First record of the predatory stink bug species Picromerusgriseus (Dallas) (Hemiptera, Heteroptera, Pentatomidae, Asopinae) in Japan, with an illustrated key to the Japanese species of the genus Picromerus Amyot & Serville

Abstract Background The predatory stink bug genus Picromerus Amyot & Serville, 1843 (Hemiptera, Heteroptera, Pentatomidae, Asopinae) comprises 11 species found in the Northern Hemisphere. In Japan, two species have been recorded to date. However, an easy-to-understand identification method, such as an illustrated key, is lacking. Currently, Picromerusgriseus (Dallas, 1851) has been recorded in Bangladesh, Bhutan, China, Indonesia, Myanmar, Pakistan and Taiwan, but not in Japan. New information Picromerusgriseus was recorded in Japan for the first time, based on a single individual collected from grasslands around the fields of Ishigaki Island of the Ryukyu Islands, which belong to the Oriental Region. This discovery represents the easternmost record of the species. An illustrated key to the species of Picromerus occurring in Japan is also provided.

The genus Picromerus Amyot & Serville, 1843 (Asopinae) comprises 11 species from the Northern Hemisphere: P. bidens; P. brachypterus Ahmad & Önder, 1990;P. conformis Herrich-Schäffer, 1841;P. elevatus Zhao, Liu & Bu, 2013;P. fasciaticeps Zheng & Liu, 1987;P. griseus (Dallas, 1851);P. lewisi;P. nigridens (Fabricius, 1803); P. orientalis Rishi & Abbasi, 1973;P. pseudobidens Ahmad & Önder, 1990;and P. viridipunctatus Yang, 1934(Thomas 1994, Rider 2006, Zhao et al. 2013. In Japan, two species, P. bidens and P. lewisi, were only recorded from Japan proper and its surrounding islands, which belong to the Palearctic Region (Ishikawa 2016), while no species have been recorded from the Ryukyu Islands, whose central and southern parts belong to the Oriental Region. These two species are so similar that it is difficult to distinguish them at first glance. Although some field guides and pictorial books dealing with Japanese Asopinae have been published (e.g. Yasunaga et al. (1993), Takai and Ishikawa (2012)), no identification key illustrating the diagnostic characteristics of the two species, based on the Japanese populations, has been provided for easier and more accurate identification. Therefore, field surveys on the Ryukyu Islands and publication of an illustrated key for Japanese Picromerus are needed to elucidate Asopinae in Japan and make this genus known to the public.
In autumn, 2022, the second author collected a single individual of an indeterminate species of Picromerus from the grasslands around the fields of Ishigaki Island of the Ryukyu Islands (Oriental Region). After the first author examined its morphological characteristics, we concluded that it belonged to P. griseus, which is currently known to occur in Bangladesh, Bhutan, China, Indonesia, Myanmar, Pakistan and Taiwan (Thomas 1994, Rider 2006, Zhao et al. 2013, Zheng and Lin 2013. Herein, we report P. griseus in Japan for the first time, representing the easternmost occurrence of this species. In addition, we provide an illustrated key for all three Japanese species of Picromerus.

Materials and methods
Morphological characteristics of dried specimens were observed using a stereoscopic microscope (SZ60; Olympus, Tokyo, Japan). To examine the genital characteristics, the male terminalia were removed from the body after softening the specimens in hot water. The removed genital capsule was immersed in hot 15% potassium hydroxide (KOH) solution for 5 min. For further observations, parameres were removed from the genital capsule soaked in 99% ethanol. Male genitalia were preserved in small polyethylene vials containing a 50% glycerine and 50% water solution. A polyethylene vial was mounted on the pin with the specimens. The specimens were photographed using a digital microscope (Dino-Lite Premier M; Opto Science, Tokyo, Japan) and a compact digital camera (Tough TG-6; Olympus) and image stacks were processed using Adobe Photoshop 2021 ver. 22.5.1 (Adobe Inc., CA, USA) when using the digital microscope. Measurements were obtained using a stereoscopic microscope equipped with an ocular grid and a digital microscope. Morphological terms were assigned as described by Tsai et al. (2011).
The single specimen of Picromerus griseus examined in the present study was deposited at the Laboratory of Entomology, Faculty of Agriculture, Tokyo University of Agriculture, Kanagawa, Japan (TUA). Specimens of the Japanese species of Picromerus that were used for creating the identification key and for comparison with P. griseus were deposited in the Entomological Laboratory, Faculty of Agriculture, Kyushu University, Fukuoka, Japan (ELKU) and TUA.  Breddin, 1902-Breddin (1902: 96, new species and description; Thomas (1994): 192, synonymised with Picromerus griseus. Gaedike (1971): 100, designation of lectotype.

Diagnosis
Picromerus griseus can be distinguished from other species of the genus using a combination of the following characteristics: head, pronotum, scutellum and femora uniformly brown (Fig. 1a, b); humeral angle of pronotum strongly protruding laterad, acute at apex, posteriorly with a distinct subapical prominence (Fig. 2a); posterior margin of genital capsule weakly curved inwards in middle part (Fig. 4a); and paramere weakly curved inwards in apical part in dorsal and caudal views, distinctly concave along inner margin in dorsolateral view (Figs 3a, 5a, 6a).
The discovery of Picromerus griseus from Japan represents the easternmost record of the species.

Biology
Picromerus griseus was collected from grasslands surrounding fields in Japan. In Japan, adults are collected in November; however, the nymphs are unknown.

Taxon discussion
The specimen recorded above (Figs 1a, b, 2a, 3a, 4a, 5a, 6a) matched the photographs and descriptions (Dallas 1851, Zhao et al. 2013 of Picromerus griseus in terms of morphological characteristics, including the humeral angle and male genitalia. The Japanese specimen was identified as P. griseus using a key for the East Asian species of Picromerus (Zhao et al. 2013), based on its morphological characteristics. However, the colouration of the connexivum of the Japanese specimen (yellow and black) did not match the above-mentioned key (entirely black). To the best of our knowledge, Japanese populations of P. bidens and P. lewisi show a high degree of Habitus of three species of Picromerus from Japan. First record of the predatory stink bug species Picromerus griseus (Dallas) ...
intraspecific variation in the colouration of the connexivum (yellow and black to entirely black) (Figs 1c, d, 7). In conclusion, we did not use the colouration of the connexivum as a diagnostic characteristic of Picromerus and identified the Japanese specimen as P. griseus, based on the shape of the humeral angle and male genitalia.  (Thomas 1994, Kanyukova and Marusik 2006, Rider 2006, Takai and Ishikawa 2012, Aukema et al. 2013, Zhao et al. 2013, Ishikawa 2016, Roca-Cusachs et al. 2020. Heads and pronota of three species of Picromerus from Japan, dorsal view.  (Thomas 1994, Rider 2006, Zhao et al. 2013, Zheng and Lin 2013.

Key to the species of Picromerus occurring in Japan
The key below is based on morphological characteristics and colouration that are stable in the Japanese populations and the distribution of three species in Japan.