I came across a large aggregation of Bindweed Turret bees building their nests on May 28, 2020. I was mountain biking on Albertson Fire Road and was riding back down when I heard the sound of a swarm. The aggregration is located on a section of Albertson Fire Road that runs through National Park Service land. The spot is beyond the intersection with China Flat, but before the climb to the powerline tower.
What seemed like a thousand or more bees were flying, digging and tending to nests in a wide open, level space on either side of the fire road. Some were digging on the road itself, but most were outside of the human traffic zone.
The Bindweed Turret bee, Diadasia bituburculata, is a solitary bee that nests in the ground. Each female digs her own nest and provides sustenance in the form of pollen and nectar packets that she leaves in the nest for the larvae to eat when they hatch.
I encountered a smaller but very active aggregation earlier in the month on the Hidden Meadow trail in Conejo Open Space. Since late May, I have also observed this bee species in the Paramount Ranch area and heard of their presence in several other locations in the Santa Monica Mountains. I was saddened to see that the Paramount Ranch site appeared to be trampled by the time I visited there to see it. A small group of bees was still active and nesting together, but the main aggregation looked like a stampede had come through it.
If you encounter a nesting site, slow down a little and be careful where you step or roll to avoid destroying the nests.
With so many bees flying about, it can be hard to focus. I noticed one bee trying repeatedly to break ground and focused on her and a neighbor, whose nest seemed to be nearly finished. I was careful where I stepped and stood near the middle of the swarm. These bees are not aggressive or territorial. They will sting only if handled or threatened. I can testify that standing near their nests to observe is not perceived by them as an existential threat. In fact, they did not appear to take much notice of my presence at all, except to modify flight trajectories so as not to collide with a large obstacle.