Redescriptions of two parasitoids, Metapelma beijingense Yang (Hymenoptera, Eupelmidae) and Spathius ochus Nixon (Hymenoptera, Braconidae), parasitizing Coraebus cavifrons Descarpentries & Villiers (Coleoptera, Buprestidae) in China with keys to genera or species groups.

Abstract Two parasitoids, Metapelma beijingense Yang (Hymenoptera, Eupelmidae) and Spathius ochus Nixon (Hymenoptera, Braconidae) are redescribed and illustrated. Both were reared from Coraebus cavifrons Descarpentries & Villiers (Coleoptera, Buprestidae) boring in Symplocos stellaris Brand (Symplocaceae). Metapelma beijingense is a solitary parasitoid with a parasitism rate of about 13.5% and S. ochus is a gregarious parasitoid with a parasitism rate of about 21.2%. A revised key to Oriental and Palaearctic species of Metapelma Westwood and a key to the species of the Spathius labdacus-group are provided.

introduction Symplocos stellaris Brand (Symplocaceae) is common landscape ornamental tree in South China. It is popular for its beautiful clusters of small white flowers in the spring. In addition, the wood is made into kitchen tools, furniture, etc., the seed oil is used to make soap, and the leaves and roots are used in traditional Chinese medicine. During recent investigations of woodborer biodiversity and their natural enemies in Guizhou Province, South China, we found a beautiful but little-known beetle, Coraebus cavifrons Descarpentries & Villiers (Coleoptera, Buprestidae) (Fig. 1D), which feeds on this tree and can cause serious damage (Fig. 1A). According to our investigation, this pest infests healthy trees rather than stressed trees. It bores into the main trunk making long longitudinal galleries (Fig. 1C). In the worst observed instance, the whole trunk was crowded with galleries bored by dozens of individual larvae (Fig. 1B). The pupal chambers are constructed about 5-10 mm under the bark in the xylem and close each other. The shape of pupal chamber is elongate-oblong. The young pupa is yellow but turns blue before emergence.
Coraebus cavifrons was described based on one female from Tonkin, northern Vietnam (Descarpentries and Villiers 1967), and nothing new has been reported about it except for the occurrene records by Bellamy (2008) in several provinces of southern China (Zhejiang, Fujian, Guangdong, Hainan, Sichuan). Here, we newly report this species from Zunyi City in Guizhou Province, and, more importantly, for the first time we report Symplocos stellaris as its host plant.
During our investigations on the biology of C. cavifrons, two parasitoid species belonging to different families of Hymenoptera were discovered parasitizing the buprestid larvae.
One of the parasitoid species belongs to Metapelma Westwood (Eupelmidae). Members of this genus are solitary parasitoids, with one larva parasitizing a single host larva (Yang 1996). Prior to this study, 38 valid species were reported (Noyes 2019), including 11 species from the Oriental region, five species from the Palaearctic region (including one extinct species from Baltic amber), 13 species from the Afrotropical region, six species from the Australian region, two species from the Nearctic region, and one species from the Neotropical region. In the Palaearctic region, Yang (1996) described two species parasitizing bark beetles in Beijing, China. The specimens found on C. cavifrons belong to M. beijingense Yang despite some minor differences with the original description by Yang (1996). The detailed redescription is given below, including the observed variation.
The second discovered parasitoid belongs to Spathius Nees (Braconidae), which is a huge cosmopolitan genus of the subfamily Doryctinae. The genus includes about 425 described species, of which 299 are known from the Oriental region and 91 from the Palaearctic region (Nixon 1943;Belokobylskij 2003;Chen and Shi 2004;Belokobylskij and Maeto 2009;Tang et al. 2015;Yu et al. 2016). Nixon (1943) tried to arrange the species in almost 40 species groups to facilitate identification. Our specimens belong to the S. labdacus species-group in the sense of Nixon (1943). Prior to this study, eight species have been included in this group, including three species occurring in China.

Survey methods
Dying Symplocos trees were cut down and cleaned of all small branches because borers are only present in the tree trunk. The trunks were cut into logs of 50 cm length. Each log was dissected and all parasitized hosts were collected and reared individually in vials (with diameter 12 mm and length 50 mm) in the laboratory at 25 °C and 65%-85% humidity. After the parasitoids emerged, they were collected, killed, and glued to triangle cards for taxonomic study. Some newly killed specimens were used for imaging. The parasitism rates were based on the number of beetles found in these dissected logs.

Identification and photography
The parasitoid specimens were examined with a Nikon SMZ1500 stereomicroscope, and redescription of the parasitoids is based on naturally dried specimens. Photographs of fresh specimens of all the species were taken with a UV-C Optical Totally focused System (Beijing United Vision Technology Co. Ltd.) mounted on an Olympus CX31 microscope. Terminology follows Nixon (1943), van Achterberg (1993) (Spathius), and Gibson (2009) (Metapelma). Measurements were obtained using a calibrated micrometer. Specimens are deposited in Insect Museum, Chinese Academy of Forestry, Beijing, China, except for three specimens of S. ochus deposited in Naturalis Biodiversity Center, Leiden, the Netherlands, and two specimens of S. ochus deposited in Shanxi Insect Herbarium, Institute of Plant Protection, Shanxi Academy of Agricultural Sciences, Taiyuan

Species determination and development of keys
The identification of Metapelma beijingense is based on the key provided by Narendran et al. (2013), the original description of M. beijingense (Yang 1996) and its type series. According to personal communication with Dr Gary Gibson (Agriculture and Agri-Food Canada, Canadian National Collection of Insects and Arachnids) the main morphological differences between the types and the reared specimens belong to intraspecific variation. We describe the Guizhou population to distinguish it from the holotype (Beijing population) and to facilitate future research on this species. The Guizhou population may be in the process of speciation considering the distinct differences. The key to Oriental and Palaearctic species of Metapelma provided is based on Narendran et al. (2013), to which M. beijingense, M. zhangi Yang, and M. nobilis (Förster) have been added. The inclusion of the latter species is based on the original description and additional published information only (Yang 1996). Spathius ochus Nixon was identified based on Nixon's (1943) original description, Chao's (1957) redescription, and the keys by Belokobylskij and Maeto (2009) and Tang et al. (2015). The key to species of the Spathius labdacus-group provided here is based on Belokobylskij and Maeto (2009) and Tang et al. (2015), plus original descriptions and collected specimens.  Recognition. Metapelmais one of four extant genera described for Neanastatinae (Eupelmidae). The genus is differentiated from the other three genera using the keys by Gibson (1995Gibson ( , 2009), but indivduals can be recognized uniquely by the following combination of characters: head lenticular with short scrobe above each torulus but scrobes not united into a common scrobal depression ( Redescription (based on specimens from Guizhou; differences between Beijing and Guizhou populations are shown in the key below).
Color. Body generally dark with metallic tints (Fig. 3A). In frontal view, head with lower half of frons and entire face, gena, and occiput bright metallic green, but upper half of frons and vertex with slight red tint (Fig. 3C). Propleuron, apical half of mesopleuron in lateral view with metallic green tint (Fig. 4A); V-shaped sulcus on mesonotum, apical half of axilla, metanotum bright metallic green (Fig. 3E), basal 1/3 of 1 st gastral tergite in dorsal view ( Figure 4C), and basal 2/3 of visible ovipositor sheath in lateral view with metallic green tint (Fig. 4B). Lateral stripe on metacoxa and tergites 2-5 in lateral view varies in color from base to apex in metallic red, golden and green, successively (Fig. 4B). Apical spur and basal 1/2 of 1 st tarsal segment of mid leg white to pale yellow. Basal 1/5 of mesofemur yellow to brown. Outer margin of metatibia with basal 0.3-0.4 length white (Fig. 4C). Fore wing subhyaline, infuscation paler posteriorly and extend beyond medial fold toward posterior margin, as well as along medial fold and along posterior margin of discal area basally; veins and setae dark brown; hind wing subhyaline (Fig. 4E). Head. Head with sparse long white setae. In dorsal view, head width 1.75× its median length, eye occupy 1/3 of maximum width in dorsal view (Fig. 3B), pilose, nearly as long as head; ocelli small, POL: OD: OOL = 2: 3: 4. In frontal view, head as wide as high; height of eye 0.6× and mandible width 0.3× head width; minimum width of frons 1/3 head width; face, gena, frons and vertex with tiny transverse strigate-rugose stripes. Scrobes short, no more than 1/3 length of scape. In lateral view, malar sulcus straight, 1/2 eye height. Mandibles bidentate (Figs 3B, C, 4A). Dorsal margin of torulus slightly above lower ocular line, but its ventral margin distinctly below it (Fig. 3C); antenna with a very short radicle, almost not evident; scape 3.8× its maximum width, 3× length of pedicel, 4.7× length of anellus, and 2.2× length of 1 st funicular segment; pedicel 0.8× of length 1 st funicular segment; 1 st funicular segment 0.9× length of 2 nd funicular segment (Fig. 3D).
Wings. Fore wing extending beyond apex of metasoma to about middle of visible part of ovipositor sheath; basal cell bare but disc with dense setae except for slender, oblique bare band behind parastigma; submarginal vein 2.3× length of marginal vein, marginal vein 0.7× length of postmarginal vein and 2.25× length of stigmal vein; R fold and Cu fold visible. Hind wing about 0.8× as long as fore wing (Fig. 4E).
Remarks. Metapelma beijingense is a solitary parasitoid with a parasitism rate of about 13.5%, based on seven individuals together with 34 buprestid pupae. The ratio of females to males is six. Metatibia with a dorsal forked expansion (Mani et al. 1973: fig. 38E)

Spathius labdacus-group (sensu Nixon 1939)
Main history of S. labdacus-group. Nixon (1939) described the first species of this group, S. labdacus from Coimbatore in South India, with its host as the cotton stem weevil, Pempheres affinis (Faust). Nixon (1943)  Recognition. Body slightly depressed to distinctly depressed dorso-ventrally. Eyes obliquely placed, transverse diameter usually longer than length of temple. Gena smooth. Vertex, face, and temple usually sculptured. Pronotal carina free, distinct, prominent or sharp. Setae on mesoscutum sparse and erect, posteriorly mesoscutum always with raised rugosity. Propodeum elongate, medio-longitudinal carina 0.5-1.0× anterior fork of areola. Forewing strongly infuscated, subbasal cell distinctly constricted just beyond middle and crossed by a broad subhyaline fascia at its narrowest part, base of marginal cell with an oblong subhyaline spot, a broad subhyaline fascia from base of pterostigma to posterior margin of wing. Hind coxa simple, hind femur narrowed basally. First metasomal tergite densely rugulose with short rugulae, tergite 2+3 evenly shagreened all over. Ovipositor sheath less than, equal to or longer than metasoma.
Head. Median length 0.8× of its width in dorsal view; with transverse striae. Length between posterior margin of lateral ocellus and occipital carina 1/2 of length of head in dorsal view; occipital carina distinct, median portion concave, reversed V-shaped; length of eye: length of temple in dorsal view = 11: 14 (Fig. 6B); eyes rather large (Fig. 7A); OOL: OD: POL = 3: 2: 1.Width of head 1.2× height in anterior view and width of face 1.1× height of eye; clypeus with transverse thin carina, face covered with sparse white setae; malar space 0.4× height of eye; height of clypeus 0.4× its width, exterior margin of clypeus slightly concave; length of maxillary palp 0.6× head width, 1.5× height of eye and 3.2× length of malar space; hypoclypeal depression deeply concave (Fig. 6C); antennae 36-segmented, scape 1/3 length of first flagellar segment, and 0.65× its maximum width; first flagellar segment 7.5× its maximum width, 1.3× as long as second segment; last antennal segment acute (Fig. 6D).
Remarks. The mesosoma is variably depressed; usually 2.4-2.9× longer than high, but in some specimens up to 3.7-4.0×. Obviously, this character is useless to separate S. tereus Nixon, 1943. Therefore, we agree with Chao (1957) that the latter cannot be separated. Spathius ochus is a gregarious koinobiont ectoparasitoid like most other Spathius, each buprestid larva can feed 3-9 individuals. From one tree 11 borer larvae were parasitized by 42 individuals of S. ochus, together with seven borer larvae were parasitized by M. beijingense and 34 live buprestid pupae, resulting in a parasitism rate of about 21.2% for S. ochus. The sex ratio is about 14:1 (71 females to 5 males).
The very interesting phenomenon of synparasitism (Tobias 2007) is shown in Figure 2D; one individual of M. beijingense and four individuals of S. ochus were together parasitizing the same woodborer larva. Likely, these two ectoparasitoid species laid their eggs near the host at about the same time, and the larvae did not start fighting each other because the host was large enough to avoid severe food competition. Of course, this is only circumstantial evidence that is in need of corroboration.
The species is very similar to S. parochus Belokobylskij & Maeto and can be recognized with the key below. First metasomal tergite 1.5× as long as propodeum (Fig. 7A), medio-longitudinal carina of propodeum about as long as anterior fork of areola (Fig. 6E)

Discussion
During the investigation, we found that the host C. cavifrons boring in Symplocos stellaris has only one generation per year in Zunyi, Guizhou Province. From the end of May to early June, this buprestid begins emerging and it will last for about 2 weeks. We chose 20-30 days before its emergence to cut and dissect logs when there are no emergence holes of parasitoids in the trunk. The best time for collecting these parasitoids proved to be the first week of May. We guess that both two parasitoids are at least oligophagous, because there are no C. cavifrons larvae available for laying eggs after their emergence. However, they may search for another host to lay their eggs. Both parasitoids seem to have two generations per year in Zunyi, but this needs to be further investigated.
Combined study of the stressed tree (host), the woodborers (pest), the parasitoids (natural enemies) and their relationships is interesting, and biological traits may be useful in their taxonomy. For the identification of Metapelma beijingense we used only morphological and biological evidence, but a molecular data analysis study with fresh material of the Beijing and Guizhou populations might be helpful for the identification and determination of the systematic status of these two populations.