This was one of the acorn galls that was secreting honeydew and attracting an amazing number and variety of insects. The oak is Quercus emoryi
Does anyone know which species of cynipid induces this gall?
Weld's 1960 Cynipid Galls of the Southwest
lists one gall that might be this one: Callirhytis balanella
Weld. The host oak is Quercus emoryi
. Weld's photos of two galls of this species look exactly right except for one thing: The image that shows an acorn cap shows a very stunted acorn seed inside the cap. On the tree by the SRER parking lot, all of the seeds had fallen out. Instead there were lovely acorn caps with galls attached both on the insides and outsides, and no seeds at all. This image shows one of the galls attached to the outside of a cap. It has a drop of honeydew on it.
I found Weld's original 1957 description of Callirhytis balanella
in the "Proceedings of the United States National Museum" Vol. 107 No. 3384, New American Cynipid Wasps from Oak Galls
. Someone else collected the galls for Weld that he used in describing the species. It's hard to know if the "undersized acorn" that he mentions is really typical of the species he describes or not. Here's part of the description:
"GALL: Similar to a "pip" gall in shape but produced on the side of the cup of an undersized acorn in June and July. Gall slightly flattened, 4 mm. high by 3 mm. in diameter.
HABITAT: The type and one paratype are from galls collected at Cherry, Ariz., by Mrs. N. W. Capron on Oct. 4, 1935. One adult emerged April 23 and the other May 1, 1937. One paratype is from a gall Mrs. Capron collected at Prescott, Ariz., Sept. 12, 1950, and the living adult was cut out Apr. 11, 1952. Galls were seen in the Santa Catalina and Huachuca Mountains and at Patagonia, Nogales, and Bisbee, Ariz., and at Alpine, Tex."
Another image one of these galls, attached to the inside of a cap: