I don't know insect species, but the location of observation and the ecology (preference for Asclepias sp.) seems to fit. Observed here along Lockwood - San Ardo Road on the west side of Williams Hill. Pollinating Asclepias eriocarpa. About 50 individuals total observed on several Asclepias individuals. Numerous Pepsini sp (tarantula hawk) individuals observed pollinating the Asclepias also.