Andricus mendocinensis was formally described by Weld (1957b), with the description based on two dead adults cut from a gall in 1922; that gall had been collected five years previously, in May 1917, at an unspecified location in Mendocino County, California. The description of the gall was superficial, and a second publication by Weld indicated some uncertainty about its exact location on the host plant (“thought to be a root gall” in Weld’s description [Weld 1957b]; “stem swelling at the crown” in Weld 1957a). At the time of formal description, the type gall had been lost. Other than the original collecting event, no further records of this species have been mentioned in the primary literature or on citizen science websites (e.g. BugGuide [https://bugguide.net], iNaturalist [https://www.inaturalist.org], or Gallformers [https://www.gallformers.org]). Furthermore, to the authors’ best knowledge, no specimens beyond the types have been deposited in the collections of any scientific institutions, with the holotype being in the Smithsonian National Museum of Natural History, Washington, D.C. (catalog number USNMENT00802192) and paratypes at the California Academy of Sciences, San Francisco, California (catalog numbers CASENT8590680, CASENT8590681).

The authors have searched extensively, and until recently unsuccessfully, for the gall of A. mendocinensis on multiple field trips over the past 18 years to numerous locations within the native range of N. densiflorus in northern California. However, in July 2023, we re-discovered galls of this species within a stand of N. densiflorus trees at Lord-Ellis Summit, Northern Coast Ranges, along State Route 299, Humboldt County, California (40.9306°N, 123.8599°W; altitude 690m). Eight galls were discovered by digging in the soil around the base of the trees. Herein we provide a detailed description of the gall, including its location on the host plant, plus brief information about its taxonomic affinities and life history. At the time of publication, these galls were still under rearing conditions awaiting the emergence of adult wasps; all gall rearings, dissected larvae used for DNA sequencing, and emerged parasitoids are currently housed at the Institute of Ecology and Evolution, University of Edinburgh, U.K., with a future plan to deposit reference specimens (galls, inducers, and parasitoids) at the Smithsonian National Museum of Natural History.

Galls of A. mendocinensis are approximately spherical integral swellings of the subterranean portions of suckering shoots that originate from the root stock of N. densiflorus plants (Fig. 1A, 1B). Galls were only found on suckering shoots with a diameter of 1–2 cm growing from smaller trees that were typically 10–15 cm in diameter at the base; despite extensive searching, they were not found on larger trees. Most galls had been induced on sections of shoots that had grown 10–20 cm away from the tree trunk (Fig. 1C), although several had been induced very close to the point at which the galled sucker diverged from the main root stock (Fig. 1D). Once fully developed, the galls appear to kill the shoot above the gall. All the galled suckers found in this study had died back to within a few centimeters beyond the gall itself (arrowed in Fig. 1C & 1G), and as a result, there was no obvious sign of the presence of a gall from above the surface of the soil and overlaying leaf litter. Most of the galls observed were 10–15 cm below the surface of the soil (Fig. 1C, 1D).

Mature galls range in diameter from 2 cm to about 10 cm. They are multilocular, with the larval cells (Fig. 1F, 1G) distributed across the outer surface of a hard core of solid woody tissue (Fig. 1E). Larval cells are embedded in the gall surface in sockets rather like teeth in gums (Fig. 1H; arrow in Fig. 1E), although each cell can be detached relatively easily from its socket; these cells and empty sockets correspond to the surface detail included on part of Weld’s illustration of the gall (Fig. 2, reproduced from fig. 182 in Weld 1957a). Each larval cell has a blunt conical shape (Fig. 1I); the pointed end is embedded in the main body of the gall while the flatter end emerges from the gall body, forming a rounded bulge on the surface. Larval cells are approximately 3 mm in diameter and 4 mm long. Cells have a surface structure of longitudinal lines, primarily on the innermost half, similar to the surface sculpturing apparent on galls of the related A. notholithocarpi (see fig. 10–14 in Nicholls et al. 2018a).

Mature galls of A. mendocinensis were found in July. Inhabitants of some larval cells had already emerged; other larval cells from the same gall were dissected and found to contain full-sized larvae. This implies that some larvae may diapause for an extended period, with adult emergence occurring over several years, a life history trait often found for Cynipini species that induce large subterranean galls (Weld 1921). No adults of A. mendocinensis had emerged as of the February following gall collection, although three adult parasitoids emerged in late summer of the year of gall collection. These parasitoids (2 males, 1 female) were of a species of Torymus Dalman, 1820, possibly T. denticulatus (Breland, 1939), which is known to attack cynipid galls in California (Grissell 1979).

- James Nicholls, John DeMartini, Graham Stone: (2024) Re-discovery and detailed description of the gall of Andricus mendocinensis Weld, 1957 (Hymenoptera: Cynipidae: Cynipini) on the tanoak Notholithocarpus densiflorus (Hook. & Arn.) Manos, Cannon & SH Oh (Fagaceae).^©