Notes on braconid wasps (Hymenoptera, Braconidae) parasitising on Agrilus mali Matsumura (Coleoptera, Buprestidae) in China

Abstract Braconid parasitoids reared from Malus sieversii and Malus domestica trees in NW China infested by Agrilus mali Matsumura (Coleoptera, Buprestidae) are illustrated and discussed. Six species were found parasitising Agrilus mali in NW China, namely, Atanycolus ivanowi (Kokujev) (Braconinae), Doryctes undulatus (Ratzeburg), Pareucorystes varinervis Tobias, Polystenus rugosus Foerster, Spathius sinicus Chao, and Spathius brevicaudis Ratzeburg (Doryctinae). All listed species are newly recorded parasitoids of Agrilus mali. Pareucorystes varinervis and Spathius brevicaudis are new records for the Chinese fauna, but Spathius brevicaudis has been recorded from Taiwan before. Both sexes of Spathius brevicaudis are redescribed here to allow inclusion in the recent revision of the Chinese Spathius species. An identification key to the six braconid parasitoids of Agrilus mali in NW China is provided.


Introduction
The apple buprestid, Agrilus mali (Coleoptera: Buprestidae), is considered to be a dangerous pest in China of apple trees. Recently, a large area of wild Malus sieversii has been killed by Valsa mali Miyabe et Yamada and Cytospora mandshurica Miura, after infection by A. mali ( Figure 1A, B, C). Its larvae feed under the bark on the phloem, which weakens the nutrient transportation and results in depressed and dark-coloured dead bark over the affected areas ( Figure 1D). Heavy infestations result in dead branches and, eventually, in the death of the whole tree ( Figure 1D). Besides apple trees, also other fruit trees (e.g., crab-apple, pear, peach, and cherry) are attacked.
Adult, egg and larval stages of Agrilus mali are shown in Figure 2; A. mali is widely distributed in Asian Russia, Japan, the Korean peninsula, and north China, and is a common species in apple orchards. In 1995 it was reported attacking the endangered wild apple (Malus sieversii) in Xinyuan County, Ili Kazakh Autonomous Prefecture, Xinjiang Province (Wang et al. 1995). This tertiary relict is the sole ancestor of most cultivars of the domesticated apple, Malus domestica. With its high genetic diversity, the forest is seen as the most significant national gene pool of apple, even to the world. Recently, A. mali became the major pest of M. sieversii and 48.6% of the Xinjiang wild apple forest was damaged, over 4866.67 m 2 , and in some areas like Xinyuan County and Gongliu City, most trees died.
Using organic insecticides has allowed the control of A. mali in orchards, but in the wild this is ineffective as the trees are scattered over a vast area. The damage of A. mali is erratic, wide-spread, and frequent spraying of chemicals will be another threat to the vulnerable local ecosystem. Therefore, biological control is considered as the best countermeasure and a survey about the natural enemies of A. mali has been carried out in recent years.

Materials and methods
This study is based on specimens retained in the Entomological Museum of Chinese Academy of Forestry (Beijing, China) and the Naturalis Biodiversity Center (Leiden, the Netherlands). Natural enemy surveys of A. mali were conducted in Xinjiang, Shaanxi and Qinghai Provinces from 2011 to 2018. Trunk bark of stressed trees was peeled off to search for A. mali larvae and associated parasitoids. The larvae and possible parasitoid cocoons were placed singly in vials (12 mm in diameter and 75 mm in length), each containing a piece of filter paper dipped in distilled water for moisture. The vials were plugged tightly with sterilised cotton and maintained at 22-25 ℃ in the rearing room. Parasitoid cocoons were successively reared to adults. Specimens were examined with a Nikon SZH 1500 stereomicroscope. Photographs were taken with an Olympus CX31 microscope with the UV-C Optical Totally Focuses System developed by Beijing United Vision Technology Co., Ltd. Terminology and measurements follow van Achterberg (1993).
Genus Atanycolus are ectoparasitoids of wood borers, and are usually solitary ( Figure 3A), very rarely there are two individuals on one host. A. ivanowi is widely distributed in the Central and South Palaearctic region. This parasitoid is mostly found on the mature larva of A. mali. When full grown, the larva will spin a cocoon in the gallery of A. mali, ca. 10 days later the adult will emerge by biting a small round hole in the bark.  We found two generations per year of A. ivanowi in our experimental fields. The first generation lasts ca. 40 days from late March to May, and many adults can be seen during May to July. The larvae of the overwintering generation can be seen before August and later only cocoons can be found. Obviously, this parasitoid overwinters in the cocoon stage.
In total, we collected 86 females and 78 males of A. ivanowi in 2015 from one site (Xinjiang Province, Gongliu City, Mohuer County), which shows that the approximate ratio of female and male is 11/10. The natural parasitisation rate is approximately 26.7% and according to our investigations, A. ivanowi has the maximum population on A. mali. Obviously, A. ivanowi should be protected in order to increase the biodiversity of the forests, in which Malus sieversii is the main component.      Remarks. According to the detailed redescription of Japanese specimens (Belokobylskij et Maeto, 2009) and the key to this genus for China (Belokobylskij et al. 2012), our specimens from Xinjiang are quite the same, although there are still some tiny differences present, e.g., hind femur much stronger, 2.8 times as long as the maximum width in lateral view (vs. hind femur 2.9-3.2 times longer than wide in Japan); female body colour is stable reddish brown and male body is totally black (vs. reddish brown to almost black in Japan); extend of sculpture of third tergite varies from 1/4 to 1/2 basally, but semi-circular striation is always present, which is the main character to separate it from D. striatellus (Belokobylskij et Maeto 2009). After extensive comparison, we consider the differences to be intra-specific variation of D. undulatus. Agrilus mali is newly reported as host of D. undulatus. On average, a host larva is parasitised by two larvae of D. undulates as ectoparasitoid ( Figure 3C, D, E, F). Remarks. This species is mainly specialised on Agrilus species; if the population density could be enlarged, it would be a potential biological control agent. The species is also a new to the Chinese fauna, and a new parasitoid of A. mali.

Spathius sinicus
Remarks. This species is widely distributed in China and A. mali is the first reported of a host. It is peculiar that during several years of investigation, only 6 individuals have been found on A. mali at one tree, which indicates that it is an occasional parasitism.
Colour (Female). Head dark brown, basal half of antenna yellow, its apical half brown; mesoscutal lobes, scutellum dark brown, pronotum brown; metasoma dark brown except first and second metasomal segments, basal portion of third metasomal segment yellow; fore wing partly weakly darkened; legs yellow ( Fig. 15A, B). Head. Median length 0.7 times of its width in dorsal view; vertex broad, surface rough, with low (fine) sculptures and rare white setae (Fig. 15D); length between posterior margin of lateral ocellus and occipital carina half of head length in dorsal view; occipital carina median portion concave, reversed V-shaped (Fig. 15D); length of eye: length of temple in dorsal view = 4.3: 4; eyes small, slightly protruding bilaterally; OOL: OD: POL = 3: 1: 1.8; ocellar area distinctly differentiated, slightly swollen; width of head 1.1 times of height in front view, distance between eyes 1.3 × height of eye (Fig. 15C); face distinctly and irregularly striate, covered with white setae; malar space 0.6 × height of eye; height of clypeus 0.4 × its width, exterior margin of clypeus straight; basally mandible broad, apical portion black, blunt and robust; hypoclypeal depression deeply concave; antenna 28 segmented, scape twice length of first flagellar segment, and twice its maximum width; first flagellar segment 5.3× its maximum width, as long as second flagellar segment; last antennal segment acute apically.
Wings. Fore wing with faintly brown along veins in basal half, apical half of fore wing largely subhyaline, its length 3.2 times of width; pterostigma 4.0 times as long as its maximum width; vein 1-R1 1.3 times of pterostigma, vein r originates at middle of pterostigma; vein SR1 8.5 times as long as vein r and straight; vein r nearly 1/4 of  in small specimens) and basal pale spot of pterostigma weakly differentiated (basal pale spot of pterostigma distinctly differentiated in dark specimens). Although, the differences are minor, all related to the fore wing and sometimes gradual, we prefer to recognise S. brevicaudis as separate species till molecular data will become available. The main reason for this is that reared series show these minor differences to be stable enough for separation both species in north-western Europe.