A number of these large pompilids were found visiting the flowers of Asclepias erosa here on a warm sunny morning...post of others appear below. The habitat was open high desert.

   
Regarding the generic ID, the apically rounded submarginal cell, with its tip separated from wing margin (i.e. with bounding the radial vein bending back and meeting the costa at an acute angle, rather than at a right or obtuse angle), implies genus Pepsis here. The straight, relatively thick antennae indicate a male.

Regarding species ID, using the keys in Hurd(1), Townes(2), and Vardy(3)...this would go without difficulty to P. thisbe if treated as a female. But it's a male, and those keys are all much more difficult for males, as they focus mainly on traits of the subgenital plate (not visible without dissection). However, by carefully cross-referencing between keys, range maps, and descriptions and various characters visible in these photos...one can narrow the choice here down to either P. thisbe or P. pallidolimbata.

Before reading further, note the orange fore- and hind-wings both exhibit a definite darkened band apically, along with a very short and narrow apical hyaline area (see full-size image). This points to P. thisbe here.

To support the tentative ID of P. thisbe, carefully compare the photos here with the wing description for the male below from Hurd(1):

"Wings predominantly xanthochromatic, thickly clothed with minute, yellow, orange, and red tomentose hairs; exterior marginal band of forewing whitish hyaline, usually infuscated with black, narrow, broadest at wing apex, occupying about one-half of the width of area external to closed wing cells, narrowing posteriorly and disappearing near inner wing angle; exterior submarginal dark band narrow, variable in width, usually one-half, or less, of the width of area external to closed cells, extending posteriorly adjacent to closed wing cells from anterior wing edge to or near axillary excision; occasionally the band may be almost absent; humeral band of forewing blackish brown, narrow, usually one-fourth of length of median cell or may be virtually absent; whitish hyaline character of exterior marginal border of hindwing less distinct than that of forewing, narrower; exterior submarginal dark band of hind wing less intense, begins at radial vein, progresses posteriorly and marginally, ultimately broadening and joining humeral band; humeral band of hindwing similar in nature and extent to that of forewing."
One the other hand, in describing P. pallidolimbata, Hurd(1) makes no mention of a darkened wing band:

"Wings predominantly xanthochromatic; exterior marginal wing band of forewing and extreme apex of hindwing narrowly whitish hyaline; humeral band of forewing and hind wing brown or blackish brown, broad, extending fully one-third of length of median cell in forewing and nearly three-fourths of length of median cell in hind wing."
The location here is well within the areas shown on the range maps for P. thisbe and P. pallidolimbata...and not so well (or at all) within the range maps for the other species. Although this male appears to agree with P. thisbe, some of the other nectaring Pepsis I photographed during my visit to this locale might also be contrued as P. pallidolimbata...for instance, the one below:



I wonder if the two species are known to co-mingle as might appear to be the case here?

Comments, corrections, and/or confirmation on the ID here would be welcome and appreciated.