If the problem is worse than before, they say, it may be because more bee colonies are being housed and trucked by fewer beekeepers, raising the chances of infestations or infections spreading.
The official word, endorsed by many scientists and people in beekeeping businesses, is that a newly named syndrome, called colony collapse disorder,or CCD, is at work and poses a significant threat to American fruit, nut and vegetable crops.
An action plan released Friday by the Department of Agriculture used the phrase “CCD crisis” to describe the recent die-offs, even as it said it was “uncertain whether CCD is a new phenomenon” and described similar die-offs as long ago as 1898.
No one in the field doubts that commercial beekeepers in more than 20 states have seen large declines in hive populations in the last year — more than 70 percent in some cases — and that agriculture is facing problems pollinating some crops.
It is also clear that bees in the Americas, both wild native species and honeybees, which were imported long ago and are the commercial standard, have been hard hit in recent decades by mites and infectious agents.
What some scientists say is missing from the debate is historical context. “Every time there are these disappearances, the ills of the moment tend to be held accountable,” said May Berenbaum, who heads the entomology department at the University of Illinois Urbana-Champaign and led a National Academy of Sciences review of the status of North American bees and other pollinators that was published last year.
“In the ’60s it was synthetic organic insecticides,” Dr. Berenbaum said. “In the ’70s it was Africanized bee genes. In the 19th century, there is a wonderful report about this resulting from a lack of moral fiber. Weak character was why they weren’t returning to the hives.”
One thing almost everyone seems to agree on is the need for consistent, frequent censuses of the country’s bee populations, but money for monitoring has not been increased, bee experts said.
Eric Mussen, a bee expert at the University of California, Davis, said he did not understand the talk of catastrophe, noting that even after colonies are lost, beekeepers can quickly replace them.
Michael Burgett, a professor emeritus of entomology at Oregon State University, said the big honeybee losses in some regions could simply reflect unremarkable spikes above a common level of mortality of more than 20 percent in recent decades.
“In the late 1970s we had another scare similar to this,” Dr. Burgett said. “They called it ‘disappearing disease’ at the time. But we never found a specific cause for it, we continued to improve our bee management programs and ‘disappearing disease’ disappeared.”