A mysterious malady that has been killing honeybees en masse since 2005 has drastically expanded this year, wiping out as much as 40 to 50 percent of the hives needed to pollinate America’s fruits and vegetables. Scientists have been unable to determine the cause of the mass deaths, but some beekeepers and researchers blame a new class of pesticides called neonicotinoids that are now bred into plants themselves. (Not surprisingly, the pesticide industry disputes this.) Until this year, beekeepers in California’s San Joaquin Valley had only been losing a third of their bees, but the deaths have increased dramatically this year. The largest farm, in South Dakota, said it lost 55 percent of its bees this year. The EPA has sent a top official and chemical experts to California for discussions.