Research
Four bumblebee species once common across most of North America have suffered precipitous – and so far mysterious – declines, a new study shows. Within the past 20 years abundances of the bee species Bombus occidentalis, Bombus affinis, Bombus pensylvanicus, and Bombus terricola have plummeted by up to 96 percent. The finding is based on a new analysis of more than 73,000 museum collections of bumblebees, which showed where bees had been found over the last century, as well as collections of wild bees across the United States. The study examined eight of the fifty known bumblebee species in North America in thoroughly fashion. “We found that yes, indeed, these four species are seriously declining, but amazingly there are some other species doing very well,” said study co-author Sydney Cameron of the University of Illinois‘ department of entomology. The discovery makes it harder to pinpoint pesticides or climate change as singular causes for the bug die-off, because those factors wouldn’t explain why other bumblebee species in the same areas have actually survived. One possibility is that the four species in crisis may all be infected with the invasive Nosema bombi fungus, which was found in greater quantities on the dying bumblebees than on relatively healthy species that were also studied by Cameron’s team. And bees reared in Europe, where the fungus is much more virulent, were imported to California in the early 1990s – right before the American bees began to die off. But the link isn’t certain yet. “We’re at this frustrating stage where there is a lot of circumstantial evidence to say that these species are declining because of this pathogen,” Cameron said. “But we don’t have direct evidence. We have no clear-cut notion of cause and effect.”
Efficient farmworkers
Like honeybees, bumblebees rely on pollen as a source of protein. In addition to their wild work, bumblebees are widely used as pollinators for multibillion-dollar commercial crops, such as blueberries and tomatoes. Theoretically, a bee imported from Europe carrying Nosema bombi could have escaped into the wild through a greenhouse vent and infected its wild brethren. In fact, the bee die-off seen in the new study looks very much like a pattern you’d find with a newly introduced disease, said Robbin Thorp, professor emeritus at the University of California. Thorp has been studying pollinators for decades and first noticed the decline of a fifth bumblebee species, Bombus franklini, in the late 1990s. Although Thorp was not involved in the present study, he and Cameron will soon be working together to study more museum specimens “to see if we can pick up the fingerprint of the Nosema prior to and after this period of potential introduction.” The researchers also studied the DNA of the bee species and found that the declining species had less genetic diversity than the species doing well – a trait often seen in species whose populations are isolated, or fragmented, resulting in increased inbreeding. “They seem to be doing a lot of moving around,” Cameron said of the bees. “They are not fragmented. But at the same time they have lower genetic diversity, and we don’t know why.” Figuring out the exact cause of the bumblebee declines may be crucial for farmers, as bumblebees are even more efficient pollinators than honeybees. That’s because the larger bees’ high-frequency buzzing is just right for opening a flower’s pollen-holding pores.
Source: National Geographic News
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