Confirmation, consequences and causes of extensive polyphyly in the oak gall wasp genus Dryocosmus(Hymenoptera: Cynipidae: Cynipini)

Authors: TANG, CHANG-TI - George Washington University; NICHOLLS, JAMES - Commonwealth Scientific And Industrial Research Organisation (CSIRO); MELIKA, GEORGE - Hungarian University Of Agriculture And Life Science; Buffington, Matthew; Gates, Michael; LILL, JOHN - George Washington University; MAPES, CAROL - Kutztown University; YANG, MAN-MIAO - Chung Hsing University; YOSHIHISA, ABE - Kyushu University; STONE, GRAHAM - University Of Edinburgh

Submitted to: Systematic Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/3/2026
Publication Date: 3/10/2026
Citation: Tang, C., Nicholls, J., Melika, G., Buffington, M.L., Gates, M.W., Lill, J.T., Mapes, C., Yang, M., Yoshihisa, A., Stone, G. 2026. Confirmation, consequences and causes of extensive polyphyly in the oak gall wasp genus Dryocosmus(Hymenoptera: Cynipidae: Cynipini). Systematic Entomology. 10(2). Article eixag006. https://doi.org/10.1093/isd/ixag006.
DOI: https://doi.org/10.1093/isd/ixag006

Interpretive Summary: Invasive insect species cause billions of dollars per year in agricultural losses and natural habitat degradation. The density of these pests is critical for their control. The chestnut gall wasp is just one of these pests, causing millions of dollars in damage since its introduction. Originally from China, this species arrived in N. America in the 1970s and immediately started to impact chestnut production as well as American chestnut restoration. Our research group has already determined the species status of this pest and provided diagnostics for its identity; here we augment that research with the placing of the species within the broader context of gall wasp evolution (phylogeny). As we discovered, the chestnut gall wasp has been in the wrong genus for decades, and our work paves the way to remedy this. Our work will be used by biocontrol workers and forest ecologists to reduce damage from the chestnut gall wasp.

Technical Abstract: Oak gall wasps are a species-rich group in which most genera induce galls on a single Fagaceae lineage (either a section within Quercus or a single non-oak genus). Preliminary data for the few exceptions (most notably Andricus, Callirhytis, Dryocosmus, and Neuroterus) suggest that high apparent host plant diversity is an artifact of polyphyly resulting from inadequate morphological definition of genera. Here we demonstrate this issue in Dryocosmus, which (as currently defined) galls seven Fagaceae lineages, by reconstructing relationships among 35 species in the genus and 27 other Cynipini genera worldwide. Data for 1,871 base pairs of DNA sequence from 1 mitochondrial gene (cytochrome b) and 3 nuclear genes (28S D2, opsin, wingless) confirm that Dryocosmus comprises a polyphyletic assemblage of more host-specific lineages. Our analyses also strongly reject monophyly of Callirhytis. We surveyed the taxonomic issues underlying these problems in Dryocosmus and begin the taxonomic revision of this and closely related genera. We establish (i) D. asymmetricus as a junior synonym of Neuroterus asymmetricus (Kinsey), comb. nov., (ii) C. congregata as a junior synonym of Melikaiella congregata (Ashmead), comb. nov., (iii) C. clavula as a junior synonym of Andricus clavula (Osten Sacken), comb. rev., and (iv) reinstate Plagiotrochus kunugiphagus as Dryocosmus kunugiphagus Ide & Abe comb. rev. We establish a new genus, Chrysolepicynips Stone, Nicholls, Melika & Tang, gen. nov. for D. castanopsidis, D. demartinii, D. juliae and D. rileypokei that gall chinquapins (Chrysolepis sp.). Finally, we identify alternating generations in two species: asexual C. furva is a junior synonym of sexual D. quercusnotha (Osten Sacken), and asexual Zopheroteras guttatum and sexual D. quercuslaurifoliae are junior synonyms of sexual D. quercuspalustris (Osten Sacken). Our results provide a backbone for revision of Dryocosmus and highlight the need for Cynipini-wide revision of polyphyletic genera.