LOST HILLS, Calif. – Almond trees are exploding with pink and white blossoms across the vast Central Valley, marking the start of the growing season for California’s most valuable farm export.

Toiling among the blooms are the migrant workers that will make or break this year’s crop: honeybees.

The insects carry the pollen and genetic material needed to turn flowers into nuts as they flit from tree to tree. It’s a natural process that no machine can replicate. But it can’t be left to chance. Bees are too integral to the fortunes of California’s nearly $3-billion-a-year almond industry.

So each year beginning in early February, hundreds of beekeepers from around the country converge on California’s almond farms with their hives in tow. Lasting about four weeks, it’s the largest such pollination effort on Earth: 1.6 million hives buzzing with 48 billion bees across a cultivation area about the size of Rhode Island.

“Without the honeybees … the (almond) industry doesn’t exist,” said Neal Williams, an entomologist and pollination ecologist at the University of California-Davis. “We need those bees. We need them to be reliable, and we need them at the right time.”

But a mysterious malady known as colony collapse disorder has wreaked havoc on the U.S. bee population in recent years, stoking fears among almond producers and other farmers that depend on the insects for their livelihoods.

Between 2003 and 2009, the number of bee colonies in California plunged 26 percent to 355,000, according to the U.S. Department of Agriculture. Eric Mussen, another UC-Davis bee expert, said no agency has a precise count. He believes those federal hive statistics to be too low. Still, he too estimates that the state lost about a quarter of its hives over that time period.

Although California bee populations have recovered a bit, almond farmers are still feeling the sting. Prices to rent bees have tripled since 2003-2004 to as much as $160 a hive because of tight supplies and rising expenses for beekeepers to keep their colonies healthy. Collectively, California growers will spend about $250 million on bees this year.

Scientists believe that colony collapse disorder is a combination of ailments that includes mites, malnutrition, stress and fungi.

Even in relatively normal years, those factors can claim a third of a hive’s population, said beekeeper Bryan Ashurst.

A fifth-generation California beekeeper with 12,000 hives filled with about 360 million insects, Ashurst said the creatures are surprisingly delicate.

“It takes time to build a hive,” Ashurst said. “But it can collapse really quickly.”

His crews pollinate all manner of crops, including apples, alfalfa and zucchini. But his biggest job of the year is almonds.

Early reports show California on track for a record crop in 2012. California producers this year are projected to grow and ship 1.9 billion pounds of the nuts, about 70 percent of them for consumers in Asia, Europe, the Middle East and other international markets.

The source for 80 percent of the global supply, California’s almond belt stretches from Kern County in the southern San Joaquin Valley to Butte and Tehama counties in the far north of the Sacramento Valley.

There are also smaller pockets of orchards in the Sierra Nevada foothills and on the north coast. About 6,500 almond ranchers harvest 750,000 acres of trees, an increase of 42 percent over the last decade.

Nowhere is the pollination process played out on a grander scale than at Paramount Farming Co., in Kern County about 145 miles northwest of Los Angeles. A unit of Los Angeles-based Roll Global – a privately held company owned by Stewart and Lynda Resnick, a billionaire business-philanthropy power couple – Paramount is the world’s biggest almond grower with 47,000 acres under cultivation.

“Almonds are our primary crop and the most critical because they bloom for a short period; it’s early in the season and we must have bees to pollinate,” said Paramount President Joe Macilvaine.

“Lots of things can reduce almond yield – weather conditions, drought, insect infestations,” Macilvaine said. “But if you don’t have the bees, you never get to begin.”

This season, Paramount contracted with 26 beekeepers to bring in 92,000 hives from as far as Maine, Louisiana, Florida and the Carolinas. The rental expense represents 15 percent of the company’s total almond production cost.

Maintaining a supply of healthy, top-quality bees is so challenging that Paramount employs its own staff entomologist, Gordon Wardell, who holds a doctorate in the field. He works with beekeepers and research scientists to develop reliable pollinators.

On a recent warm, nearly windless day, Wardell donned a bee suit to inspect a hive smack in the middle of a mile-square Paramount almond orchard, where tiny white petals covered the ground like confetti. Hundreds of honeybees buzzed around him as he gently removed the top from a standard, two-tier hive box and pulled out one of eight wooden frames where the worker bees make honey, store pollen and feed their larvae.

The symbiotic relationship between honeybees and man goes back at least 5,000 years to ancient Egypt. Spanish missionaries brought the first honeybees to California in 1750, Wardell said.

Today, bees need more care and feeding because their once-natural environment is more polluted and threatened by urbanization.

As a result, some adult field bees die after just two or three weeks instead of their normal six-week life span. They’re replaced by younger bees who are forced to leave the hive to gather nectar and pollen before reaching optimal strength.

“It’s like sending (human) 6-year-olds to work at heavy construction,” Wardell said.

To help their bees stay strong, beekeepers give their charges special foods laced with proteins and sugars. The diet helps them survive the winter and gives them the energy needed to fly long distances and work long hours spreading pollen. Beekeepers also have become adept at rejuvenating hives by splitting the populations and replacing ailing queens.

The hard work is paying off. After hitting a low in 2007 of about 340,000 hives, according to the USDA, the number of managed bee colonies in California is rising. Other industry experts put the colonies at about 500,000, up from a low of around 400,000.

“We’re looking at the best bees we’ve seen in five years,” Wardell said as he doused the hive with smoke to distract the insects from his interfering presence. “The bees are better because the beekeepers are getting better at managing them.”

Meanwhile, researchers at the University of California and elsewhere are seeking ways to make pollinating easier on the traditional honeybees. They’re experimenting with a different bee breed, known as Blue Orchards. The Blue Orchards don’t live socially in colonies, instead raising broods individually in hollowed-out wood nests.

Paramount is hoping to use the Blue Orchard bees as a kind of “insurance in case something happens to the honeybees,” Wardell said.

Farmers have also begun planting a species of hermaphroditic, or self-fertilizing, almond trees. The trees, which are just coming into production after a four-year maturation process, still need bees.

But pollination and fertilization can occur with just one bee visit to the same flower. Traditional pollination requires the bees to carry pollen from one tree’s flower to another.

Farmers are heartened by the latest research developments. Still, beekeepers said these insects, which are so critical to California agriculture, remain something of an enigma.

It’s an inexact science “to manipulate the bees to do what they naturally do on our terms,” said Henry Harlan, who manages 2,400 hives in Yolo County near Sacramento, Calif. “If you meet a beekeeper who says he knows it all, his bees will probably be dead next year.”