The puzzling occurrence, dubbed colony collapse disorder, has been wiping out more than 40 percent of the hives needed to pollinate many of the nation's fruits and vegetables, they said.
While scientists have been unable to determine a cause, some beekeepers and researchers say growing evidence points to a powerful new class of pesticides known as neonicotinoids as possibly being involved, The New York Times reported Friday.
Research on neonicotinoids -- which become incorporated into the plants -- "supports the notion that the products are safe and are not contributing in any measurable way to pollinator health concerns," said Jay Vroom, president of CropLife America, which represents more than 90 pesticide producers.
"We stand with science and will let science take the regulation of our products in whatever direction science will guide it," he said.
Beekeepers say they've seen the die-off problem skyrocket.
"They looked so healthy last spring," Bill Dahle, who owns Big Sky Honey in Fairview, Mont., said of the bees in his hives. "We were so proud of them. Then, about the first of September, they started to fall on their face, to die like crazy. We've been doing this 30 years, and we've never experienced this kind of loss before."
Beekeepers have traditionally accepted annual bee losses of 5 percent to 10 percent but losses have risen to around one-third of all bees since colony collapse disorder surfaced around 2005, and are still increasing.
Notable deaths of 2014 [PHOTOS]