Last year, researchers identified a virus as a major cause of the die-off; the latest suspect is a class of pesticides called neonicotinoids, which are used to protect common agricultural seeds, including corn. The insecticides are systemic, which means they persist throughout the life of the plant. Scientists have demonstrated that exposure to these chemicals damages bees’ brain function, including their ability to home in on the hive.

In mid-March, environmental groups and beekeepers sued the Environmental Protection Agency to persuade it to withdraw its approval of two of the most widely used neonicotinoids. The manufacturers of these chemicals — notably Syngenta and Bayer CropScience — have claimed again and again that they are safe. And it is true that bees face other stresses. Even so, beekeepers managed to keep their hives relatively healthy before the increased use of neonicotinoids began in 2005.

Bees are essential to modern agriculture. There is no replacing them, no substitute of any meaningful kind. The E.P.A. has sent a team to central California — where more than 1.6 million hives are needed every spring — for “discussions.” That is not remotely good enough. The agency must conduct an immediate analysis of neonicotinoids. The manufacturers’ bland assurances seem empty in the face of this long-term die-off of these beneficial creatures.

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