Vincent Kessler / Reuters A coloured honeycomb from a beehive is seen in Ribeauville near Colmar Eastern France, on Oct. 5. Bees at a cluster of bee hives in northeastern France have been producing honey in mysterious shades of blue and green, alarming their keepers who now believe residue from containers of M&M's candy processed at a nearby biogas plant is the cause. Since August, beekeepers around the town of Ribeauville in the region of Alsace have seen bees returning to their hives carrying unidentified colourful substances that have turned their honey unnatural shades.

Vincent Kessler / Reuters French apiarist Andre Frieh holds a sample green colored honey at his home in Ribeauville near Colmar Eastern France, on Oct. 5.

Reuters -- Bees at a cluster of apiaries in northeastern France have been producing honey in mysterious shades of blue and green, alarming their keepers who now believe residue from containers of M&M's candy processed at a nearby biogas plant is the cause.

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.

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Vincent Kessler / Reuters The village of Ribeauville is seen near Colmar Eastern France, on Oct. 5.

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