A large survey of bumblebees in the United States shows that several species have declined substantially over the past 2 to 3 decades, verifying the suspicions of scientists who have seen local populations disappear. "We've lost a lot of bees. There are whole regions where we can't find them any more," says entomologist Sydney Cameron of the University of Illinois, Urbana-Champaign. The new research also suggests that a parasite might be driving the die-off, as declining species have higher rates of infection than do stable species.
Bumblebees pollinate many plants and crops such as tomatoes, pumpkins, and blueberries. They're particularly effective because they "buzz-pollinate," vibrating their wings fast to release lots of pollen. And their large size means that they can fly in weather that keeps honeybees, which pollinate some of the same plants, in their hives.
In the 1980s, researchers in the United Kingdom noticed that it was becoming harder to find some species of bumblebees. They realized that changes in agriculture—such as destruction of hedgerows to make larger fields—were eliminating bumblebee habitat. Declines in North American bumblebees began to get attention in the mid-1990s. On the West Coast, researchers had trouble finding a species that had been abundant during the previous century but is now feared extinct. Three species that live in the rest of the country also seemed to be in decline, but there wasn't a clear picture of what was happening nationwide.
A team led by Cameron decided to do a large survey. They picked eight of the nearly 50 bumblebee species in the United States. All eight had been historically common, but four had appeared to decline in recent decades. First, the researchers figured out the geographic ranges and relative abundance of the species over the past century by creating a database of collection records from 47 museums and other institutions, which covered nearly 78,000 bees in all. Then the researchers hit the road, caught 16,788 bumblebees in 40 states, and brought them back to the lab.
The four species suspected to be declining had in fact become rarer. Whereas 20% of the bumblebees in the museum collections were Bombus occidentalis, for example, it accounted for only 1% of the bees in the field survey. The other three species in trouble were just as scare, with the relative abundance of B. affinis plummeting by 96%. The ranges of the four bee species had shrunk by between 23% and 87%, the researchers reported online yesterday in the Proceedings of the National Academy of Sciences. In contrast, the four species thought to be stable fluctuated but showed no consistent patterns of decline.
The team also checked the bees for infection by Nosema bombi, a single-celled parasite that afflicts bees in Europe. It has also been found in the United States, where its effects on bumblebees are poorly known. The rate of infection was much higher in populations of dwindling species, averaging 37% for B. occidentalis, compared with 1% or so in the stable species. Three of the four declining species are close relatives, which suggests to Cameron that they share a genetic susceptibility to the cause of the disappearance—and that a massive die-off of all bumblebee species in North America is unlikely.
Cameron and her colleagues are now investigating whether Nosema is to blame for the bumblebee decline. The possibility that Nosema was introduced to the United States via bumblebees imported from European breeders suggests that it will be important to keep breeding facilities disease-free, Cameron says: "It's been a wake-up call for the industry."
James Cane, an entomologist with the U.S. Department of Agriculture in Logan, Utah, who was not involved in the research, calls the new study rigorous and objective. "I'm impressed by it," he says. "This confirms the reality of the decline." The next step, he adds, is to nail down the cause and whether anything can be done to halt the declines.