Trissolcus is a cosmopolitan genus in Platygasteridae (Hymenoptera) and a relatively large group including 161 described species worldwide (69 species in the Palearctic region) (Johnson, 1992). Most species of the genus are egg parasitoids of economically important pests in Pentatomoidea (Hemiptera). Therefore, they are generally considered as potential agents for biological control, and some of them had been used for biological controls in practice (e.g. Johnson, 1984; Corrêa-Ferreira and Moscardi, 1996).

Trissolcus looks very similar to Telenomus but can be readily distinguished by 1) female antennal clava compact and composed of six segments, 2) frons sculptured throughout, and with preoccellar pit located below the median ocellus, 3) scutellum with surface sculpture, 4) second metasomatic tergite wider than long, and 5) eyes seeming glabrous under low magnification (Masner, 1980; Johnson, 1991). The two genera have different biological characteristics like that the former almost exclusively parasitizes the eggs of Pentatomoidea, and the latter predominantly the eggs of Lepidoptera (Ryu and Hirashima, 1984).

To date six species of Trissolcus and their hosts have been known from Korea (Yasumastu, 1975; Ryu and Hirashima, 1984). In the present paper, we add the Samurai wasp, T. japonicus (Ashmead) as an egg parasitoid of Halyomorpha halys (Stål) (Hemiptera: Pentatomidae) to the Korean insect fauna, and newly confirm that T. itoi Ryu and T. nigripedius (Nakagawa) parasitize the eggs of Eurydema gebleri Kolenati and Dolycoris baccarum L., respectively.

Materials and Methods

Surveys for natural insect enemies of pentatomid pests were conducted mainly in soybean fields and deciduous forests in Jeonnam province, Korea from May to September, 2016. Egg masses of stink bugs laid on the underside of leaves were collected and kept individually in plastic dishes (10 cm in diameter and 4 cm in height). The egg masses were reared in the laboratory at room temperature (≒25°C) and checked daily until parasitoids emerged from the host eggs. Leica EZ4 HD binocular stereo microscope (Leica Microsystems, Wetzlar, Germany) was used for imaging and identification the parasitoids collected and CombineZM (Free software released from http://www.hadleyweb.pwp.blueyonder.co.uk/; UK) for photo-stacking. The voucher specimens are deposited in the insect collection room of Sunchon National University, Suncheon, Korea.

The key to species by Ryu and Hirashima (1984) is referred for species identification of the parasitoids, and descriptive terminology for morphological characters is taken from Talamas et al. (2015). Acronyms used in the text are as follows: JN, Jeollanam-do (Province); JARES, Jeonnam Agricultural Research & Extension Services; SCNU, Sunchon National University; TL, type locality.

Results

Trissolcus japonicus (Ashmead) 썩덩큰검정알벌(신칭) (Figs. 1-a, b; 2-a, b; 3-a, b; 4-a, b, c)

Dissolcus japonicusAshmead, 1904: 73 (TL: Japan).

Asolcus plautiaeWatanabe, 1954: 18, 22 (TL: Japan).

Trissolcus halyomorphae Yang, in Yang et al., 2009: 40 (TL: China).

Diagnosis. Body length: 1.3-1.4 mm (♀) and 1.2 mm (♂).

♀: Black. Antennae dark brown to black except for scape and pedicel which are yellowish brown. Legs yellowish brown with coxae black and femora infuscated. Antennae 11-segmented; pedicel shorter than 1^st funicular segment; 2^nd funicular segment distinctly longer than broad. Oribital furrow present. Mesoscutum with notauli sharply defined posteriorly. Scutellum smooth and shining. ♂: Antennae 12-segmented, and brown except for sixth to tenth flagellar segments which are tinged with dark brown. Legs yellowish brown except for coxae and hind femora which are somewhat infuscated.

Materials examined. 8♀♀, 1♂, emerged at 4-5. VIII. 2016, at room temperature from an egg-mass (28 eggs) of Halyomorpha halys (Pentatomidae) collected at 26. VII. 2016, Suwol-ri, Gwangui-myeon, Gurye, JN (SCNU); 8♀♀, 1♂, emerged at 6. VIII. 2016, at room temperature from egg-mass of Halyomorpha halys (Pentatomidae) collected at 2. VIII. 2016, Woesan-ri (Jicho), Sandong-myeon, Gurye, JN (JARES).

Hosts.Plautia stali Scott, P. splendens Distant, Elasmucha putoni Scott, Halyomorpha halys Stål.

Distribution. Korea (South), Japan (Kyushu, Honshu, Shikoku), China (Beijing, Shandong, Hebei).

Remarks.Talamas et al. (2015) examined four Korean specimens (3♀♀ and 1♂) that kept in the Smithsonian National Museum of Natural History, Washington DC, USA. We have given this species a Korean name in this paper.

Trissolcus itoi Ryu, 1984 천마큰검정알벌 (Figs. 1-c, d; 2-c, d; 3-c, d; 4-d)

Trissolcus itoi Ryu in Ryu and Hirashima, 1984: 37, 52 (TL: Japan).

Diagnosis. Body length: 0.9-1.0 mm (♀) and 0.9 mm (♂).

♀: Black. Antennae dark brown to black; basal and apical parts of scape tinged with yellowish brown and club somewhat darker. Legs brown with coxae black, femora remarkably infuscated. Antennae 11-segmented; pedicel about as long as 1^st funicular segment. Mesoscutum without notauli. Scutellum strongly sculptured with irregular wrinkles as in posterior part of mesoscutum. ♂: Antennae 12-segmented, and brownish to dark brown. Legs brownish except for femora which are infuscated; hind femora darker than other femora.

Materials examined. 6♀♀, 1♂, emerged at 2. VIII. 2016, at room temperature from an egg-mass of Dolycoris baccarum (Pentatomidae) collected at 28. VII. 2016, Sindae-ri, Haeryongmyeon, Suncheon, JN (SCNU); 5♀♀, 1♂, emerged at 15. VIII. 2016, at room temperature from an egg-mass of Dolycoris baccarum (Pentatomidae) collected at 6. VIII. 2016, ditto (SCNU).

Hosts.Homalogonia obtusa (Walker), Elasmucha putoni Scott, Eurydema gebleri Kolenati.

Distribution. Korea (Central, South), Japan (Honshu, Shikoku).

Trissolcus nigripedius (Nakagawa, 1900) 검은다리큰검정 알벌

(Figs. 1-e, f; 2-e, f; 3-e, f; 4-e)

Asolcus nigripediusNakagawa, 1900: 17 (TL: Japan).

Diagnosis. Body length: 1.1-1.2 mm (♀) and 1.0 mm (♂).

♀: Black. Antennae dark except for junction of scape and apex of pedicel which is somewhat yellowish brown. Legs yellowish brown except for coxae which are black; femora sometimes infuscated. Antennae 11-segmented; pedicel as long as 1^st funicular segment. Mesoscutum without notauli. Scutellum weakly tessellate, much less strongly sculptured than in posterior part of mesoscutum. Mediotergite I with distinct longitudinal striae. ♂: Antennae 12-segmented, and brownish black; sixth to tenth flagellar segments somewhat darker than other segments.

Materials examined. 22♀♀, 3♂♂, emerged at 24. VIII. 2016, at room temperature from an egg-mass of Nezara antennata (Pentatomidae) collected at 17. VIII. 2016, Namsandong, Gwangsan-gu, Gwangju, JN (SCNU); 21♀♀, 2♂♂, emerged at 5. VIII. 2016, at room temperature from an eggmass of Nezara antennata (Pentatomidae) collected at 2. VIII. 2016, Jigok-ri, Bonggang-myeon, Gwangyang, JN (SCNU).

Host.Dolycoris baccarum L.

Distribution. Korea (South), Japan (Kyushu, Honshu, Hokkaido, Shikoku).

Discussion

The most common stink bug are serious agricultural pests that has been readily causing damage to crops. To obtain their food, stink bugs use their stylets to pierce the plant tissue in order to extract the plant fluids. The most common signs of stink bug damage are pitting and scarring of the fruit, leaf destruction, and a mealy texture to the harvested fruits and vegetables. In most cases the signs of stink bug damage makes the plant unsuitable for sale in the market as the insides are usually rotten. They are a threat to soybeans, ornamental trees, shrubs, vines, weeds, and many cultivated crops. In Korea, several stink bugs such as Halyomorpha halys (Stal) and Plautia stali Scott have recently become serious pests of fruits such as persimmons and citruses (Kim et al., 2014; Park et al., 2015).

Many species of Trissolcus are egg parasitoids of economically important pests in the superfamily Pentatomoidea (Hemiptera) in Korea and neighboring countries (Table 1). Three species of the genus found in the present work are confirmed as egg parasitoids of Halyomorpha halys (Stål), Eurydema gebleri Kolenati and Dolycoris baccarum L. that are severe pests to many cultivated crops. It is expected that these solitary endoparasitoids in the many pentatomid eggs are developed as excellent biological control agents through the investigation of biological characters including sex ratio, developmental time, parasitism rate, mass rearing, and so on.

Especially, the brown marmorated stink bug (BMSB), Halyomorpha halys Stål (Hemiptera: Pentatomidae), was accidentally introduced into the United States from native area to China, Japan, Korea or Taiwan. The first documented specimen was collected in Allentown, Pennsylvania, in September 1998. A decade after its introduction into the United States, the BMSB has been detected in 39 US states as well as Cananda, Switzerland, Germany, France, and Italy, and has been intercepted in New Zealand (Xu et al., 2014). Due to the potential non-target effects of controlling the BMSB with pesticides, USDA conducted foreign exploration in Asia for natural enemies of BMSB in South Korea and China (Xu et al., 2014). As the result, two parasitoid, Trissolcus japonicus (Ashmead) and Trissolcus cultratus (Mayr) are considered with potential as classical biological control agents, and are presently studying the host preferences of these parasitoids and the ability of these parasitoids to successfully develop in BMSB eggs (Talamas et al., 2015).