A third of the honeybee colonies in America died last winter, says Dennis vanEngelsdorp, and he would like to know why. "We haven't found one smoking gun," notes the 38-year-old apiarist with Pennsylvania's Department of Agriculture, who, like other concerned researchers nationwide, is investigating a phenomenon scientists call colony collapse disorder. Mature bees are leaving their hives and disappearing. Their queens and young ones, abandoned, die soon afterward. In vanEngelsdorp's own backyard, a hive that once held 12,000 bees dwindled to less than 100 in three weeks. "I thought this colony was going to survive," he says sadly, inspecting empty combs and bee carcasses. Bees, he says, are "essential for our food and essential for our environment."

In the last 25 years, the managed honeybee population of the U.S. has declined by more than half—and as bees go, so does food: Bees pollinate as much as 30 percent of the nation's food supply—about $14 billion worth of seeds and crops, although there have been no shortages yet. Researchers are looking into pesticides, parasites and a virus that weakens bees' immune systems as possible causes. A University of Virginia study focused on air pollution, saying it destroys the fragrance of flowers, curtailing the bees' ability to follow scent trails. What's clear, says vanEngelsdorp, is that the bees are sending their keepers a message. "This has to make us aware something is seriously wrong. It needs to be a wake-up call: No bees is a bad thing."