Image caption More than 1,000 sq mi of California's Central Valley is covered by almonds

If you're eating a Bakewell slice or a macaroon as you're reading this, then there is a very good chance that you are sampling a little piece of Californian sun.

Whether ground, flaked or toasted, almonds play an important part in our cuisine and a staggering 85% of the world's almonds are grown in the Central Valley; the 450-mile-long, 40- to 60-mile-wide flat-bottomed valley that dominates mid-California.

To get to your kitchen, those almonds have been involved in the biggest single pollination event on Earth.

Californian almond pollination requires billions of honeybees travelling thousands of miles in a nationally coordinated migration operating on a scale that is almost unimaginable to most beekeepers in the UK.

This week, BBC Radio 4 is On the Trail of the American Honeybee - the story of migratory beekeeping, and a story that touches on some of the most controversial - and disturbing - aspects of modern agriculture.

Image caption Some of the biggest players have more than 80,000 hives

With over 1,000 sq mi of the Valley under almonds, the almond bloom of late spring is a spectacular sight. Almond blossom appears before the leaves emerge and the resulting white and pink flowers make great swathes of the Valley look as if heavy snow has fallen.

In fact, if you want a stunning visual demonstration of modern agricultural monoculture, then you could do worse than to take a drive down the Central Valley at this time of the year.

There are stretches of these roads where all you can see, from horizon to horizon and for hours on end, is row upon row of blossoming trees planted with absolute precision to ensure the maximum use of this prime agricultural real estate.

Hive mind

But in amongst the trees are bee hives and, once you get your eye in for their distinctive shape and colour, you start to realise that there are rather a lot of them. In fact, there are over 1.5 million hives in the Valley at this time of year.

With more than 20,000 bees in each hive, this means that there are more than 30 billion honeybees in the Central Valley and all of them have been brought there to work. Beekeepers from across the US have put their hives onto flatbed trucks and hauled their insect livestock down to the almond orchards.

Image caption Almond pollination in California constitutes the largest operation of its kind in the world

Once there, the bees work the blossom throughout the sunny spring days, collecting nectar and pollen to raise their brood - the larvae developing within the hive - and inadvertently transferring pollen between flowers.

This pollination is the job for which their owners are being paid up to $200 a hive. Once pollinated, the trees set fruit and start the process of producing almonds.

Beekeeper Randy Oliver has calculated that each bee is being rented out for a penny (US) a month. Per bee, such an amount isn't so impressive but scaled up to thousands of hives it is easy to see why beekeeping can be an attractive business to get into.

Some of the big players have more than 80,000 hives and to keep all these bees busy they migrate around the US, following the pollination season crop-by-crop.

This style of migratory beekeeping only became possible when the transport infrastructure of the US developed, and large-scale agricultural practices meant that farmers couldn't rely on pollinators based on-site.

The sheer scale of modern monoculture and the use of herbicides and pesticides contribute greatly to decreasing the number of natural pollinators on agricultural land and force farmers to buy in pollination services from beekeepers.

Expensive journey

While most migratory beekeepers today are primarily "pollinators", migratory beekeepers originally moved hives around to make extra honey. California beekeeper John Miller, president of the California State Beekeepers Association, has migratory beekeeping in his blood.

His great-grandfather is credited as being the first migratory beekeeper in the US, pioneering the practice by using the railroad to take his bees from Utah to California to get a second honey crop from California citrus. Since then migratory beekeeping has moved away from the railroad and onto the highways.

Image caption The bees make a 10,000-mile journey at exorbitant cost throughout the year

Beekeeper David Mendes is known as the "marathon man" of US beekeeping. A former president of the American Beekeeping Federation, David moves about 13 to 14,000 hives across the highways of the US from Florida to California for the almonds, moving them back to the Florida Panhandle to make Tupelo honey.

In May, bees are moved up to Maine to pollinate wild blueberries, then a month or so later they are moved to the Cape Cod region of Massachusetts to pollinate the cranberry crop.

When the cranberry bloom ends, the bees are taken back down to Florida until the following January, when their 10,000-mile annual journey begins again. Such a journey isn't cheap.

Just taking his bees to California costs David $500,000 but, as he says, "it's part of the deal". After all, beekeeping on this scale means taking an almost industrial view of managing hives.

Some of the "industrial" practices of large-scale beekeeping can, at first, seem a little shocking. For example, most beekeepers will kill the queen bee annually and replace her with a younger queen, bred on a massive scale by specialist queen breeders.

Also, to increase numbers, hives are split into two, mimicking the natural swarming process, in a procedure that has become known as "nuking the hive". The industrial nature of this is clear when you consider John Miller's "nuking machine", an industrial "line" of his own design that can split 200 hives an hour.

Beekeepers in the US have had a tough ride over the past few years. The mysterious Colony Collapse Disorder, or CCD, where bees abscond from the hive never to return, has been a major issue, as have various diseases, a lack of forage for bees outside of crop blooming periods and the use of pesticides.

Migratory beekeeping itself has been implicated in bee health. Bringing together large numbers of bees from across the country provides a perfect way to transmit diseases and spread them throughout the US such that, as Randy Oliver says, "local problems become national issues".

But, "bee guys", as they call themselves are problem solvers and they have lived through plenty of threats to their bees. As John Miller says, "I was born to keep bees in a box".

On the Trail of the American Honeybee was first broadcast on Radio 4 on Tuesday 26 March. It will be re-broadcast at 21:00 GMT on Thursday 28 March, and will be available on iPlayer.