Holocynips illinoiensis Melika & Nicholls, sp. nov.

Diagnosis: Two Holocynips species are known from the eastern half of the USA, also from Illinois in particular (see Burks 1979), H. badia and H. maxima, but their galls differ from those of H. illinoiensis, sp. nov.. In H. badia and H. maxima the galls are woody and rounded, just below the soil surface, large and polythalamous in H. maxima, small and monothalamous in H. badia; while in H. illinoiensis the galls are monothalamous, just above the soil surface and structurally complex with multiple spongy protrusions surrounding the larval cell.

Gall (Figs 14–16). Occurring singly or in closely appressed clusters of 3–6 at the base of small stems (diameter at breast height ≤ 3 cm) or shoot regrowth from cut stumps, most at ground level and either hidden by leaf litter and vegetation or partially buried in the soil. Occasional galls have been observed higher on the stem, up to 75 cm above the ground.

Galls are dull red to reddish-brown, darkening to purple-brown or black with age and elemental exposure. Surface texture is finely granulate and dull. Each individual gall consists of a single larval cell surrounded by 6–10 thin-walled parenchyma-filled protrusions in a radial or whorled configuration. Protrusions are septate, becoming hollow and brittle with age and eventually disintegrating. Overall gall morphology ranges from irregularly globular (as when protrusions are small or absent) to more elaborate shapes reminiscent of flowers or dried star anise. Individual galls range in size from 11–20 mm wide and 8–10 mm in height. Gall detachment from the stem surface requires some force and leaves a conically depressed scar 4–6 mm in diameter, paler than the surrounding tissue and enclosed by an elevated margin 1–3 mm high.

The larval cell is round or ovate to teardrop-shaped, 4–5 mm wide and 4–5 mm tall. Cell walls are woody, 0.5–0.6 mm thick and lighter in color than the surrounding protrusions. Adult emergence holes observed on old galls are ovate, approximately 2 mm wide by 3mm tall.

Biology. Only the asexual generation is known, which induces galls on Q. macrocarpa Michx. (subgenus Quercus, section Quercus). Galls mature in November–December; adults were cut out in December and January under laboratory conditions.

- James Nicholls, Nick Furlan, George Melika: (2023) Holocynips illinoiensis, sp. nov., a new species of oak gall wasp (Hymenoptera: Cynipidae) from the USA^©