Might be a Copestylum mexicanum (Mexican cactus fly). If so, it is fittingly on a cultivated California wild rose flower.
distortions.net
I'm Ryan Mikulovsky. Tech geek. GIS pro. Avgeek. California Native plant gardener. All photos on this blog, unless stated otherwise, are mine. Cameras: Fujifilm X-T5 and X100V. Views are my own.