Worldwide decline in honeybees puzzles and worries scientists

May 2007. Scientists are working hard to understand the sources of a staggering decline in honeybees in countries in Europe and Asia as well as in as many as 27 U.S. states this winter, according to Cornell associate professor of entomology Nicholas Calderone.

In the United States, half a million to a million colonies out of a total 2.4 million colonies have died this winter. Both tracheal mites (Acarapis woodi) and varroa mites (Varroa destructor) have threatened the bee industry since the 1980s, causing similar catastrophic die-offs to bee populations in the winters of 1995-96 and 2000-01. The mites feed on U.S. honeybees and act as a carrier for a number of bee viruses, though varroa mites are especially deadly. While many bees this year exhibit symptoms of mite damage, about 25 percent of the deaths this year cannot be attributed to mites or any other known honeybee pest, Calderone said.

Threat to world agriculture
Finding the cause of the problem is vital for U.S. agriculture. Many fruit, vegetable and seed crops, worth between $8 billion and $12 billion each year, rely on honeybees for pollination.

‘Just like in the immediate aftermath of a natural disaster,’ Calderone said, ‘there are a lot of conflicting and inaccurate reports’ circulating in the media. ‘What we do know is that there are an awful lot of dead bees. We are looking for patterns.’

Genetically modified foods, mites, pathogens, pesticides and electromagnetic radiation from cell phones have all been proposed as possible causes of the bees' demise. But, Calderone said, the actual causes are unknown at this time.

Parasites and insecticides
A number of studies suggest that a protozoan, a single-celled parasite called Nosema ceranae, may be playing a role. The protozoan infects the midgut of honeybees. Some beekeepers have noted that treating bee boxes with gamma rays used for food irradiation has allowed healthy hives to return to the boxes, leading to speculation that a pathogen like Nosema could be involved in the some bee deaths this year, Calderone said.

Another possible culprit is a class of insecticides known as neonicotinoids, which has been widely detected on pollen at low concentrations in other countries experiencing die-offs. At certain levels these insecticides may impair the bees' abilities to learn, leading some scientists to believe exposed bees may leave the hive and get lost. ‘The studies don't seem to indicate that the doses they [the bees] are encountering are having any detectable effect on foraging behavior,’ though more research is needed, said Calderone.

As yet, there is little evidence that insecticides or electromagnetic radiation have led to bee deaths, Calderone added. But, he said, even if the cause of the new deaths were ‘cured tomorrow, we would still be operating in crisis mode due to mite damage.’