A new documentary seeks to unravel the mystery of why billions of honey bees have been disappearing from hives across the United States, and concludes that the chief suspect is pesticides.

“Vanishing of the Bees,” which has a limited theatrical release in Britain from next week, follows the fate of a group of U.S. beekeepers hit by Colony Collapse Disorder (CCD), which first struck in 2004 and made U.S. headlines three years later.

Countless bees would suddenly vanish, leaving an empty hive but few bodies, and the phenomenon has variously been linked to mites, disease, genetically modified crops, mobile phones and, in the words of one beekeeper, “PPB,” or “piss-poor beekeeping.”

While the cause has yet to be established, the film suggests there is a link to pesticides, and particularly those applied to seeds as opposed to sprayed on existing plants.

Other factors could also contribute, it added, including the fact that bees are being transported long distances to pollinate single crops, or monocultures, rather than producing honey.

The dominance of monocultures in U.S. agriculture means crops flower only once a year, and so cannot support indigenous insects. So devastating were the effects of CCD that beekeepers started shipping bees from Australia to meet U.S. demand.

U.S.-based directors George Langworthy and Maryam Henein argue that the problem goes beyond the disappearance of the insects. One third of everything we eat is pollinated by bees and without them farming could be thrown into chaos.

“They are one of our most ancient allies,” Henein said in an interview in London. “We actually depend on honey bees to eat. May be out of selfishness it raises a red flag.”

Langworthy added: “It’s a broader issue about the system of agriculture. People are going to have to rethink it and maybe they don’t want to. It really will have to be driven by the general public’s call for change.”