For more than two decades crop dusters have buzzed the skies of Colombia showering bright green fields of coca with chemical defoliant as part of a US-funded effort to stem the country’s production of cocaine. Farmers across the country have long complained that indiscriminate spraying also destroys legal crops, and that the chemical used – glyphosate – has caused everything from skin rashes and respiratory problems to diarrhoea and miscarriages.