Bees at a cluster of apiaries in northeastern France are producing honey in mysterious shades of blue and green.
Beekeepers believe they have been bringing back residue from a plant 4km away which processes waste from the production of M&Ms - chocolates in brightly coloured shells, Reuters reports.
Mars operates a chocolate factory near Strasbourg, around 100km away from the affected apiaries.
The honey has been turning into unnatural shades of colour since August. It is unsellable.
Apiculturist union president Alain Frieh said a dozen beekeepers are affected. They are already dealing with high bee mortality rates and dwindling honey supplies following a harsh winter.
The biogas company says it is now storing the waste inside.
There are about 2400 beekeepers in Alsace who tend some 35,000 colonies and produce about 1000 tonnes of honey per year, according to the regional chamber of agriculture.