Ananova:

A study of genetically modified corn has stirred new concerns about the crop after finding that it spreads enough pollen to nearby weeds to kill the monarch butterfly caterpillars that are feeding on them.

Iowa State University scientists found that one in five monarch larvae died after being exposed to the toxic corn pollen for two days. Three days after the initial two day exposure, more than half the larvae died.

The biotech corn, known as Bt corn after a bacterium gene that makes it toxic to the European corn borer, became controversial last year after a laboratory study at Cornell University showed it was toxic to monarch butterflies.

The Iowa State study was intended to examine how the pollen would affect butterflies that encountered it in the field.

"In the field you may have higher mortality rates because of the cumulative effect of being exposed to the toxin throughout the larval stage," said researcher John Obrycki. "Coming up with a good ecological assessment of this technology probably needed to be done before planting it across the Midwest."

The Cornell finding produced a public outcry in Europe and calls from environmental groups to curb the spread of gene-altered crops, but biotech supporters have criticised that study and the one from Iowa State as being unrealistic.

"Much of what the Iowa State study reports is based on analysis taking place in laboratory manipulations rather than field conditions," said Val Giddings, vice president of food and agriculture for the US Biotechnology Industry Organisation.

But Jane Rissler, a biotech expert with the Union of Concerned Scientists, said more research needs to be done.

"This is a study that confirms another study but this isn't the end of the story. It still hasn't resolved the issue," she said. "All of this should have been done five years ago instead of having this research come out in dribs and drabs."