Bird experts are worried about the effects of growing transgenic winter oilseed rape.© SPL

Commercial use of some genetically modified crops could alter the balance of weed species that thrive on British farmland. Such a shift could harm bees and butterflies, warn researchers.

Butterfly numbers were cut by up to two-thirds and bee populations by half in fields of transgenic winter oilseed rape (canola), according to the final results of a three-year study commissioned by the UK government.

Researchers behind the £6-million (US$11-million) study say that the project's weed-control system is to blame. The crops are engineered to resist a particular herbicide, which hits broad-leafed weeds harder than grassy varieties. Bees and butterflies suffer because they prefer the former type of weed.

The scientists add that this would have a knock-on effect on animals higher up the food chain. "If this crop were commercialized we'd be concerned about the implications for birds such as sparrows and bullfinches," says David Gibbons, a conservationist from the Royal Society for the Protection of Birds and a member of the committee that oversaw the experiment.

Crop fans

Supporters of transgenic crops stress that most insect species were not affected by the rape's herbicide and say the overall impact on biodiversity is minimal. "As with all weed-management systems, some weed and insect species will be positively affected while others may be negatively affected, but the vast majority are unaffected," says Tony Combes, deputy chairman of the Agricultural Biotechnology Council, a London-based lobbying group.

Bayer CropScience, headquartered in Monheim, Germany, already markets the winter oilseed rape used in the trial in the United States and Canada. Although the crop is grown widely in the two countries, Bayer says it has no intention of applying for a licence to sell it in Europe.

But Bayer officials point out that the biggest difference in butterfly and bee numbers is seen in July, when the crop is just about to be harvested and there is little green material. "There's nothing in the field at that point for bees and butterflies," says spokesman Julian Little. "You wouldn't get very many there anyway."

The results will, however, be felt as a further blow to advocates of transgenic crops. In 2003, two of the three other transgenic varieties covered by the study, spring oilseed rape and beet, were shown to harm biodiversity by reducing overall levels of weeds.

Impact factor

Release of the results marks the end of what has been the largest ever study into the ecological impact of transgenic crops. More than 150 people worked on the experiment, which involved counting a million weeds and 2 million insects at sites across Britain. The report is published in the Proceedings of the Royal Society B¹.

Although none of the crops tested is likely to be licensed in Europe, researchers behind the study say that the data will inform agricultural policy for years to come. They point out that the ecological impacts of previous changes in farming practice, such as increasing herbicide use, were not properly investigated at the start.

"Now we have a rational and scientific basis for managing change," says Chris Pollock, director of research at the Institute for Grassland and Environmental Research in Aberystwyth, UK, and chairman of the study committee. "We've demonstrated in enormous detail just how tight the association is between agriculture and the environment." 

Royal Society for the Protection of Birds

• References

  1. Bohan D. A., et al. Proc. Roy. Soc. B, 272. 463 - 474 (2005).