I assume that these bees are in the Diadasia australis species group, since its from the same spot where I photographed them last year, & the females look very similar. Interesting is the greater hairyness, certainly on the abdomen, of the males.
The bees are just beginning to emerge, & mating is in full swing. Both the Opuntia and Cylindropuntia are beginning to flower.
I've taken specimens of a mating pair & will try to get them identified by Doug Yanega.