Holocynips humicola is a gall wasp (family Cynipidae) described in 1922 by Alfred Kinsey. The wasps on which Kinsey based his description were found in swellings on Quercus kelloggii roots in Ashland, Oregon. From the original description of the species, a "cave-in of a railroad embankment ... exposed these galls."

The galls are abrupt, potato-like swellings on roots and contain multiple larvae. The galls can't easily be removed without cutting or tearing the root. Small, round holes on the galls where wasps have exited can help confirm that they are wasp galls. Most gall wasps are considered to be harmless to trees. They are part of a fascinating and complicated ecosystem on oaks.

Holocynips humicola is also thought to gall Quercus agrifolia and Q. wislizeni (see Weld 1957). Q. agrifolia (coast live oak), Q. wislizeni (interior live oak), and Q. kelloggii (black oak) are the three main species of red oak that grow in California. Usually (but not always) a wasp that galls one of these oaks will gall all three, but a wasp that galls red oaks will never gall white oaks (ie., Q. lobata or Q. douglasii) or golden oaks (ie., Q. chrysolepis or Q. vacciniifolia). So, one thing we know is we won't find Holocynips humicola on anything except a red oak. White and golden oaks host a completely different set of gall wasp species.

Q. wislizeni and Q. agrifolia are host to another gall wasp, Callirhytis quercussuttoni, which makes integral (= not easily detachable) stem galls on branches above ground similar to those of H. humicola just on a different part of the tree.

Callirhytis quercussuttoni gall with exit holes where wasps emerged. On Quercus agrifolia (coast live oak).

There are museum specimens of Holocynips humicola galls and wasps collected decades ago but as far as I'm aware nothing recently collected. The specimens I've seen from the California Academy of Sciences and the Essig Museum are all from Q. wislizeni.

Galls labeled Holocynips humicola from Quercus wislizeni (interior live oak) at the California Academy of Sciences.

In November 2021, Diane Erwin and I found what we thought to be H. humicola galls on roots of a large Q. agrifolia. The galls were between half an inch to 6 inches under the soil. The surface of the galls was reddish-brown. The tree also had a lot of integral stem galls above the soil. We assumed the galls above ground to be the commonly-seen Callirhytis quercussuttoni galls. The galls above ground were not reddish-brown -- the surface was like that of the branches -- but otherwise those galls looked similar to the galls on the roots.

Root galls from Quercus agrifolia (coast live oak), Oakland, CA, November 2021.

I collected a gall from a root under the soil. I dissected it on the same day and found fully formed adult wasps and also some very small larvae. The adults appeared to be the inducers of the galls. Other options for wasps inside galls are parasitoids or inquilines (= "uninvited guests").

Root gall cut in half, from Quercus agrifolia (coast live oak), Oakland, CA, November 2021.

Root gall cut in half, with larval chambers visible. From Quercus agrifolia (coast live oak), Oakland, CA, November 2021.

Adult wasp from root gall on Quercus agrifolia (coast live oak), Oakland, CA, November 2021.

In January 2022, some wasps emerged on their own from the remainder of the gall that I had not dissected. The wasps were identical to those that I dissected out the previous November.

I also collected some Callirhytis quercussuttoni galls in November 2021 and dissected out some adults. The adults looked quite like those from the Holocynips humicola galls. Of course, adult cynipids all tend to look similar no matter what the species .... small and brown.

Adult wasp from Callirhytis quercussuttoni stem gall on Quercus agrifolia (coast live oak), Oakland, CA, November 2021.

In February I sent some adult wasps from the Holocynips humicola galls to James Nicholls (CSIRO, Australia). James and his colleagues are working on a phylogeny of Cynipidae of the world. James sequenced them, and they came out so close to Callirhytis quercussuttoni that he concluded my Holocynips humicola wasps were really Callirhytis quercussuttoni wasps. What were their galls doing below ground?

I then went to the Essig Museum and photographed the four Holocynips humicola specimens that are in the museum. They looked nearly identical to the wasps from my Q. agrifolia root galls. James and I also looked at photos his colleague took of another Holocynips humicola specimen. That specimen, too, looks very much like Callirhytis quercussuttoni. In addition, images of the Holocynips humicola holotype (specimen from which the species was described) at the American Museum of Natural History look similar to Callirhytis quercussuttoni.

Is it possible that Holocynips humicola is really the same species as Callirhytis quercussuttoni?

Since root galls on oaks are hard to find due to their location underground, people have rarely seen them and there aren't many in museum collections. It would be interesting to see images of root galls from any fallen red oaks. If there are two species that gall their roots, do the galls look different? In the few museum specimens I've seen, the galls do not look obviously different to me. Are there adults in any of these root galls that look different from Callirhytis quercussuttoni adults?

California red oaks also host a gall wasp Callirhytis apicalis whose galls grow at the base of trunks, just above and just below the soil line. This gall is distinctly different from Holocynips humicola and Callirhytis quercussuttoni galls in that it only contains one larva/wasp inside a centrally located chamber. The area between the larval chamber and outer edge of the gall is somewhat spongy and soft, versus very hard and woody in Holocynips humicola and Callirhytis quercussuttoni.

Here is Kinsey's 1922 description of Holocynips humicola galls: "Large, irregular, tuber-like, woody swellings of the bark and new wood of roots. Agglomerate. The swellings are very irregular, smooth and rounded when fresh, roughening with age; the fresh bark covering is more reddish brown than on the normal roots; length up to 10.5 cm., the diameter 7.5 cm. on large galls. Affecting both wood and bark, including wood below the normal line of the bark; larval cells lying toward the bark, oval 3.5 by 4.5 mm., with a thin, distinctive tissue lining the cells, but not separable from the wood. On the sides of large roots, well below the surface of the ground, on Quercus kelloggii."

References:

Kinsey, Alfred C. (1922) New Pacific Coast Cynipidae (Hymenoptera). Bulletin of the AMNH ; v. 46, article 3

Russo, Ronald (2021) Plant Galls of the Western United States

Weld, Lewis H. (1957) Cynipid Galls of the Pacific Slope

Web sites:

Note that the identification of any Holocynips humicola galls on the sites below does not mean they are really Holocynips humicola. They are just best guesses based on gall location on the host and what little is known about this species. I thought my Quercus agrifolia root galls were Holocynips humicola, too, until I found out they weren't.

iNaturalist Callirhytis quercussuttoni images

iNaturalist Holocynips humicola images

Gallformers web site Callirhytis quercussuttoni page

Gallformers web site Holocynips humicola page

My web page with photos of many California oak galls.