the advocates of transgenic crops recently received a big blow from the findings of the largest ever study into the ecological impact of these crops, published recently in Proceedings of the Royal Society B:Biological Sciences . The three-year study commissioned by the uk government suggests that some genetically modified crops can harm bees and butterflies by altering the balance of weed species that thrive on farmland. This would subsequently impact animals higher up in the foodchain, it warns.

The study found that the number of butterflies decreased by up to two-thirds and bee populations by half in the fields of transgenic winter oilseed rape (canola). It blames the weed-control system for this. The crops are resistant to a particular herbicide, which hits broad-leafed weeds harder than grassy varieties. Bees and butterflies suffer because they prefer the former. "If this crop were commercialised, 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.

Germany-based Bayer CropScience already markets the winter oilseed rape (used in the trial) in the us and Canada. It says it has no intention of applying for a licence to sell it in Europe. But its officials point out that the biggest drop 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 and it is not justified to blame transgenic crops,' says Bayer spokesperson Julian Little.

Over 150 people worked on the experiment conducted at sites across the uk . 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. "Now we have a rational and scientific basis for managing change,' says Chris Pollock of the Institute for Grassland and Environmental Research, Aberystwyth, the uk, who was chairperson of the study committee. "We've demonstrated in enormous detail just how tight the link is between agriculture and the environment,' he claims.