Introduction
The Seguin chestnut, Castanea seguinii Dode (Fagaceae), is a shrubby species native to the mountainous regions of central and southern China. While less known than the widely cultivated C. mollissima, C. seguinii possesses high disease resistance and serves as an essential germplasm reservoir for chestnut breeding and forest genetic diversity (Lang et al., Reference Lang, Dane, Kubisiak and Huang2007; Nie et al., Reference Nie, Zhang, Chu, Yu, Liu, Yan, Zhao, Gao, Li, Shi, Zheng, Fang, Qin and Xing2024). However, the survival and productivity of this species are increasingly threatened by specialised gall-forming pests, most notably the stem gall wasp Synergus castaneus Bernardo (Hymenoptera: Cynipidae) (Bernardo et al., Reference Bernardo, Gebiola, Xiao, Zhu, Pujade-Villar and Viggiani2013).
Synergus castaneus induces multi-chambered, woody, spherical galls exclusively on the stems of C. seguinii. These galls severely disrupt the host plant’s vascular system, frequently leading to branch withering or the death of the entire plant (Bernardo et al., Reference Bernardo, Gebiola, Xiao, Zhu, Pujade-Villar and Viggiani2013; fig. 1). Previous observations further indicate that S. castaneus is a biparental univoltine species: adults emerge from April to early May, new galls become evident in May–June, and the species overwinters inside the galls (Bernardo et al., Reference Bernardo, Gebiola, Xiao, Zhu, Pujade-Villar and Viggiani2013). Managing gall-forming pests presents a significant challenge; conventional chemical control is often ineffective because the larvae are physically protected within the thick, lignified gall tissues (Price, Reference Price2005; Ferracini et al., Reference Ferracini, Pogolotti and Alma2022). Furthermore, the excessive use of pesticides poses risks to environmental health and non-target organisms. Consequently, integrated pest management strategies – specifically biological control through the use of natural enemies – have become the most sustainable approach for regulating gall wasp populations (Matošević et al., Reference Matošević, Lacković, Kos, Kriston, Melika, Rot and Pernek2017; Ferracini et al., Reference Ferracini, Ferrari, Pontini, Saladini and Alma2019). In chestnut-associated gall wasps, the successful use of specialised parasitoids such as Torymus sinensis against the Asian chestnut gall wasp Dryocosmus kuriphilus has highlighted the value of host-specific natural enemies in achieving sustainable suppression (Moriya, Reference Moriya2003). Identifying functionally similar specialist parasitoids in native Chinese chestnut systems is therefore of both ecological and practical importance.
Host plant Castanea seguinii and the morphology of stem galls induced by Synergus castaneus. (A) A healthy, fruiting shrub of C. seguinii in its natural habitat; the top inset provides a detailed view of the characteristic spiny cupule (burr) and small nut, while the bottom inset highlights a fresh, spherical woody gall induced by S. castaneus on a live stem. (B) Impact of severe infestation on the host plant, showing significant branch withering and dieback; the inset displays a cluster of old, dark galls remaining on the desiccated twigs, illustrating the long-term damage to the plant’s vascular system.
Recent studies on chestnut gall wasp systems have shown that parasitoid communities can vary markedly across regions and through time. For example, differences in native and introduced chalcid parasitoid communities recruited by D. kuriphilus have been documented between Iberian territories (Gil-Tapetado et al., Reference Gil-Tapetado, Cabrero-sañudo, Gómez, Askew and Nieves-aldrey2021), and temporal changes in parasitoid assemblages associated with invasive chestnut gall wasps have also been reported in recent surveys (Jara-Chiquito et al., Reference Jara-Chiquito, Oliva, Lobato-vila and Pujade-Villar2024; Vinagrero-Conde et al., Reference Vinagrero-Conde, Muñoz-adalia, Pujade-Villar and Fernández-fernández2025). These studies highlight the importance of considering both phenological structure and geographic stability when evaluating the biological control potential of parasitoid assemblages. However, such information is still lacking for the S. castaneus–C. seguinii system. In particular, the previous study did not provide detailed rearing observations or describe seasonal emergence patterns of the associated wasp community (Bernardo et al., Reference Bernardo, Gebiola, Xiao, Zhu, Pujade-Villar and Viggiani2013).
Host-obligate parasitoids are especially important in this context because they are favoured to avoid non–target impacts and to strongly suppress a particular pest (van Lenteren et al., Reference van Lenteren, Cock, Hoffmeister, Sands, Bigler, Babendreier and Kuhlmann2006). Previous investigations into the S. castaneus complex identified a parasitoid assemblage comprising several families, including Eupelmidae, Eurytomidae, Ormyridae, and Torymidae (Bernardo et al., Reference Bernardo, Gebiola, Xiao, Zhu, Pujade-Villar and Viggiani2013). While these generalist parasitoids contribute to pest mortality in established ecosystems, they often fail to provide reliable regulation during the geographic dispersal or assisted migration of host plants. In such scenarios, generalist natural enemies typically remain associated with alternative hosts in their original habitats. This leads to an ‘enemy release’ effect, where pests like S. castaneus escape top-down control in new environments (Mlynarek et al., Reference Mlynarek, Moffat, Edwards, Einfeldt, Heustis, Johns, MacDonnell, Pureswaran, Quiring, Shibel and Heard2017). Therefore, identifying obligate parasitoids is critical for the sustained protection of C. seguinii germplasm. Unlike generalists, obligate specialists are more likely to track pest populations across environmental transitions, ensuring stable, long-term regulation of pest populations (Evans, Reference Evans2018).
During our field investigations into the natural enemy complex of S. castaneus across Zhejiang, Jiangxi, and Guizhou provinces in China, we discovered a distinct parasitoid that was consistently reared from S. castaneus galls across all three provinces and appeared to be obligately associated with the host gall wasp. Initial observations revealed that this species belongs to the genus Aulogymnus. The genus Aulogymnus Förster (Hymenoptera: Eulophidae) comprises approximately 37 species worldwide, most of which are solitary ectoparasitoids associated with galls on Quercus and Castanea (Schauff and Gates, Reference Schauff and Gates2005). To date, the Chinese Aulogymnus fauna remains poorly characterised; only four species (A. elevatus, A. hyalopterus, A. insculptus, and A. longicalcar) have been recorded in the country, with no new additions or detailed biological reports published in over two decades (Zhu et al., Reference Zhu, LaSalle and Huang1999).
Based on an integrative taxonomic approach combining morphological characterisation and mitochondrial COI barcoding, we determined that this population represents a previously undescribed species. This species is an obligate endoparasitoid: a rare life-history trait within this predominantly ectoparasitic genus (Askew, Reference Askew1961). The primary objectives of this study are to formally describe Aulogymnus liuchunhuensis sp. nov. and to report essential aspects of its biology, thereby providing a new candidate for the biological control of chestnut gall wasps in China.
Materials and methods
Insect sampling and biological observation
To investigate whether a geographically consistent parasitoid was associated with C. seguinii galls, we conducted field surveys and rearing experiments in three widely separated regions within the natural range of C. seguinii. Field collection was concentrated from May to September 2022 in Zhejiang (Longyou County), Jiangxi (Linchuan District), and Guizhou (Pingba District), China (table 1), whereas the subsequent period was devoted to laboratory rearing and emergence monitoring. During sampling, galled twigs bearing mature or nearly mature woody galls of S. castaneus were collected directly from as many different host individuals as possible within each study region and transported to the laboratory for rearing. Host identification followed Bernardo et al. (Reference Bernardo, Gebiola, Xiao, Zhu, Pujade-Villar and Viggiani2013). Family-level identification of parasitoids was conducted using keys provided by Goulet and Huber (Reference Goulet and Huber1993).
Collection localities of Synergus castaneus galls used in this study
Table 1 Long description
Collection localities for Synergus castaneus galls are listed by locality name, province, and geographic coordinates. Pingba District in Guizhou is at 26 degrees 29 minutes 42 seconds north, 106 degrees 12 minutes 25 seconds east. Longyou County in Zhejiang is at 28 degrees 45 minutes 07 seconds north, 119 degrees 04 minutes 20 seconds east. Linchuan District in Jiangxi is at 27 degrees 17 minutes 45 seconds north, 115 degrees 59 minutes 48 seconds east. Across the three sites, latitude ranges from the mid 26s to the high 28s north, while longitude ranges from about 106 to 119 east. The easternmost site is Longyou County, and the westernmost site is Pingba District. Coordinates are presented in degrees, minutes, and seconds and reflect point locations rather than sampling area or sample size.
Collected galls were inspected individually and placed in separate plastic rearing tubes sealed with sponge plugs to avoid cross-contamination among emergence records. The tubes were maintained under laboratory ambient conditions, and the plugs were moistened regularly to reduce desiccation of the galls during rearing (fig. 2A). Emerged adults were collected immediately after emergence and preserved in absolute ethanol for subsequent morphological and molecular analyses. Voucher specimens were deposited in the Insect Specimen Room of Nanjing Forestry University (NJFU), Nanjing, China.
Experimental workflow and seasonal emergence phenology of the wasp community. (A) Methodological pipeline for the study: field collection of galls followed by laboratory rearing and monitoring of adult wasp emergence under controlled conditions. (B) Relative emergence abundance of adult wasps throughout the year. The spring peak (April–May) illustrates the host–parasitoid synchrony between the host Synergus castaneus (green) and the newly described species Aulogymnus liuchunhuensis sp. nov. (light beige). The autumn peak (October–November) shows the emergence of a generalist community consisting of Eurytomidae, Pteromalidae, and Eulophidae across three sampling sites (Jiangxi, Zhejiang, and Guizhou). Shaded areas represent the relative abundance of each taxa/family over time.
Figure 2 Long description
Panel A shows a three-stage workflow arranged left to right. The first stage is labeled Field collecting and shows galls on a branch. The second stage is labeled Rearing and shows galls placed in individual laboratory tubes. The third stage shows a close-up of a single rearing tube containing an emerged adult wasp. Arrows connect each stage in sequence from left to right. Panel B is a line and area graph. The x-axis is labeled Month and spans from Jan through Dec. The y-axis is labeled Relative Emergence Abundance and has no numeric tick values visible. Two distinct shaded peak regions appear on the graph. The first peak is labeled Spring peak and occurs approximately between March and June, reaching its highest point around April to May. Within this peak, two overlapping shaded areas are visible. One represents S. castaneus labeled as Host and the other represents A. liuchunhuensis sp. nov. The two shaded areas overlap closely and the label Host-Parasitoid Synchrony points to this region. A small inset photograph of a wasp is placed near this peak with the label All three sites, indicating the spring peak is observed across all three sampling locations. The second peak is labeled Autumn peak and occurs approximately between August and December, reaching its highest point around October to November. This peak is labeled Generalist Community. Three inset photographs of wasps appear to the right of this peak, each labeled with a site name: Jiangxi, Zhejiang and Guizhou. The autumn peak area represents Eurytomidae comma Pteromalidae comma and Eulophidae as separate overlapping shaded areas stacked within the peak region. The taxa listed in the figure are S. castaneus left parenthesis Host right parenthesis, A. liuchunhuensis sp. nov., Eurytomidae comma Pteromalidae, Pteromalidae comma Eurytomidae and Eulophidae. Each taxon corresponds to a distinct shaded area visible in the graph.
Because our aim was to compare emergence patterns among sites using a standardised sampling unit, 20 galls were randomly selected from the collected material at each locality for detailed emergence monitoring. These galls were checked regularly, and all emerged insects were recorded monthly for up to 1 year after collection. The remaining collected galls were used for bulk rearing to obtain additional adult specimens, as well as for dissection and molecular analyses. Because our aim was to characterise seasonal emergence composition rather than directly estimate chamber-level parasitism, community structure was quantified on the basis of emerged adults from the standardised gall samples. Relative abundance was calculated for each taxon as the proportion of all emerged adult wasps recorded at a given locality. Community composition was then compared among sites according to the presence, absence, and proportional dominance of the host and associated parasitoid taxa.
To visualise seasonal phenology, monthly emergence records were pooled by taxon and locality and used to describe temporal changes in community composition over the rearing period. Particular attention was paid to the degree of synchrony between S. castaneus and the newly discovered Aulogymnus species during the spring emergence period, as well as to the taxonomic composition of the autumn-emerging parasitoid assemblage.
Morphological examination and imaging
To diagnose the new species (the obligate parasitoid), morphological characters were compared with established keys and descriptions for Palearctic and Nearctic Aulogymnus species (Zhu et al., Reference Zhu, LaSalle and Huang1999; Schauff and Gates, Reference Schauff and Gates2005). All morphological terminology follows Gibson et al. (Reference Gibson, Huber and Woolley1997).
Specimens were examined using an Olympus SZX10 stereomicroscope and photographed with a Nikon D5600 digital camera. For detailed analysis of micro-morphological features, slide-mounted specimens were imaged at high resolution. Final figures were generated by processing multifocal image stacks through Helicon Focus software.
Molecular species delimitation
To complement morphological identification, genomic DNA was extracted from six adult females (two individuals per site; see table 2) using the Chelex-100 method (Nishiguchi et al., Reference Nishiguchi, Doukakis, Egan, Kizirian, Phillips, Prendini, Rosenbaum, Torres, Wyner, DeSalle, Giribet, DeSalle, Giribet and Wheeler2002). A fragment of the mitochondrial cytochrome c oxidase subunit I (COI) gene was amplified for molecular species delimitation. PCR amplification was performed using the universal primers LCO1490 and HCO2198 (Folmer et al., Reference Folmer, Black, Hoeh, Lutz and Vrijenhoek1994). Thermal cycling conditions were as follows: initial denaturation at 95°C for 3 min; 40 cycles of denaturation at 94°C for 30 s, 54°C for 30 s (annealing), and 72°C for 40 s; and a final extension at 72°C for 7 min. PCR products were sequenced by Sangon Biotech (Shanghai) Co., Ltd.
Specimens of Aulogymnus and out-groups used for phylogeny analysis
Table 2 Long description
Specimen list for a phylogeny dataset, giving species name, voucher ID, sampling location, data source, and GenBank accession. Aulogymnus liuchunhuensis is represented by six vouchers collected in China, with two each from Guizhou, Zhejiang, and Jiangxi, all sourced from this study and assigned accessions PX845606 through PX845611. Nine additional taxa are included from GenBank as comparative ingroup or outgroup material: Pnigalio soemius from Russia; Cirrospilus diallus from Finland; and Aulogymnus japonicus with an unknown location. Other Aulogymnus species come from several European countries, including Finland, France, Spain, and Hungary, each with its own GenBank accession. Voucher IDs for GenBank records incorporate the accession and species name, while the new study samples use short province-based codes. Locations reflect collection metadata and include one unknown entry, so geographic comparisons are limited by incomplete provenance.
Note: Zhejiang, Jiangxi, and Guizhou are the provinces in China.
Resulting sequences were quality-checked and compared against the GenBank database using NCBI BLASTn. Sequences obtained in this study were aligned with those of other Aulogymnus species and two out-group species, Pnigalio soemius (Walker, 1839) and Cirrospilus diallus Walker, 1838 (Hymenoptera: Eulophidae), retrieved from GenBank (accession numbers in table 2) using Geneious v.4.8.2.
Phylogenetic reconstruction was performed via Bayesian Inference (BI) in MrBayes v.3.2 (Ronquist and Huelsenbeck, Reference Ronquist and Huelsenbeck2003). The best-fitting nucleotide substitution model (GTR + G) was selected using Modeltest v.3.7 (Posada and Crandall, Reference Posada and Crandall1998) based on hierarchical Likelihood Ratio Tests. The BI analysis ran for 2 × 106 generations using Markov chain Monte Carlo sampling, with trees sampled every 1,000 generations. Pairwise genetic distances between the new species and its congeners were calculated using the Kimura 2-parameter (K2P) model with 1,000 bootstrap replicates in MEGA v.7.0.26 (Kumar et al., Reference Kumar, Stecher and Tamura2016).
Results
Seasonal emergence patterns and community composition
Rearing experiments of field-collected galls revealed a distinct bimodal emergence pattern for the wasp community across all three sampling locations (fig. 2B). The emergence phenology was characterised by a ‘Spring peak’ occurring between April and May, followed by a secondary, ‘Autumn peak’ centred around October.
The spring emergence was dominated by the host gall wasp, S. castaneus, and its obligate parasitoid, A. liuchunhuensis sp. nov. These two species exhibited high host–parasitoid synchrony, appearing simultaneously at all three study sites (fig. 2B). Synergus castaneus represented the highest relative abundance in all locations, accounting for 41.18% of the total emergence in Jiangxi, 52.53% in Guizhou, and 41.67% in Zhejiang (table 3). The new species, A. liuchunhuensis, consistently constituted approximately one-quarter of the emerged community, with relative abundances ranging from 25.00% to 27.27% across the sites (table 3). The autumn emergence period consisted of a ‘Generalist Community’ comprising three chalcidoid families: Eurytomidae, Pteromalidae, and Eulophidae (fig. 2B). Unlike the spring peak, the community composition in autumn showed significant regional variation (table 3). In Jiangxi, the autumn community was the most abundant, led by Pteromalidae (21.57%) and Eurytomidae (11.11%). In Guizhou, the autumn emergence was exclusively represented by Eulophidae (20.20%). In Zhejiang, the community was split evenly between Eurytomidae (16.67%) and Pteromalidae (16.67%). Overall, the data indicate that while the host and its primary parasitoid maintain a highly synchronised and stable presence in the spring, the late-season parasitoid complex is more diverse and geographically variable.
Number of adult wasps emerging from rearing experiments across three sampling locations during spring and autumn peaks
Table 3 Long description
Adult emergence counts are reported for the host wasp Synergus castaneus and four associated parasitoid groups at three locations, with each cell giving a count and the within-location percentage. In Guizhou, spring peak emergence was 52 host adults (52.53%), 27 Aulogymnus liuchunhuensis (27.27%), no Eurytomidae or Pteromalidae, and 20 Eulophidae (20.20%). In Zhejiang, spring peak emergence was 20 host adults (41.67%), 12 Aulogymnus liuchunhuensis (25.00%), 8 Eurytomidae (16.67%), 8 Pteromalidae (16.67%), and no Eulophidae. In Jiangxi, spring peak emergence was 63 host adults (41.18%), 40 Aulogymnus liuchunhuensis (26.14%), 17 Eurytomidae (11.11%), 33 Pteromalidae (21.57%), and no Eulophidae. Across locations, the host is the largest single category in spring at all sites, while Eulophidae appear only in Guizhou and Pteromalidae are strongest in Jiangxi. Percentages are calculated within each location and rounded, so totals may not add to exactly one hundred percent.
Notes: Values are numbers of emerged adults; percentages were calculated within each location. Percentages were calculated within each location and rounded to two decimal places; therefore, totals may not sum to exactly 100%.
Morphological examination and biological observations
Rearing results indicate that the spring-emerging parasitoid is likely an obligate specialist, as evidenced by its consistent occurrence across a wide geographic range. Moreover, the emergence timing closely overlaps with the host gall wasp, indicating A. liuchunhuensis most likely targets host eggs or very early first-instar larvae. In this study, all 79 emerged specimens (table 3) were female; the total absence of males suggests thelytokous parthenogenesis or a highly female-biased sex ratio. Generic placement was confirmed as Aulogymnus Förster, distinguished from the closely related genus Cirrospilus Westwood by the presence of three funicular segments in the female and the specific reticulate sculpture of the scutellum. Detailed morphological comparisons indicate that this species is distinguished from all other described Aulogymnus species. We herein describe this new species as follows.
Aulogymnus liuchunhuensis sp. nov. (figs 3 and 4)
Holotype female of Aulogymnus liuchunhuensis sp. nov. (A) Habitus, lateral view. (B) Head and antennae, lateral view. (C) Mesosoma and base of the wings, lateral view. (D) Habitus, dorsal view. (E) Head, dorsal view. (F) Detail of the mesosoma, dorsal view, showing the reticulate sculpture of the mesoscutum and scutellum. Scale bars: A, D = 500 μm; others = 100 μm.
Morphological details of Aulogymnus liuchunhuensis sp. nov., female. (A) Head, frontal view. (B) Antenna. (C) Mesosoma, dorsal view. (D) Mesosoma, lateral view. (E) Forewing and hindwing. (F) Abdomen, the ventral view. Scale bar in C, D = 100 μm, E = 200 μm.
Chinese common name: Liuchunhu ao jixiaofeng (六春湖奥姬小蜂)
Diagnosis: Aulogymnus liuchunhuensis sp. nov. is distinguished from other Palearctic and Oriental congeners by the following combination of characters. Distinctive bicoloration: The mesosoma is predominantly bright orange-testaceous, contrasting sharply with the dark metallic blue-green propodeum and the metallic metasoma (fig. 3A, D). Antennal formula: The female funicle consists of three distinct segments (F1–F3) followed by a three-segmented club (fig. 4B). This distinguishes it from many Aulogymnus species that typically possess only two funicular segments. Wing characters: Speculum is open; the ratio of the marginal vein (MV) to the stigmal vein (STV) is approximately 3:1 (table 4; fig. 4E), which is higher than in most related species such as A. longicalcar (2.0:1) or A. skianeuros (1.5:1). Sculpture: The scutellum features raised reticulate sculpture (fig. 3F), lacking the longitudinal lines found in A. gallarum.
Comparison of key morphological characters between Aulogymnus liuchunhuensis sp. nov. and related Palearctic congeners
Table 4 Long description
Comparison across ten Aulogymnus species summarizes wing speculum state, female funicular segment count, marginal-to-stigmal vein length ratio, scutellar sculpture, and a key color trait. A. liuchunhuensis is distinguished by an open speculum, female funicle ending at the third segment, a marginal vein about three times the stigmal vein, raised reticulate scutellar sculpture, and a sharp orange mesosoma contrasted with metallic blue-green propodeum and metasoma. Most species share the same female funicular segment count, so separation relies mainly on speculum state, vein ratio, sculpture, and color notes. Open speculum occurs in A. liuchunhuensis, A. longicalcar, A. elevatus, and A. gallarum, while the remaining listed species have a closed speculum. Reported vein ratios range from about one and a half times to about three times, with A. skianeuros lowest and A. liuchunhuensis highest; A. arsames has no vein-ratio data. Scutellar sculpture varies from raised elongate or isodiametric patterns to strongly reticulate, hexagonal, or engraved and etched appearances, providing additional diagnostic differences. Color features further separate taxa, such as yellow tibiae in A. gallarum, two wing darkenings in A. skianeuros, yellow pleural or pronotal patches in A. balani, and a yellow basal stripe on the first gastral tergite in A. elevatus. Interpretive caveat: several color notes include qualitative descriptors and one species lacks vein-ratio information, so identification should combine multiple characters rather than rely on a single field.
FSC, funicular segment count; F1, first funicular segment; T1, first gastral tergite; MV, marginal vein; STV, stigmal vein.
Description of female (fig. 3). Body length: 2.3 mm.
Colour: Head coppery-brown with distinct metallic green reflections, particularly on the vertex and frons; ocelli surrounded by metallic blue-green patches. Eyes dark red; ocelli translucent yellow. Antennae with scape and pedicel yellowish-brown; funicle and club darker brown. Mesosoma vivid orange-testaceous (honey-coloured), except for the propodeum, which is dark metallic blue-green. Legs entirely pale yellow to whitish, except for the hind coxa, which is dark metallic brown basally. Metasoma (gaster) dorsally dark with strong metallic blue, green, and purple iridescence. Wings hyaline; venation light brown.
Head: Head slightly wider than high (approx. 1.1×). Vertex with reticulate sculpture and scattered short, dark setae. Frons and lower face with distinct reticulate/alveolate sculpture (fig. 4A). Inner eye orbits slightly diverging ventrally. Malar space distinct; eye height approximately 1.9× the malar space.
Antenna: Antenna inserted slightly above the level of the ventral eye margin. Scape cylindrical, not reaching the median ocellus. Pedicel conical, approx. 1.8× as long as broad. Funicle three-segmented; segments subequal in length. Club three-segmented, the terminal segment distinct with a small spicule (fig. 4B).
Mesosoma: Pronotum conical, narrower than the mesoscutum, with reticulate sculpture. Mesoscutum midlobe with strong, raised reticulate sculpture and sparse dark setae (fig. 3F). Notauli curved and complete. Scutellum convex, with raised reticulate sculpture similar to the mesoscutum; sublateral grooves absent. Propodeum smooth to weakly coriaceous, distinct from the coarse sculpture of the scutellum; median carina complete and distinct (fig. 3F). Spiracles small, rounded.
Wings: Forewing broad (length approx. 2.2× width). Speculum open, extending slightly below the MV; wing disc densely setose beyond the speculum (fig. 4E). MV long and slender; STV short and petiolate. The MV:STV ratio is 3.0 (table 4). Postmarginal vein length subequal to the STV.
Metasoma: Petiole short and transverse. Gaster ovate to lanceolate, slightly longer than the mesosoma (fig. 3A). First gastral tergite (Gt1) with a brilliant blue-green sheen; surface of tergites smooth and shiny with weak reticulation visible under high magnification (fig. 4F). Ovipositor sheaths not exserted.
Male: Unknown.
Host: Synergus castaneus.
Etymology: The specific epithet liuchunhuensis refers to Mt. Liuchunhu, the mountain in Longyou, Zhejiang Province, where the galls were first collected.
Distribution: China (Zhejiang, Jiangxi, and Guizhou Provinces).
Material examined: Holotype: ♀, China: Zhejiang Province, Longyou, Mt. Liuchunhu, ex S. castaneus galls on C. seguinii, 2022, Huayan Chen, deposited in Insect Specimen Room, NJFU, Nanjing, China. Paratypes: 2♀, same data as holotype. Voucher specimen accessions for holotype and paratypes: NJFU-Aulogymnus liuchunhuensis, NJFU-Aulogymnus liuchunhuensis2, NJFU-Aulogymnus liuchunhuensis3.
ZooBank URLs: https://zoobank.org/References/766b9338-73a6-42e2-86db-775ec6e00c6e; https://zoobank.org/NomenclaturalActs/4C5D951D-1D5B-41BF-8DCF-DEC55284D21E.
Molecular delimitation
The mitochondrial COI gene was successfully sequenced from six specimens of the new species. The aligned sequences exhibited high intraspecific conservation, with negligible nucleotide divergence (K2P distance: 0–2%) observed between populations from Guizhou, Zhejiang, and Jiangxi provinces (table 5). BLAST comparisons against the GenBank database yielded no high-similarity matches, further supporting the molecular novelty of the species.
Pairwise genetic distances (Kimura 2-parameter model) for the mitochondrial COI gene among Aulogymnus species and out-groups
Table 5 Long description
Pairwise genetic distances for the mitochondrial COI gene are listed as proportions of base substitutions among 15 taxa, including two outgroups, several named Aulogymnus species, and six specimens representing a newly described species. Distances between the new specimens (10 to 15) are extremely small, ranging from 0.00 to 0.02, indicating minimal variation within the new species across sampled provinces. The new specimens are consistently more distant from all other taxa, typically around 0.14 to 0.19, supporting clear separation from the named species and outgroups. Among the named Aulogymnus species, the smallest distances occur within the cluster of taxa 7 to 9, with values from 0.01 to 0.03, suggesting very close relationships there. Outgroups show larger distances to Aulogymnus overall, roughly 0.16 to 0.21, with the largest value being 0.21 between taxon 2 and taxon 6. Values are symmetric pairwise distances and should be interpreted as sequence divergence under the stated model, not as direct measures of time or taxonomy by themselves.
Notes: Numbers in the top row of the matrix correspond to the taxa: Pnigalio soemius (out-group); Cirrospilus diallus (out-group); Aulogymnus obscuripes; Aulogymnus japonicus; Aulogymnus skianeuros; Aulogymnus arsames; Aulogymnus trilineatus; Aulogymnus gallarum; and Aulogymnus testaceoviridis. Specimens 10–15 represent the new species described in this study, collected from Guizhou (GZ1–GZ2), Zhejiang (ZJ1– ZJ2), and Jiangxi (JX1–JX2) provinces. Values represent the proportion of substituted bases between sequences. Genetic distances between the new species and other taxa are shown in red; intraspecific distances are highlighted in yellow.
Phylogenetic analysis using BI strongly supported the placement of the new species within the genus Aulogymnus (fig. 5). The six sampled individuals formed a well-supported, distinct monophyletic clade (BI posterior probability = 1.00). The genetic distance analysis (K2P) corroborated the phylogenetic results. The interspecific distance between the new species and other congeners ranged from 14% to 19% (table 4). Specifically, the distance between A. liuchunhuensis sp. nov. and its closest molecular relatives – A. trilineatus, A. gallarum, and A. testaceoviridis – was substantial (14–16%). This significant ‘barcoding gap’ between the low intraspecific variation (≤2%) and the high interspecific divergence (≥14%) provides robust evidence for the status of A. liuchunhuensis as a distinct species.
Phylogenetic relationships of Aulogymnus liuchunhuensis sp. nov. and related congeners. Bayesian inference (BI) tree based on mitochondrial cytochrome c oxidase subunit I (COI) sequences. Numbers at the nodes represent Bayesian posterior probabilities. Out-group taxa (Pnigalio mediterraneus, Cirrospilus diallus, and Zagrammosoma latilineatum) are indicated. Terminals labelled GZ, ZJ, and JX represent specimens of A. liuchunhuensis sp. nov. collected from Guizhou, Zhejiang, and Jiangxi provinces, respectively. Sequences for congeners are labelled with their corresponding GenBank accession numbers.
Figure 5 Long description
The diagram is a Bayesian Inference tree illustrating the phylogenetic relationships among Aulogymnus species and related taxa. The tree is oriented vertically, with branches extending from top to bottom. At the top, the outgroup taxa are labeled as OP292571.1Pnigaliosoemius and MZ633973.1Cirrospilusdiallus. Below these, PP826464.1Aulogymnusobscuripes branches off. The main clade labeled Aulogymnusliuchunhuensis includes six specimens: GZ2, GZ1, ZJ1, ZJ2, JX2 and JX1, with Bayesian posterior probabilities at nodes ranging from 0.56 to 1. The tree further branches into other Aulogymnus species, including PQ776751.1Aulogymnusjaponicus and PP829061.1Aulogymnusarsames, with a posterior probability of 0.94. The clade continues with PP826467.1Aulogymnustrilineatus, HM573874.1Aulogymnusgallarum and PP826466.1Aulogymnustestaceoviridis, all supported by a posterior probability of 1. The final branch is PP826465.1Aulogymnus_silvaticus. The tree visually represents evolutionary relationships, with posterior probabilities indicating the confidence of each node.
Discussion
The discovery of A. liuchunhuensis sp. nov. expands the known distribution of the genus within the Oriental region and marks the first addition to the Chinese fauna since 1999 (Zhu et al., Reference Zhu, LaSalle and Huang1999). Validated by both morphological and molecular evidence, this new species also holds significant practical value for the conservation of C. seguinii. As an obligate endoparasitoid of the gall-forming cynipid S. castaneus, A. liuchunhuensis serves as a key natural regulator of pest populations. By targeting the host during its early developmental stages, this species has the potential to suppress pests rapidly compared to parasitoids that exploit later host stages that are more dependent on local adaptation.
In this study, we employed an integrative taxonomic approach, combining morphological and molecular evidence to validate the species’ novelty. Morphologically, A. liuchunhuensis is readily diagnosable by its striking bicoloration – the bright orange-testaceous mesosoma contrasts sharply with the metallic blue-green propodeum and gaster. Furthermore, the presence of three funicular segments in the female antenna is a critical diagnostic feature. This antennal formula not only distinguishes the new species from the closely related genus Cirrospilus (which consistently possesses two funicular segments) but also separates it from the majority of Aulogymnus congeners (Zhu et al., Reference Zhu, LaSalle and Huang2002). Molecular analysis further confirms that A. liuchunhuensis represents a distinct evolutionary lineage. The species exhibits a substantial ‘barcoding gap’, with an interspecific genetic divergence (K2P distance) of 14–19% from its closest relatives (Miralles et al., Reference Miralles, Puillandre, Vences and DeSalle2024), including A. gallarum, A. testaceoviridis, and A. trilineatus. Phylogenetic reconstruction reinforces its placement within Aulogymnus. The high posterior probability support and significant molecular divergence provide robust evidence for the establishment of A. liuchunhuensis as a new species.
The biological characteristics of A. liuchunhuensis provide profound insights into the evolution of life-history traits within the genus. While the vast majority of the 37 known Aulogymnus species are solitary ectoparasitoids, A. liuchunhuensis is an obligate endoparasitoid. Previously, A. gallarum was the only congener reported to exhibit endoparasitism (Linné, Reference Linné1761). The phenology of A. liuchunhuensis is characterised by a high degree of synchrony with its host, the gall-forming cynipid S. castaneus. Our observations demonstrate that the spring emergence of the parasitoid is precisely timed with the early developmental stages of the host, since the adults typically complete their reproductive cycle within 2–3 weeks of emergence and then perish (Godfray, Reference Godfray1994). By targeting host eggs or early-instar larvae, the parasitoid likely encounters minimal physiological resistance from the host’s immature immune system (Vinson, Reference Vinson1990). This high level of specialisation likely explains the biological stability and consistent occurrence of A. liuchunhuensis across diverse regions (Zhejiang, Jiangxi, and Guizhou), contrasting with the generalist parasitoid community observed in the autumn. The autumn assemblage showed significant local adaptation and regional variation, likely due to a co-evolutionary ‘arms race’ with late-instar hosts that possess more robust defences (Wu et al., Reference Wu, Monnin, Lee and Henry2022).
From an applied perspective, this distinction is important. Biological control of gall-inducing insects is often constrained by the physical protection provided by gall tissues, especially once galls have enlarged and lignified. A parasitoid that attacks the host before or during the earliest phases of gall development may therefore provide a more effective form of natural regulation than parasitoids exploiting later host stages. In this respect, A. liuchunhuensis is of particular interest because its emergence pattern suggests a rapid-response interaction with S. castaneus. Its consistent occurrence across multiple provinces further indicates that this association is not restricted to a single local population. These characteristics make A. liuchunhuensis a promising candidate for future evaluation in biological control and conservation management programmes aimed at protecting native C. seguinii germplasm. Future work should assess its host specificity, field parasitism efficiency, and feasibility for incorporation into conservation-oriented biological control programmes.
Acknowledgements
We thank Prof. Xinqiang Xi from Nanjing University for his assistance with the identification of the parasitoid wasp specimens. We also sincerely thank Mr Xiaoyue Lin of Xikou Forest Farm, Longyou County, Quzhou, Zhejiang Province, China, for his continuous support and assistance during our fieldwork.
Competing interests
The authors have no relevant financial or non-financial interests to disclose.
