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A bumblebee leaves a flower after gathering pollen. ((Tony Dejak/Associated Press))

Four of the 50 species of bumblebees found in the U.S. are "significantly in trouble," University of Illinois entomology professor Sydney Cameron said Tuesday.

Cameron, lead author of the first in-depth national study of wild bees in the U.S., said the analysis only covered eight species, so "this could be the tip of the iceberg."

The finding echoes similar ones in other reports that documented huge declines in honeybee populations.

Using historical records from the late 1800s, Cameron and her colleagues found the relative abundance — the proportion of the studied species out of all bumblebees — of the four species decreased by up to 96 per cent since that period, and their geographic range contracted by between 23 and 87 per cent.

The study said that the declining populations had significantly higher infection levels of the pathogen Nosema bombi than unaffected populations. Separate research has shown that the parasite can render bumblebees helpless and unable to function, and eventually kill them.

The infected populations also had lower genetic diversity.

"Higher pathogen prevalence and reduced genetic diversity are, thus, realistic predictors of these alarming patterns of decline in North America, although cause and effect remain uncertain," the abstract of the study said. 

Bumblebees vs. honeybees

Bumblebees are larger, often with yellow and black body bands, whereas honeybees tend to be orange and black. Honeybees are more aggressive, whereas bumblebees give the impression of being lazy, bumbling and dozy. Both bees make honey, but the bumblebee's output is tiny compared with the huge amounts honeybees produce. Honeybees live in large colonies while bumblebees live in small groups or alone. Honeybees survive over the winter, but bumblebees, except for the queen, which hibernates, die off.

Research last year on colony-collapse disorder,  which led to the mass deaths of honeybee colonies, suggests the collapse might have been caused by a combination of viruses and the fungi Nosema apis and Nosema ceranae.

Cameron said there is no proven cause of the bumblebee declines, but climate change has contributed to European losses and habitat loss may affect certain specialized species.

The Committee on the Status of Endangered Wildlife in Canada last May said the rusty-patched bumblebee should be listed as endangered. Its rapid decline is a mystery, but pesticides, disease and a loss of habitat could be contributing factors.

The three-year U.S. study compared more than 73,000 museum records with recent U.S. national surveys of more than 16,000 specimens from about 400 sites.

The report was published in the Proceedings of the National Academy of Sciences.