BAKERSFIELD, Calif. — A mysterious malady that has been killing honeybees en masse for several years appears to have expanded drastically in the last year, commercial beekeepers say, wiping out 40 percent or even 50 percent of the hives needed to pollinate many of the nation’s fruits and vegetables. A conclusive explanation so far has escaped scientists studying the ailment, colony collapse disorder, since it first surfaced around 2005. But beekeepers and some researchers say there is growing evidence that a powerful new class of pesticides known as neonicotinoids, incorporated into the plants themselves, could be an important factor. The pesticide industry disputes that. But its representatives also say they are open to further studies to clarify what, if anything, is happening. “They looked so healthy last spring,” said Bill Dahle, 50, who owns Big Sky Honey in Fairview, Mont. “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.” ...... In the valley, where 1.6 million hives of bees just finished pollinating an endless expanse of almond groves, commercial beekeepers who only recently were losing a third of their bees to the disorder say the past year has brought far greater losses. ...... Bret Adee, who is an owner, with his father and brother, of Adee Honey Farms of South Dakota, the nation’s largest beekeeper, described mounting losses. “We lost 42 percent over the winter. But by the time we came around to pollinate almonds, it was a 55 percent loss,” he said in an interview here this week. ...... Annual bee losses of 5 percent to 10 percent once were the norm for beekeepers. But after colony collapse disorder surfaced around 2005, the losses approached one-third of all bees, despite beekeepers’ best efforts to ensure their health.