A rare, elusive bee endemic to the Bay Area has appeared on Mount Tamalpais, the first documented sighting in nearly 45 years.
The San Francisco leafcutter bee, also known as Trachusa gummifera, was discovered as part of a collaborative scientific effort by One Tam to locate the fast-flying pollinator. The bee has been documented fewer than 100 times and has not been seen since 1980. The discovery could help uncover previously unknown details about its ecosystem.
“When you’re doing this work and you haven’t seen something for so long, you expect it to be difficult to find it again,” said Sara Leon Guerrero of the Golden Gate National Parks Conservancy. “Often the reason we haven’t discovered or found something is because we haven’t looked for it.”
The One Tam partnership – a program consisting of the National Park Service, California State Parks, Marin Municipal Water District, Marin County Parks and the Parks Conservancy – manages more than 60,000 acres of forestland in Marin County.
Guerrero directs the Tamalpais Bee Lab, which monitors and studies bees and other pollinators on the mountain as part of the One Tam partnership.
From 2017 to 2022, the Bee Lab conducted a bee survey in the Mount Tamalpais area in collaboration with a lab at San Francisco State University. The project also included a list of historical species and bee groups to look for, either because of their suspected distribution, rarity, or because they were documented in the area at some point. The list included the San Francisco leafcutter bee.
“When the experts tell you to look at something, you think, OK, maybe we should look at it,” Guerrero said.
The bee is considered the species with the greatest need for protection in California’s wildlife plan. According to Guerrero, there are more than 1,600 species of bees in California and an estimated 300 to 400 in Marin County.
Carl Sanders, natural resources program manager for the Marin Municipal Water District, said the discovery is intriguing because it could provide information on how best to manage the land on Mount Tamalpais.
“It’s a very helpful sign to find a species, any species – in this case a very small, hard-to-find, ground-nesting bee that hasn’t been seen or heard from in decades. It’s a hopeful sign that we’re properly managing the lands that we currently manage through the various agencies,” Sanders said.
There is little information about the bee other than a general description, the types of plants it frequently visits, and its general range. Still, Guerrero read everything she could. The solitary bees collect resin from conifers in the area and use leaves to build brood cells. The team would look for the bees near fresh sap and in small oval or round cutouts in leaves.
She presented the idea to One Tam to use some time and community volunteers to search for the bee, and the organization gave her the green light.
“They were very excited,” Guerrero said. “You can’t often work on just one species, but this species tells us something about the larger ecosystem in which it exists.”
The search party was assembled in the spring. The team conducted some preliminary site visits in the south to central part of the Lagunitas Creek watershed. On the first day, Guerrero was stationed near a chaparral pea plant – a purple-flowered plant that the bee had been documented visiting in the past – and was thinking about a search plan when something caught her eye.
“I saw a bee flying very fast and thought, that can’t be that bee,” Guerrero said.
Just in case, she grabbed her net and caught the bee after a few tries. She placed the bee in a sterile vial and put it in a cooler on ice – a harmless technique that puts the bee into a calming, sleep-like state.
“When I took pictures of this bee, I just thought, oh my god, oh my god,” Guerrero said.
The next day, a taxonomist from the U.S. Department of Agriculture confirmed that the bee was a male Trachusa gummifera.
“I think it’s a very hopeful story,” Guerrero said. “Sometimes we’re just not at the right time or the right place to document things that we think are in trouble or missing.”
Max Korton, director of the Marin County Parks Department, said the discovery will contribute to a better understanding of the status of key wildlife species in Marin, how ecosystems are adapting to climate change and how to best manage these areas.
“This discovery is so cool,” Korton said. “Finding a rare insect, flower or amphibian is also inspiring for us and the community. The more we learn about the complexity and richness of our natural systems, the more we are inspired to care for them.”
Although the peak season for bee work is over, Guerrero said the next steps are to work with the state Fish and Wildlife Department to get the necessary permits to continue studying the bee. Guerrero hopes to fill in some gaps in its life history, habitat preferences and phenology.
Sanders said the county is working to obtain funding from the Department of Fish and Wildlife to increase monitoring efforts for the species. He said the bee has now been spotted at several locations, and any information gained from studying the bee will inform land management actions.
“In my eyes, it’s a powerful symbol and yet another piece of missing information about our local natural network,” Sanders said.
Guerrero said that without the volunteers in the community science program, exciting discoveries like this would not be possible, especially with so many bees and not enough staff and scientists.
“Without the idea that we can just go out with volunteers and look because no one is looking, this discovery would not have happened,” Guerrero said. “In the future, the things we’re going to learn will be achieved through community science.”
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