earth, said GM beet was now being grown at 16 farm-scale trial sites in England. "Once again scientists are discovering new impacts of GM crops," he said. "The government always emphasizes the importance of a sound scientific approach to GM crop safety, so they should look at this research seriously and question whether or not we should be testing GM crops out of doors."

Two years ago government research reported that GM crops could cross-pollinate with ordinary crops over larger distances than had been thought. The government is in its final year of trials to investigate the effect of growing GM crops on the countryside.

© Guardian Newspapers Limited 2002

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