Thanks again for your compliments and for positively identifying P. vespoides not only to species but to sex! I though the top right abdominal tip looked different. I'm sorry about the incorrect plant name. Your are right--I confused it with Apodemia virgulti!
In the Krombein et al 1979 catalog, neither Pseudomasaris vespoides nor any other Pseudomasaris is listed on Penstemon virgatus, but it does grow right along side P. secundiflorus and blooms maybe a week or two later.
I envy you all your species of these wonderful pollen wasps!

P. virgatus was one of the spp. I had thought of. Your posts caught me about a week after starting to compile a list of Penstemon spp. recorded for P. vespoides. I don't have the time to travel as much as I used to a few years ago, otherwise I would have liked to check on a number of places, not just for this sp. but other, Phacelia using Pseudomasaris as well.
I wouldn't be too much concerned with whether these wasps are seen only on the reported plant species. We're not always in the woods when a tree falls.
If you travel to higher altitudes early this month, possibly into August, you might be able to find Pseudomasaris marginalis in Phacelia, i.e. P. sericea.
P. zonalis should be seen this month (saw some yesterday in the San Gabriels), also into August depending on altitude and flowering of Phacelia food source. This species is generally found above 1800m (~6000ft), with records for Boulder County.

When you mentioned P. marginalis and P. zonitis in Boulder Co, I thought, I've never seen a Phacelia around here. So I looked it up in my CO books. Sure enough, 5 species are recorded for Boulder Co. Four are subalpine to montane (and we live at about 6500' in the foothills). But one of them I did recognize--Scorpion Flower. I thought it was genus Hydrophyllum along with H. fendleri which we do have here. But no, it is Phacelia heterophylla. So, we could have P. marginalis and P. zonitis right here! Thanks for pointing this out to me! I don't see it very often (and I can't believe I have no photograph of it). Flowers are white instead of the usual Phacelia purples. Plants --and insects--are a lot easier to find if I'm actively looking for them. And so I shall.

would be great to see some photos from your area. There are records for both Pseudomasaris zonalis and P. marginalis for Boulder County. I've e-mailed some site suggestions.

Such a beautiful subject! My especial enthusiasm is insect/plant relationships in all forms, feeding, nesting, sleeping. . . .

We are enjoying them so much. We are getting overcast here, so I went out a few minutes ago to see if they were settling in for the night. No, but a solitary female was visiting flowers collecting pollen.

The one that has pulled its head of the flower is a female. The male antennae are much longer and even more clubbed. Great pictures!

How interesting that both sexes are sleeping in these Penstemon flowers. Tonight we just came in from doing a census of sleeping wasps. Only one flower was occupied! Either adults have a very short life or they have moved uphill somewhere where flowers are fresher or more abundant. The few flowers we have came in on their own.