A study has found that genetically modified corn can spread enough toxic pollen to nearby weeds to kill the monarch butterfly caterpillars feeding on them.

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

The field trial followed concerns raised earlier about a biotech crop known as Bt corn in a Cornell University laboratory study.

Researcher John Obrycki said: “Coming up with a good ecological assessment of this technology probably needed to be done before planting it across the Midwest.”

GM supporters questioned the study, saying it created unrealistic field conditions.