SAN JOAQUIN VALLEY, Calif. On a cool January day in the foothills of the Sierra Nevada mountains, Steve Ellis culled his sick bees. The only sounds were their steady buzz and the chuffing of the smoker he used to keep them calm as he opened the hives, one by one, to see how many had survived. The painful chore has become an annual ritual for Ellis, and, hardened now like a medic on the front lines, he crowned another box with a big rock to mark it.

"This one is G.A.D.," he said. "Good as dead."

Ellis, of Barrett, Minn., is one of some 1,300 commercial beekeepers from across the United States who migrate to California each year, along with nearly 2 million hives, for the single largest pollination event in the world. Below him in the sprawling valley, nearly 1,400 square miles of almond trees three-fourths of the global supply were ready to burst out into a frothy sea of pink and white. To grow into a nut, every single blossom would need at least one American honeybee.

Ever since the ominous phrase "colony collapse disorder" first surfaced in 2006, scientists have struggled to explain the mysterious mass die-offs of honeybees. But here in America's food basket the escalating stakes are laid out as clearly as the almond trees that march in perfect rows up to the horizon.

Modern farm economics have created an enormously productive system of genetically engineered, chemically dependent agriculture. But it relies on just one domesticated insect to deliver a third of the food on our plate.

And that insect is dying, a victim of the very food system that has come to depend on it.

A rush of recent research points to a complex triangle of causes: pervasive pesticides, a flowerless rural landscape dominated by cash crops, and the spread of parasites and diseases. Together they inflict enormous damage on the honeybees that crisscross the country each spring and summer, like migrant laborers, to pollinate everything from almonds in California to apples in Maine.

In the past several decades, the number of crops that depend on bees for pollination has quadrupled, even as the number of hives available to pollinate them has dropped by half. Every winter, beekeepers on average continue to lose a fourth to a third of their hives, raising fears that the gradual decline of these remarkably resilient insects will soon limit the production of foods that Americans now take for granted.

Most consumers are insulated from the threat as long as the aisles of America's grocery stores are resplendent with apples, lemons, coffee, cocoa, peanuts, grapes, onions, cucumbers and watermelons. But not Ellis and his sometimes partner Jeff Anderson, a third-generation beekeeper from Eagle Bend, Minn., whose family has made the annual trek to and from the California almond bloom since 1961.

For them, catastrophe could be just one harvest away.

This winter, Ellis lost about 1,200 of the 2,200 hives he had in the summer. Last winter, Anderson lost 65 percent of his 3,000 hives and didn't have enough bees to supply all his almond growers.

Ellis and Anderson are among the most outspoken of beekeepers nationally in federal court, in the news media, before regulators, legislators and anyone else who will listen to their warning about the American food system's mortal reliance on pesticides and monoculture.

Yet they see this unfurling crisis through entirely different prisms. Anderson, slightly round and solid as a fireplug, is a creationist who believes that God made the world and all its creatures in seven days, and that mankind was put here to take care of it. That includes using chemicals to grow food. "But we should be judicious in how we do it," he said.

Ellis, courtly and rail thin, is entranced by the evolutionary role that pollinators have played over millions of years in the creation of flowers, color and scent. Without insects, he says, the world would be a place much more like Dorothy's Kansas than the Technicolor Land of Oz that nature has produced.

"We would go back to black and white," he said.

For the first time, Ellis and Anderson are thinking what was once unthinkable perhaps the foraging grounds in central Minnesota, where they have made honey for decades, have become too barren and toxic for their bees.

Springtime used to be a time of rebirth. It was the season when beehives grew fat and healthy on dandelions, wildflowers, and the sweet clover and alfalfa that farmers once grew to add fertilizer to the soil. It's what once made Minnesota part of a Midwestern mecca for beekeepers and long one of the top five honey-producing states in the country.

But in the space of a few decades, the central Minnesota landscape around Ellis has been transformed into one that has no room for bees.

When he looks out over the edge of the old gravel pit near Elbow Lake where he keeps his hives, Ellis sees what he calls a vast agricultural desert of corn and soybeans two plants that don't need bees for fertilization. Synthetic fertilizers have replaced the natural ones, farming has become increasingly specialized and now about a third of Minnesota's land and much of the Midwest is covered with just those two crops.