During the research period (2007–2017), we observed a total of six adults during four consecutive years: one male in 2014, one female in 2015, one male in 2016, and one male and two females in 2017 (Figure 4A–F, Tables 1–2).

The body length (from apex of mandible to apex of elytra) of the male found on 19 August 2014 was 88.0 mm (Figure 4A). At the time of finding, the right elytron was missing, and much of the left elytral pubescence was missing. The individual was in a state of exhaustion and died the next day.

The body length of the female found on 27 July 2015 was 78.0 mm (Figure 4B). At the time of its finding the pronotum was severely damaged, the left elytron was missing, and much of the right elytral pubescence was missing. The mesonotum was partially detached and the individual was gathered by numerous ants, dying within a few hours of discovery.

The body length of the male found on 10 August 2016 was 98.0 mm (Figure 4C). At the time of finding, there was a crack running longitudinally in its pronotum, much of the pubescence was missing, and some claws were also missing. The individual was in a state of exhaustion and died the next day.

The body length of the female found on 20 July 2017 was 78.0 mm (Figure 4D). At the time of finding, there were no particularly damaged parts; some of the pubescence was missing, but the insect was moving actively. After artificial egg collection, the individual died on 31 July 2017.

The body length of the female found on 11 August 2017 was 75.6 mm (Figure 4E). At the time of finding, its right tibiae and tarsi were missing, much of the pubescence was missing, and the ovipositor was prolapsed. The individual was in a state of exhaustion and died in under an hour.

The body length of the male found on 14 August 2017 was 60.0 mm, making it the smallest of all the males discovered to date (Figure 4F). At the time of discovery, the individual was already dead; the left elytron, some legs, and the whole abdomen were missing, and there were ants gathering on the inside of the pronotum.

Figure 4. 

Adults of Callipogon relictus discovered in Gwangneung forest: A male in 2014 B female in 2015 C male in 2016 D female in 2017 E female in 2017 F male in 2017.

In the survey in Gwangneung forest, we found a total of 22 emergence holes on four species of dead tree (Carpinus cordata Blume, C. laxiflora (Siebold & Zucc.) Blume, Quercus aliena Blume, and Q. mongolica Fisch). The largest number of holes was found on C. laxiflora and as many as six holes were found on a single tree. The holes were observed at heights of 0.9–3.1 m, and six holes faced north and five holes faced northeast (Table 3). The shapes of the emergence holes were subcircular to elliptical, with most holes having a slanted elliptical shape; the mean width and height were 32.1 mm and 26.1 mm, respectively (Figure 5A–D, Table 4).

In Ussurisky nature reserve, we observed a total of 56 emergence holes, on Ulmus davidiana var. japonica (Rehder) Nakai only. The DBH of the dead trees with emergence holes was 56–130 cm, and there were as many as 18 holes on the only one tree (that could be observed with naked eyes). The emergence holes were found at various heights in the range of 1.0–24.0 m (Table 3). The shapes of the holes were very similar to those found in Korea, albeit a little larger, with a mean width and height of 33.4 mm and 28.2 mm, respectively (Figure 6A–B, Table 4).

Figure 5. 

Emergence hole of Callipogon relictus in Gwangneung forest, Korea: A–B emergence hole on Carpinus laxiflora C emergence hole on Quercus sp. D emergence hole on Quercus aliena.

Figure 6. 

Emergence hole of Callipogon relictus in Ussurisky nature reserve, Russia: A–B emergence hole on Ulmus davidiana var. japonica.

Larval feeding scars of C. relictus were mostly observed on old (DBH: ≥ 35 cm), less vigorous Carpinus spp. and Quercus spp., and were observed more commonly on standing dead trees than on fallen trees. Most of the trees with feeding scars have fungal growth (Figure 7A, B) or fungal infection around or on the surface of the feeding scars (Figure 8A, B).

The feeding scar tunnels stretched from below the bark to deep within the trunk. In general, the tunnels had a gradually widening shape oriented vertically in relation to the ground, but the shapes were highly irregular. Some tunnels were horizontal to the ground, and sometimes vertical and horizontal tunnels intersected. Larval excreta varied according to the type and maturity of the host plant and according to the age of the larvae but they were typically slightly thicker than those of the other cerambycids and similar to those of Dorcus hopei binodulosus Waterhouse, 1874 (Coleoptera: Lucanidae) (Figure 9A–D).

Two larvae were found, one in 2010 and one in 2016. The larva collected in a C. laxiflora tree on 29 July, 2010 had a length of 11.6 cm, a head width of 1.14 cm, and was found dead, infected with a pathogenic organism (Figure 10A). The larva discovered in a Q. aliena tree on 1 August, 2016 was very healthy, with a length of 10.5 cm, a head width of 1.14 cm, and a weight of 25.8 g (Figure 10B–D).

Two larval exuviae were collected, one in 2012 and one in 2016. One pupal exuvia was found in a C. laxiflora tree on 26 July 2012 (Figure 11A) and one female pupal exuvia was found in the same Q. aliena of dead tree where the larva was discovered on 1 August, 2016 (Figure 11B). In addition, in 2017, the right mandible of a larva was found in a dead C. laxiflora tree.

Figure 7. 

Emergence hole and fungus on host plant (red circle, emergence hole; blue arrow, fungus): AUlmus davidiana var. japonicaB Carpinus cordata.

Figure 8. 

Larval gallery and pupal chamber of Callipogon relictus in Gwangneung forest.

Figure 9. 

Larval excreta of Callipogon relictus in gallery: A–B larval excreta in Carpinus laxiflora C–D larval extra in Quercus aliena.

Figure 10. 

Larvae of Callipogon relictus discovered in Gwangneung forest: A larva in 2010 B–D larva in 2016.

Figure 11. 

Pupal exuvium of Callipogon relictus in Gwangneung forest: A exuvium in Carpinus laxiflora in 2012 B exuvium in Quercus aliena in 2016.

During the study period (2007–2017), we found a total of nine pupal chambers of C. relictus in three species of dead trees in Gwangneung forest (Carpinus cordata, C. laxiflora, and Q. aliena). The highest number of pupal chambers was found in C. laxiflora trees and as many as three pupal chambers were found in one C. laxiflora tree. The DBH of dead trees containing pupal chambers was in the range 39–69 cm, with a mean DBH of 57.1 cm. The pupal chambers were either ellipses with a long horizontal axis, or long tunnel shapes. They were 13–20 cm in length, located 4.5–10.0 cm deep inside the bark, parallel to the ground and perpendicular to the tree trunk (Figure 12A, B, Table 5).

In the Gwangneung forest in Korea, direct identification of individuals and signs of habitation occurred in four species of tree: Carpinus cordata, C. laxiflora, Quercus aliena, and Q. mongolica (Figs 5A–D, 7B). In addition, we observed a number of feeding scars in other Quercus spp. that could not be precisely identified at the species level, due to the high extent of decay in these dead trees. Most of these trees were located in areas on the northern slope, close to the valley, where the terrain was not too steep.

The species in which we directly observed larvae were C. laxiflora and Q. aliena. The state of decay of Q. aliena was as follows: trunk left standing; approximately 80–90% bark remaining; bark tightness tight to loose; wood texture hard; shape of cross section round; branches absent (Figure 13A).

At the Ussurisky nature reserve in Russia, we identified various host plants such as Fraxinus mandschurica, Quercus mongolica, Ulmus laciniata, and U. davidiana var. japonica – but emergence holes of C. relictus and feeding marks were observed only in U. davidiana var. japonica (Figure 13B); overall, this region contains several tributaries and the mountain wetlands are well developed (Figure 13C).

Figure 12. 

Pupal chambers of Callipogon relictus in Gwangneung forest: A pupal chamber in Carpinus laxiflora B pupal chamber in Quercus aliena.

Figure 13. 

Host plants and habitat of Callipogon relictus: A Quercus aliena in Gwangneung forest BUlmus davidiana var. japonica in Ussurisky nature reserve C forest in Ussurisky nature reserve.

Callipogon relictus has so far been reported from four countries in East Asia: Russian Far East, China and Korean Peninsula (Kuprin and Bezborodov 2012; Bezborodov 2016; Yi et al. 2018).

In the Russian Far East, which is type locality of this species, its distribution was summarized by Kuprin and Bezborodov (2012). They reported that C. relictus were distributed across four federal subjects of Russia (Primorsky Krai, Khabarovsk Krai, the Jewish Autonomous Oblast, and Amura Oblast) and distinguished four local populations: Ussuriiskaya, Khoro-Bikinskaya, Khingano-Bure-inskaya, and Selemdzhinskaya populations (Kuprin and Bezborodov 2012).

The distribution of C. relictus in China has been reported to span eight provinces: Tianjin, Gansu, Hebei, Heilongjiang, Inner Mongolia, Jilin, Liaoning, Shaanxi, and Shanxi (Li et al. 2014). Of these, the largest number of individuals has been recorded in the three provinces (Heilongjiang, Jilin, and Liaoning) that border North Korea and Primorsky Krai, the type locality in Russia (Bezborodov 2016).

Recently, Bezborodov (2016) and Yi et al. (2018) reassessed the distribution of C. relictus based on sampling data. Based on current reports, the eastern limit of the distribution of C. relictus is the Imeni Lazo district, Khabarovsk Krai, Russia, located at 135°E (Kuprin and Bezborodov 2012); the northern limit is the Selemdzhinsky district, Amur Oblast, Russia, located at 53°N; the southwestern limit is Houyugou city, Shaanxi, China, located at 35°N, 110°E (Hua 2002); the northeastern limit is Mt. Si-fang-shan in A-er-jin city, Inner Mongolia, China, located at 49°N, 123°E (Li et al. 2013); and the southeastern limit is Sogeumgang of Mt. Odaesan, Gangwon province, South Korea, located at 37°N, 128°E (Yi et al. 2018). So far, C. relictus has been observed in the range of latitudes 35°N to 53°N and longitudes 110°E to 135°E. It is possible that the known range will expand if more precise surveys are conducted in Northeast Asia.

The first record of Callipogon relictus in South Korea was provided by Saito (1932), who misidentified the species as Macrotoma fisheri, after which Cho (1934) collected and reported C. relictus at Namjangdae in Mt. Bukhansan, Seoul.

Cho (1955) added a record of distribution in two regions in Gyeonggi province (Pocheon-gun and Paju-si) and four regions in Gangwon province (Chuncheon-si, Hwacheon-gun, Yanggu-gun, and Gangneung-si) in the north of South Korea. In addition, Cho (1961) reported the following specific locations from where specimens were collected: Chujeon-ri (Buksan-myeon, Chuncheon-si), Georye-ri (Hanam-myeon, Yuchon-ri, Gandong-myeon, Hwacheon-gun), Eupnae-ri (Yanggu-myeon, Yanggu-gun) (Gangwon province), Bukhan-san (Seoul) and Gwangneung (Gyeonggi province) (Figure 14).

Kim (1978) reviewed specimens of C. relictus deposited in various Korean research institutions and found that, after the addition of records for Gangwon province (Cheongpyeong-ri, Buksan-myeon, Chuncheon-si, Sogeumgang, Odae-san, and Gangneung-si), the only official collection records came from Gwangneung forest in Pocheon-si, Gyeonggi province (Byun et al. 2007: Table 2).

Figure 14. 

Distribution map of Callipogon relictus in the Korean Peninsula (An 2010; Bezborodov 2016; Yi et al. 2018; per. obs.): 1 Gyeonggi Province , Seoul-si, Mt. Bukhansan, Namjangdae 2 Gangwon Province, Gangneung-si, Mt. Odaesan, Sogeumgang 3 Gyeonggi Province, Pocheon-si, Gwangneung forest 4 Gangwon Province, Yangyang-gun, Seo-myeon (new record) 5 Gangwon Province, Chuncheon-si, Buksan-myeon, Chujeon-ri 6 Gangwon Province, Chuncheon-si, Buksan-myeon, Cheongpyeong-ri 7 Gangwon Province, Hwacheon-gun, Gandong-myeon, Yuchon-ri 8 Gangwon Province, Hwacheon-gun, Hanam-myeon, Georye-ri 9 Gangwon Province, Yanggu-gun, Yanggu-eup 10 Hwanghaebuk Province, upper region of the Imjingang River 11 Gangwon Province, Sepo-gun, Sambang-ri (Chuga-ryeong, Sambang-hyeop) 12 Hamgyeongnam Province, Geumya-gun 13 Pyeonganbuk Province, Mt. Myohyangsan 14 Pyeonganbuk Province, Mt. Cheonmasan 15 Hamgyeongnam Province, Sinheung-gun 16 Yanggang Province, Mt. Nangnimsan 17 Hamgyeongnam Province, Mt. Bulgaemiryeong 18 Jagang Province, Mt. Baekamsan 19 Jagang Province, Mt. Hwangsuryeong 20 Yanggang Province, Gapsan-gun 21 Hamgyeongnam Province, Mt. Duryubong 22 Jagang Province, Baekam-gun 23 Jagang Province, Mt. Bukpotaesan 24 Hamgyeongbuk Province, Yeonsa-gun 25 Hamgyeongbuk, Seonbong-gun.

In this study, we found adults of C. relictus in the Gwangneung forest for four consecutive years from 2014 to 2017 (Table 1) and we also added a new location to the Korean distribution of the species in Yangyang-gun, Gangwon province, based on photographic evidence (Figure 15). This region is thought to be a suitable alternative conservation site (habitat), as it has not only a much wider habitat area than Gwangneung forest and little human interference, but also a large population of host plants. Habitation of Korean individuals has also been verified recently in this region.

In North Korea, C. relictus was first recorded by Nakane (1974) and up to now it has been reported to have a wide distribution in the following provinces: Gangwon province, Yanggang province, Jagang province, Pyeonganbuk province, Hamgyeongnam province, Hamgyeongbuk province, and Hwanghaebuk province (Bezborodov 2016; Yi et al. 2018). Of these, the largest numbers of individuals were reported in Yanggang province, Jagang province, Pyeonganbuk province and Hamgyeong province, which are adjacent to Russian Primorsky Krai and China northeastern regions (Jilin, Liaoning), with as many as 31 individuals being collected in a single survey (Mt. Bukpotaesan, Yanggang province) (Yi et al. 2018), suggesting a stable population inhabiting the area.

Figure 15. 

Adult male of Callipogon relictus in Yangyang-gun of Gangwon province (photograph taken by Jeong-Ok Kim)

Our study recognized that, except for the new record from Yangyang-gun (Gangwon province), Gwangneung forest is the only collecting site of C. relictus. Kim et al. (1976) performed 20 surveys between June and September when adults of C. relictus occur, but they did not observe the adults, indicating that the population density was already very low at that time.

During our study, through focused surveys on habitation, we directly found six adults, two larvae, and various other signs of C. relictus (22 emergence holes, nine pupal chambers, tunnels, and feeding scars) (Table 1), providing that the Korean population, although low in number, is still persisting in Gwangneung forest and is not extinct in the South Korea.

Of the six adults discovered in this study, except one individual, all five were either exhausted from severe damage and died within a day, or were already dead. Three of which individuals were missing an elytron (Figure 4A–B, F) and another was found with a crack in the pronotum (Figure 4C); these damages are considered to be the result of attacks by natural enemies (woodpeckers, squirrels, wild animal, etc.)

In previous studies, adults were observed in Gwangneung forest between June and September and particularly, high numbers were observed in August (Byun et al. 2007: Table 3). Similarly, in our study, we found two adults in July and four adults in August, meaning that the majority of adults were observed in August, consistent with the findings of other studies (Table 2). However, the individuals that we discovered in August were severely weakened, to the extent that they could not cling to a tree or were already dead. Given that they were also missing much of their pubescence, we expect that these individuals were also active as adults in July.

During this study, we discovered a total of six adults in Gwangneung forest but, apart from a male that flew over from a nearby factory, we found the other five adults in regions with a high density of Quercus spp. with flowing sap. Like other Prioninae species, C. relictus adult is known to be attracted by light at night (Lee 1987; Byun 2007). The male discovered at a factory in 2016 was also thought to have been attracted by bright light at nighttime, while the other five individuals were probably found during egg-laying or feeding activity.

We succeeded in collecting 16 eggs from a female found under a Quercus sp. tree through artificially induced oviposition; the hatched larvae are being reared to increase their population. Given that we consistently found a small number of adults over the last four years, although the population size is small, the population of C. relictus appears to remain more stable here than in the other parts of Korea.

The signs of habitation by immature stages of C. relictus (emergence holes, pupal chambers, tunnels, and feeding scars) were mostly found on Carpinus spp. and Quercus spp. of dead trees with DBH ≥ 39 cm, in forest areas where C. laxiflora were dominant and there were a mixture of Quercus spp. Because fallen trees have a large surface in contact with the ground, they decay rapidly and there is secondary invasion by wood boring insects (e.g. Lucanidae, Tenebrionidae). It was difficult to distinguish the signs of habitation of C. relictus from those of other species. In the natural state, the immature period is very long, over 4–5 years (Li et al. 2012), and so, rapidly decaying dead trees are predicted to be disadvantageous for survival.

The shapes of the emergence holes we found were mostly elliptical, but some were close to being circular (Figs 5A–D, 6A–B). Differences in the hole shape have been reported to be correlated with mandible length (Yi 2014), but this has still not been clearly elucidated. In this study, the majority of emergence holes were elliptical, consistent with that of other prognathous species in the subfamily Prioninae (e.g. Prionus insularis Motschulsky and Aegosoma sinicum White), suggesting that differences arise according to structure of head and mouthparts (prognathous, hypognathous, or retrognathic) rather than mandible length. The size of the holes was clearly larger than that of other large other Korean cerambycids (e.g. Batocera lineolata Chevrolat; Neocerambyx raddei Blessig & Solsky; A. sinicum White) and this is considered to be related to the body width (Korean averages: width 32.1 mm, length 26.0 mm; Russian averages: width 34.6 mm, length 28.2 mm). We found that the emergence holes have towards the north or northeast, and this is thought to be a strategy to minimize moisture loss due to excessive sunlight.

Pupal chambers were most commonly observed in C. laxiflora, with lengths in the range of 13–20 cm (Figure 12A, B). The length and size of the pupal chambers was correlated with the DBH of the host plant, where a larger DBH was associated with larger and longer pupal chambers (Table 5). This is because, in host plants with a larger DBH, there is more space available to make the pupal chamber as well as sufficient feeding resources for a favorable nutritional state, and therefore, the body can grow larger (Karino et al. 2004). We assume that the size of the pupal chamber increases in proportion with the size of the body.

The depth of the pupal chambers from the bark varied according to the level of decay and the hardness of the wood. In dead trees that were highly decayed and softer, pupal chambers were deeper; conversely, in dead trees that had harder wood, or that were less decayed, pupal chambers were shallower (Table 5). This suggests that the depth from the bark is adjusted to account for the properties of the wood, in order to facilitate emerge after exuviation.

Gwangneung forest, South Korea, is located in the center of the Korean Peninsula (latitude 37°42'36"–37°47'41"N, longitude 127°8'20"–127°11'58"E), with a total area of 2,400 ha and an altitude of 40–620 m above sea level (Cho et al. 2007) (Figure 2A). The average annual rainfall is 1,433.8 mm and the average annual temperature is 11.7 °C (Korea National Arboretum 2015). Gwangneung forest not only contains approximately 20% of the recorded plant species in South Korea (Korea National Arboretum 2004) but is also reported to contain 3,794 species of insect, representing 28% of all Korean insect species (Korea National Arboretum 2006).

Managed by the Korea National Arboretum, Gwangneung forest is composed of 54% natural forest and 42% artificial forest. The natural forest consists mostly of broadleaf trees centered on Soribong, including Carpinus laxiflora, Quercus mongolica and Q. serrata, while the artificial forest, mostly on low-lying ground, consists of a needleaved plantation, Abies holophylla, Pinus rigida, and Larix kaempferi, and some planted broadleaved trees (Korea National Arboretum 2015).

In this study, adults of C. relictus were discovered in the natural stands in Gwangneung forest (Soribong, Mt. Jukyeop-san), while larvae and most signs of habitation were discovered from Carpinus laxiflora and Quercus spp. in north of Mt. Soribong. Host plants with signs of habitation were located on the northern slope, especially close to the valley, where the terrain was not steep. Because the northern slope receives less sunlight and has a lower rate of moisture evaporation, the temperature is cooler, the humidity is higher, and, being close to the valley, the host plants retain moisture better; this indicates that appropriate humidity in the surrounding environment is a very important factor for habitation of C. relictus.

Gwangneung forest is protected as a UNESCO biosphere reserve, but it is located very close to Seoul and is becoming increasingly isolated due to road construction and the development of the surrounding land as part of the expansion of the greater capital area. Habitat disruption caused by this human interference will have a significant negative effect on maintaining a stable C. relictus population.

The Ussurisky nature reserve in Russia, the type locality for C. relictus, is located at latitude 43°33'–43°47'N, longitude 132°15'–132°47'E, and has a total area of approximately 40,432 ha, with a width of 40 km and a length of 19.5 km; this is 17 times the area of Gwangneung forest (Figure 2B). The terrain is mostly flat and low-lying, with the tallest peak at an altitude of 600 m. The mean annual temperature is 2.7 °C and the mean annual rainfall is 750–800 mm (Institute of Biology and Soil Sciences 2003).

The survey area was a flat forest dominated by Ulmus davidiana var. japonica, which is a host plant for C. relictus. We observed large, old U. davidiana var. japonica trees that immature stages of C. relictus could inhabit, as well as trees producing sap, which is main food source for adults. The area contains many tributaries, and so, despite the relatively low rainfall, humidity is well maintained within the forest, which is thought to provide a suitable environment for C. relictus habitation (Fig. 13C). Moreover, like Gwangneung forest, the nature reserve is designated as a nationally protected area, making it safe from indiscriminate felling and collection, and, because it has a broad area and is located far from urban areas, it is thought to be largely unaffected by disruption of the surrounding environment.

There are seven known species of host plant for Callipogon relictus in Korea: Carpinus cordata, C. laxiflora, Fraxinus mandschurica, F. rhynchophylla, Quercus aliena, Q. mongolica, and Ulmus davidiana var. japonica. In our survey, we have found larvae and signs of habitation of C. relictus in Gwangneung forest in the following four species of tree: Carpinus cordata, C. laxiflora, Quercus aliena, and Q. mongolica (Figs 5A–D, 7B). These were most commonly discovered in Carpinus laxiflora, though we also observed a large number of feeding scars in Quercus spp. of dead trees that could not be accurately identified at the species level, due to the high extent of decay. If these dead trees could be identified, it is possible that new host plants could be reported.

In Russia, ten host plants of C. relictus have been known (including Quercus mongolica, Ulmus laciniata, Ulmus davidiana var. japonica and Fraxinus mandschurica), but although we identified various host plants in the survey in the Ussurisky nature reserve, C. relictus emergence holes and feeding scars were only found in Ulmus davidiana var. japonica (Figure 13B), suggesting that C. relictus prefer this species.

The host plants where individuals and signs of habitation of C. relictus were found were either dead trees that had low vigor or had died recently and were not in a state of advanced decay (Figure 13A, B). Meanwhile, given that fungi or mold infection were observed on host plants with signs of habitation (Figure 6A, B), we assume that habitation is closely related to fungi, and although there has been a study showing that fungus aid larval growth in indoor rearing (Lee et al. 2017), there has been no detailed research on the association with fungi in the natural environment.