All forms of agriculture have an impact on non-target organisms

Dr May Berenbaum

That earlier work was seized upon by green groups as an example of the dangers posed to friendly creatures by novel plants engineered to contain their own insecticides.

But in a new report by scientists at the University of Illinois at Urbana-Champaign, US, the pollen from BT-corn was shown to have no impact on black swallowtails (Papilio polyxenes).

The team involved believe their work underlines the need for a more sophisticated approach to GM technology in which farmers choose the crops and management systems - both novel and conventional - that best suit the local environment.

Non-target impact

"All forms of agriculture have an impact on non-target organisms," Dr May Berenbaum, the lead researcher, told BBC News Online. "There is no way to reduce the impact of agriculture to zero - even ploughing has a non-target impact.

"So the ideal objective is to minimise those impacts using the best tools available. It would be nice if the farmers making pest management decisions had as much information as possible on non-target impacts."

May Berenbaum has studied swallowtails for 25 years

Both studies sought to understand the impact of Bt-corn on the countryside. The popular crop has been modified to incorporate a gene from the bacterium Bacillus thuringiensis (Bt), which makes the plant tissue toxic to a major pest, the European corn borer.

But whereas the monarch work, carried out by Cornell researchers, was merely a lab-based study - a source of some criticism at the time - the Illinois research was done in both the lab and the field.

Dr Berenbaum and colleagues put swallowtail larvae on potted plants placed along the edge of a GM cornfield during a seven-day period last July when the crop was pollinating. They also placed gelled slides with the pots to monitor pollen spread.

Corn toxin

Although many of the larvae died - which is common at any time - the researchers could find no relationship between their deaths and their location by the field or the amount of pollen covering the weeds on which they were feeding.

Most of the deaths were clearly the result of predators, said Dr Berenbaum, who has worked with swallowtails for 25 years. "It's a tough life for a caterpillar - that's nature," the entomologist added.

"On average 98% of them die, but none of the mortality we observed could be attributed to the corn pollen."

Black swallowtails are common across the Midwest

Monarch caterpillars died after eating BT-corn pollen in lab experiments

John Losey, who led the Cornell study, said the corn variety used in the Illinois research was similar in toxicity to the type that he tested and it is the most common variety planted by farmers.

"There's a large number of butterfly species that are potentially at risk. This is one and the monarch is another,'' Losey said. "Our original lab study pointed out the need to do studies like this one.''

Farmers were expected to plant about 14 million acres of-Bt corn in the US this year, 18% of the total corn acreage.

The Illinois research is published in the Proceedings of the National Academy of Sciences (PNAS).