The western bumblebee hasn’t had much good luck in recent years.
Once one of the most common bee species in the northwestern United States and Canada, the large and charismatic insect has disappeared from much of its original range. Over the past 20 years, the bumblebees have vanished from vast areas of California, Oregon, and other states, mostly likely owing to habitat loss and the spread of an invasive parasite called Nosema bombi.
Conservationists have been warning about the decline of the western bumblebee for several years, but now a team of researchers has some unexpected good news. They have found small populations of the species at sites in Washington and Idaho where the bees had not previously been documented. Their research was published this week in the Journal of Insect Science.
Even at reduced population levels, the western bumblebee plays a major role in the pollination of native plants and agricultural crops, including apples, cherries, pumpkins, and raspberries. Unlike other bee species, it is a generalist pollinator, allowing its presence to benefit multiple plant species at once.
Lead author Paul Rhoades, a graduate research fellow at the University of Idaho, said some of the locations in which the bee was found were a bit of a surprise. For example, he said, the Palouse Prairie of northern Idaho and eastern Washington “is almost entirely agriculture, with just really small habitat fragments that are sometimes really degraded.” The paper describes the remaining habitat as “a mosaic of fragments mostly on land too steep or rocky to farm.”
The findings help to fill what appears to be a major knowledge gap. “One thing that surprises me continuously as I do this research is how little information we have about biology in general and bees in particular,” Rhoades said. “The bees in the inland Northwest haven’t been surveyed in any careful way.”
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That limits scientists’ understanding of how the western bumblebee and other species are faring, because they don’t always know where the pollinators are and where they used to be. “There aren’t clear, explicit records of past abundance for this species,” Rhoades said. “It was described as being extremely common in the Northwest and parts of the Northern Rockies. Now it’s definitely not.”
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The team’s findings don’t dispute that. The researchers found several small populations that appeared to be mostly distinct and separate from one another. “Even if the western bumblebee still persists in some areas, it doesn’t necessarily mean that the populations are sustainable or the trajectory is stable,” Rhoades said.
The geographic distance between each population poses a new threat: genetic isolation. This is particularly problematic for bumblebees because inbreeding increases the number of sterile males in a population, said Sarina Jepsen, endangered species program director for the Xerces Society for Invertebrate Conservation, who was not associated with the research. Sterile males not only can’t contribute to the next generation of the species but also don’t serve as workers, so they are useless to a nest.
The question remains: Why have western bumblebees persisted or possibly even increased in the newly surveyed areas? “It does seem like whatever pressures are impacting the species so seriously in western Oregon possibly aren’t as present in northern Idaho,” Rhoades said.
Although the paper does not go into reasons why the pressures may be less at these sites, one possible explanation is that the bees or the pathogen are changing. One of the paper’s coauthors, James Strange of the U.S. Department of Agriculture’s Agricultural Research Service, speculates that either the bumblebees have developed a resistance to the Nosema bombi parasite or the parasite has become less virulent. “Evolution of resistance is, in my opinion, the most likely explanation,” he said.
Rhoades said it will be important to keep looking for these rare bumblebees to see how they fare over time. “It’s not clear if we caught them on a good year or a bad year,” he said. “We need to see if these populations are sustainable.”