This, and "another (mostly) orange wasp" (see below), were seen fleetingly and intermittently while exploring a lovely semi-vegetated desert dune area on a warm, calm, spring day.

I got only poor quality photos of this one, which I saw repeatedly visiting flowers of Palafoxia arida, as in the image above. I also got just a few semi-decent shots of the other "(mostly) orange wasp" as it visited flowers of Desert Gold (Geraea canescens):

 

Fortunately one of my field companions, Alice Abela, had...unbeknownst to me at the time...gotten some very nice photos of the same species (also on Palafoxia, though a different individual):

   

From my photo (showing a diagnostic long, tapering, mandible...and an even longer "tongue"!), and Alice's much better resolved photos showing other characters (full-size versions here and here), I was able to key this to family Vespidae, subfamily Eumeninae in Goulet & Huber(1)(1993). (The parategula visible in Alice's photo clinched family Eumeninae for me.)

Then using Carpenter(2)(2004) narrowed the genus down to either Leptochiloides or Pterocheilus, from the strongly fringed hairs of the palpus (discernible in Alice's 1st photo...after prompting from the key). Carpenter's key indicates Leptochiloides has the 1st flagellomere less than half the length of the scape, whereas in Pterocheilus the 1st flagellomere is subequal to the scape...that pointed to Pterocheilus here.

Using the key to Pterocheilus in Bohart(1940a), I was able to get the tentative ID of a female P. diversicolor here. Seaching out the description of P. diversicolor in Bohart(1940b), and the original 1911 description by Rohwer here...the putative ID of P. diversicolor began to look more promising.

When I ran my photos of the "other (mostly) orange wasp" through the keys...lo and behold...it went to the male of P. diversicolor! :-) The match up of the descriptions of the male and female with my (rather poor quality) photos and Alice's (excellent quality) photos made me quite confident that we'd succeeded in getting images of both female & male for an interesting new sexually dimorphic species of eumenid for BugGuide :-)

The Essig Museum has a specimen record for P. diversicolor...I'll try to look at it next time I visit UC Berkeley to help confirm (or correct) the ID here.