It’s been 44 years since scientists last spotted the elusive San Francisco leaf-cutter bee, a rare species found only in the temperate marine climate of the Bay Area.
This month, a team of researchers with One Tam confirmed a sighting of the insect, which is listed among the region’s endangered invertebrates, in the Lagunitas Creek watershed.
The rediscovery comes at a time when the global decline in the bee population has sparked concern, leading to the passage of over 30 state laws aiming to protect pollinators. California has restricted the use of bee-killing pesticides and enshrined their protection under the California Endangered Species Act. Some states have introduced special tax breaks for beekeepers, and others have allocated millions to researching the vanishing colonies.
Recognizing the importance of pollinators in assessing ecosystem resilience, One Tam began efforts to gather data on bee species critical to the unique flora of Mount Tamalpais. One Tam is a collaborative partnership between the mountain’s key land management agencies—the National Park Service, California State Parks, Marin Water, Marin County Parks and the Golden Gate National Parks Conservancy.
A 2017 survey of the region’s wild bees recorded 32 genera and 122 species across 20 sites managed by Marin Water and California State Parks. The survey eventually led to the launch of the San Francisco Bay Area Leafcutter Bee Search Party, an initiative that culminated in the recent rediscovery of the species.
Remarkably, the search party’s efforts paid off on their very first day in the field. “That immediate gratification rarely happens in conservation,” said Carl Sanders, the natural resources program manager for Marin Water.
During their first site visit, Sara Leon Guerrero, program manager of the Bee Lab, a One Tam community science initiative, spotted a bee glide by. She swiftly captured it in her sweep net and placed it in a sterile vial, which was then cooled to temporarily sedate the bee for closer examination.
Using specialized photography protocol, the team captured high-resolution images of the bee during this moment of stillness before it awoke and flew away.
When Ms. Leon Guerrero reviewed the photos, she was astonished by what she saw.
“It feels disingenuous to claim we found it on the first try,” Ms. Leon Guerrero told the Light. “I’m really standing on the shoulders of those who had done this work in the ’60s and ’70s.”
Ms. Leon Guerrero was referring to the pioneering work of her mentor, the late Robbin Thorp, an internationally recognized entomologist who first named and described the San Francisco leaf-cutter bee, Trachusa gummifera, in 1963.
“While we don’t know a lot, we do have a taxonomic description that tells us what it is and how to distinguish it from other species, a historical record of where it may have existed, and information about where it might be foraging,” she explained.
The Bee Lab’s survey sites include Millerton Point in Tomales Bay State Park, Lagunitas Meadow and Sky Oaks Meadow in Fairfax, Dipsea Moors in Stinson Beach, Muir Beach and Roy’s Redwoods.
To safeguard the San Francisco leaf-cutter bee and its habitat, the Bee Lab has kept the exact location of the discovery confidential, although it confirmed that it was within the Lagunitas Creek watershed, an area already under close observation by environmentalists due to its importance as a breeding ground for endangered coho salmon.
Once the species was confirmed, the team stopped netting in the area. Since the California Department of Fish and Wildlife has designated the species as one of “greatest conservation need,” the lab is required to obtain a permit from the state agency for any further research and monitoring.
Though darker in color than honeybees, leaf-cutter bees are similar in size, with females slightly larger than males. They can be identified by their triangular abdomens that taper to a point, while males have smaller, blunt-tipped abdomens and notably long antennae.
Leaf-cutter bees get their name from the precise, semicircular holes they cut along the edges of leaves from plants and trees like chaparral peas, roses, ash, trident maples, red twig dogwoods and lilac shrubs. Although they have stingers, they are non-aggressive, and their stings are far less painful than those of honeybees or wasps. One of the most distinctive features of leaf-cutter bees is their relatively large head, which houses powerful muscles designed for chewing through leaves. Females even have teeth.
The leaf cutouts are used for both insulation and interior decorating—to line cylindrical nests that the mother bee constructs within pre-existing cavities, like hollow stems or rotting wood, or in the ground. Inside the nest, she lays up to 12 eggs, each carefully sealed with tree resin in its own leaf-lined cell. These cells are stocked with a mixture of pollen and nectar to nourish the larvae when they hatch.
Unlike their honey-producing counterparts, leaf-cutter bees are solitary creatures that do not form colonies—a trait shared by most bees. Only 10 percent of over 20,000 described species worldwide are “social” bees, forming hives.
The bee lab is encouraged by what the finding indicates for the greater ecology of the Mount Tam region and other fauna endemic to the Bay Area.
“There’s so much potential for other rare and sensitive species out there,” Ms. Leon Guerrero said. “We just kind of assume we know enough, but sometimes it takes getting out there and observing firsthand to see what’s truly happening.”
In addition to the San Francisco Bay Area Leafcutter Bee Search Party, the Bee Lab is participating in two other community science projects: the California Bumble Bee Atlas, a collaborative effort to track and conserve California’s native bumblebee species, and ground nesting bee surveys, an initiative to fill critical knowledge gaps in the behaviors of ground-nesting bees.
Overall, 276 bee species are known to be present in Marin, Ms. Leon Guerrero said. Common types in the coastal region include the solitary and ground-nesting long-horned bee, Eucera Synhalonia, and the yellow-faced bumblebee, Bombus vosnesenski, native to the West Coast and locally the most abundant species of bee.
Ms. Leon Guerrero believes that the leaf-cutter bee was never truly absent; it has always been there, simply unnoticed.
“This discovery is so hopeful,” she said. “These bees have been completing their lifecycle year after year—living its life, procreating, foraging, pollinating. We just hadn’t taken the time to look for them.”