By Mark Ward
BBC News website
The US is in danger of running out of honey bees to pollinate its almond crop - the country's number one horticultural export.
The varroa mite has devastated bee colonies all over the world
February and March are the crucial months for almond growers, as this is when trees blossom and need pollinating.
At this time of year, owners of commercial hives take their valuable cargos to California, where almost 80% of the world's almonds are grown, to service the blossom.
Annually the crop is worth more than $2.5bn and a lot of jobs depend on a good harvest, explains Dan Cummings, one of the directors of California's Almond Board and head of its bee task force.
Currently about 222,000 hectares are under production to grow almonds. Mr Cummings expects this to grow to 330,000 hectares over the next five years.
But, said Mr Cummings, that growth presented a real problem.
"Roughly two-thirds of the bees in the US need to come to California for almond pollination," said Mr Cummings. "Beekeeping in the US is very much migratory."
The danger is that as the demands of almond growers for healthy hives grow, America will simply not have enough commercial colonies available to travel. Bees travel from as far away as North Carolina to California just so they can be used at the key pollination season.
"Last year we were a little short," said Mr Cummings.
Already, he said, demand for colonies was driving up the price that beekeepers charged for renting out their colonies.
In 2004, beekeepers could get, on average, $54 for every hive they sent to almond groves in California. Last year, prices peaked at about $85, and in 2006 there are reports of owners charging more than $150.
Mites stunt bee growth and make them vulnerable to disease
To make matters worse, American bees are suffering a resurgence of debilitating attacks from the varroa mite. These tiny parasites stunt the growth of bees, sap hive resources and slowly kill off the colony.
Unfortunately, said Mr Cummings, bee colonies badly affected by varroa typically collapsed at about the same time as almond trees came into flower.
While chemical treatments can help manage the problem, many pesticides have been so widely used that some mites have developed resistance.
Finding a better way to manage mites had become a pressing problem, said Mr Cummings, because of the tight relationship between the health of beehives and the size of the almond crop.
American beekeepers are now turning to a British development to help them tackle resistant varroa mites.
Developed by Vita Europe, the thymol-based treatment is derived from thyme, and vapours from oil extracted from the herb have proved useful in killing the varroa mites.
Dr Max Watkins, technical director of Vita Europe, said: "Thymol works in a very different way from traditional pesticides which target specific points on the nervous system."
By contrast, he said, thymol has a much wider effect on varroa physiology.
In tests, thymol had been able to knock out more than 90% of the mites in a colony, said Dr Watkins.
"It's a little more difficult in theory for something to become resistant to that," he added.
Dr Watkins explained that thymol tended to knock out both resistant and non-resistant varroa mites, so beekeepers could use it in rotation with established treatments to keep the numbers of parasites under control.
Vita's anti-varroa treatment is now undergoing certification in the US.
Although certification will come too late for the 2006 almond pollinating season, Dr Watkins expects it to be in wide use to prepare bees for the 2007 crop.