This series illustrates nesting behavior of Podalonia argentifrons. It starts with the profile image above...which is a detail from the next photo in the series showing the wasp carrying the entire caterpillar.

Subsequent images show the wasp: temporarily caching the caterpillar on some low plants; visiting some flowers to nectar; grooming while making final preparations on the nest; dragging the caterpillar into the nest; and then kicking sand with its fossorial legs to fill in the burrow...finally covering it with a rock for camouflage! This is all in good accord with the descriptions in "Observations on the Nesting Behavior of Podalonia argentifrons (1983)", by M. F. O'Brien [available as a PDF here].

Though I didn't capture it in images...the wasps apparently are somehow able to search out the caterpillars by running along and tapping their antennae against the ground. Once found, they dig out the caterpillars from their refuge in the substrate, flip them over, and deliver a series of stings which paralyze the caterpillar. The stings are said to be precisely located...correlated with principal nerve centers of the caterpillar.

The photos were taken at restored dunes in the Presidio (Golden Gate National Recreation Area). The dunes were just a few hundred yards inland from the ocean, but relatively sheltered from harsh winds by intervening topography and woods.

The wasp keyed unambiguously to P. argentifrons and agreed well with its description in Fernald's 1927 paper "The Digger Wasps of North America of the genus Podalonia (Psammophila)" [available online here]. In fact, Fernald mentions (on pg. 29) that his descriptions were based on specimens collected "along the edges of the sand dunes just inside the seashore line about a mile south of the Cliff House, San Francisco". That's only a few miles from where these photos were taken...and very similar habitat.

I was unable to find the later revision of the genus by Murray (1940), and apparently Murray did describe a number of new species in California, beyond those treated in Fernald (1927). (P. mexicana and P. caerulea might be possibilities here.) But, based on Fernald's location comments quoted above, I'm presuming this is P. argentifrons. (If anyone has access to Murray's work, I'd love to know how it pans out there.) At any rate, P. argentrifrons is listed in Menke & Bohart(1), Krombein(2), and the current catalog of Pulawski(3)...so it hasn't been subsumed in synonomy with something else. Fernald seems to indicate (on pg 7, and the first couplet of his key on pg 13) that female Podalonia with "abdomen deep blackish-blue and glistening" can be assigned to species argentifrons...that character alone presumably being sufficient to distinguish it from females of other species of Podalonia.

PS: I tried (perhaps not too successfully) to discipline myself in the number of images I posted in this series, choosing ones I felt showed interesting behavior or morphological aspects. Apologies if it seems excessive. I actually took 124 shots...she was a very cooperative wasp!