A federal review of existing data unveiled an alarming trend for the western bumblebee population: its numbers have dwindled by as much as 93% in the last two decades.
The U.S. Geological Survey finding will help inform a species status assessment by the U.S. Fish and Wildlife Service this fall, which may ultimately add the insect to its endangered species list.
Tabitha Graves, senior author of the study and a research ecologist with the survey, said the trend with the western bumblebee documented between 1998 and 2018 is troubling because of their important role as pollinators.
“They contribute a lot of money in terms of pollination services for our food crops,” she said. “Seventy to 80% of flowering plants and crops are pollinated by animals overall. Pollination contributes to $20 billion in agriculture in the United States.”
Bumblebees also pollinate plants in the wild, such as huckleberries, which are a staple food source for bears.
Researchers say there are multiple factors at play that are contributing to the demise of the bumblebee, including pesticides, habitat fragmentation, a warming climate and pathogens.
“People started to notice these declines in the 1990s. This bumblebee that was once very widespread and common is something that people started to see less frequently,” said Diana Cox-Foster, research leader and location coordinator at the U.S. Department of Agriculture’s Pollinating Insects Research Unit at Utah State University.
“There are localized populations where it is still happy and healthy, but there have been declines in large parts of its previous distributions. ... Asking why these declines are happening is very important.”
There are concerns that other species of bumblebees used in commercial pollination are spreading pathogens to the western bumblebee, Cox-Foster added.
“The role of pests and pathogens is of particular concern,” she said. “There is also climate change and how that has affected the distribution of the bee. Agri-chemicals are also part of the stress issue.”
Graves said the research doesn’t point to one conclusive cause for the decline, which will be the focus of another research effort to better quantify particular threats.
“We have a sample design, and have identified where we have gaps in knowledge,” Graves said. “There are a lot of places in western North America where we have not done sampling for bumblebees for a long time. We need to support this kind of monitoring and research.”
To that end, residents can get in on the action by downloading an app at bumblebeewatch.org and documenting what bumblebees they may come across. There have been an estimated 14,000 submissions from all 49 states where bumblebees occur.
Cox-Foster also added that people can plant bee-friendly vegetation to encourage their presence around homes.
“Planting for bumblebees, or all bees, is really important,” she said. “One of the major issues facing pollinators is lack of floral resources.”