Since August, beekeepers around the town of Ribeauville in the region of Alsace have seen bees returning to their hives carrying unidentified colorful substances that have turned their honey unnatural shades.
Mystified, the beekeepers embarked on an investigation and discovered that a biogas plant 4 km (2.5 miles) away has been processing waste from a Mars plant producing M&M's, bite-sized candies in bright red, blue, green, yellow and brown shells.
Asked about the issue, Mars had no immediate comment.
The unsellable honey is a new headache for around a dozen affected beekeepers already dealing with high bee mortality rates and dwindling honey supplies following a harsh winter, said Alain Frieh, president of the apiculturists' union.
Agrivalor, the company operating the biogas plant, said it had tried to address the problem after being notified of it by the beekeepers.
"We discovered the problem at the same time they did. We quickly put in place a procedure to stop it," Philippe Meinrad, co-manager of Agrivalor, told Reuters.
He said the company had cleaned its containers and incoming waste would now be stored in a covered hall.