American bees are dropping like flies, victims of a mystery ailment that threatens to put a massive dent in the nation’s horticultural production.

US commercial beekeepers from at least 22 states are reporting that bees have either mysteriously died or disappeared in huge numbers, with some beekeepers reporting a loss of between 30 and 70 percent of overall beehives. Experts are calling this mystery syndrome ‘colony collapse disorder’(CCD), in which bees leave their hives in droves, never to return.

Since October 2006, 35 percent of the US population of the Western honey bee (apis mellifera) - billions of individual bees - simply flew from their hives and disappeared.

Researchers are scrambling to make sense of the development, which is said to have a sizeable effect on the one third of US growers who rely on bees for pollination.

Yet this is not the first time hives have mysteriously dwindled. As far back as 1896, hive declines going under names like 'fall dwindle' disease’, 'May dwindle', 'spring dwindle', 'disappearing disease', and 'autumn collapse', have all caused alarm, then dissipated. But ‘colony collapse disorder’ is by far the largest hurdle ever faced by US beekeepers.

In hives hit by CCD, adult workers simply fly away and disappear, leaving a small cluster of workers and the hive's young to fend for themselves. Adding to the mystery, nearby predators such as the wax moth refrain from moving-in to pilfer honey and other hive contents.

Some suspect that CCD may be due to the general immune-system stresses of the commutes that US bees are taken on by truck, in order to catch various pollination seasons.

Others believe that new neonicotinoids, neuro-active pesticides, may be causing bees to lose their sense of direction, and simply be unable to return to the hive. Yet another hypothesis is that sick adult bees may be self-sacrificing: flying away to die in order to protect the hive from further infection.

Still others suggest that bees may be contaminated with a toxin-producing fungus called Aspergillus, which affects the gut of the bee. There is a strong minority who blame intensive modern beekeeping methods: in nature, a bee colony lives only about five years. The honeycomb acts as a sort of ‘liver’ for the colony, filtering out toxins, then the hive flies away and the comb is replaced. Now, they suggest, the health of colonies is flagging as bees are forced to live on combs that are decades old.

Whatever the cause, with $14 billion worth of agriculture at stake, the mysteries of the hive need to be unravelled, and fast.