You probably wouldn’t recognise a bee farm in transit if you saw one on a US highway. All you would likely see is a number of boxes stacked on a tractor trailer, wrapped in netting. There wouldn’t be obvious clues as to the contents: 400+ honeybee hives, each colony home to a queen and 15,000–30,000 worker bees.

If it’s springtime, these hives could be travelling thousands of miles to the almond orchards of California’s Central Valley, joining 1.5 million other hives (and 1,600 beekeepers) in what has been dubbed “the biggest single pollination event on Earth”. About half of all honeybees in the US will be present. Farmers pay commercial beekeepers by the hive, and after five or six weeks of diligent work, the surviving bees move onto the next flowering plant species.

Such “outapiaries” developed in the US in the early 20th Century. The practice grew with the sweeping industrialisation of agriculture, and was aided by the development of road networks as well as advances in transport. (Though there were some early experiments with transporting bees by boat and bicycle, they didn’t take off.)

The system has become entrenched in the US due to factors including a catastrophic decline in wild pollinators and an increase in monoculture agriculture, especially enormous tracts of corn and soy, that depends on pesticides. Less healthy soils and less diverse food sources make for weaker pollinators, and the die-offs are accelerated when huge numbers of diseased bees are brought together to pollinate the same crop. There’s a vicious cycle involving Varroa mites, poor nutrition and insecticides, all of which weaken bees’ ability to respond to the other threats.

Intensive travel isn’t helping; these aren’t the light travellers tended by Šercelj in Slovenia. Like many humans, honeybees travelling long distances tend to experience stress and have poor diets. Bees weakened from travel may need to be fed sucrose syrup before they leave the hive and get to work. Samantha Alger, a biologist at the University of Vermont and the director of the Vermont Bee Lab, describes the staggering sight of bees unloaded from truck beds into Californian holding yards with “a huge tanker filled with syrup and then a hose coming out. They’re just going from hive to hive, giving them some supplemental nectar before they can get them out to the almonds. It doesn’t have all the nutrition and amino acids and vitamins that real flower nectar has. It’s like corn syrup or sugar water.” These hard-working bees are also more exposed to pesticides, especially the neonicotinoids now banned in the EU. All of these factors may impair bees’ disease resistance.