Trombicula mediocris Berlese, 1912

Stekolnikov, Alexandr A., 2021, A checklist of chigger mites (Acariformes: Trombiculidae) of Southeast Asia, Zootaxa 4913 (1), pp. 1-163 : 148-150

Trombicula mediocris Berlese, 1912

Trombicula mediocris Berlese, 1912: 93 ; Thor & Willmann 1947: 348, fig. 415 (adult); Gunther 1952: 6. Trombicula (Eutrombicula) mediocris: Wharton & Fuller 1952: 49 .

Type deposition. Stazione di Entomologia , Florence, Italy .

Type data. Indonesia, Java Is .

Hosts. No data.

Distribution. Indonesia.

Remarks. This species was described from adult stage.

Discussion

Diversity and distribution. The above checklist includes 49 genera and 450 species of trombiculids, which is almost equal to the size of the known fauna of China – 48 genera and 453 species ( Li et al. 1997). These numbers are also comparable with those for the chigger fauna of the African continent – 61 genera and 443 species, according to the last review ( Stekolnikov 2018). The diversity of chiggers in India is about half – 28 genera and 204 species ( Fernandes & Kulkarni 2003). The Australian chigger fauna includes 19 genera and 107 species ( Domrow & Lester 1985). The number of currently recorded species by countries are as follows: Malaysia 202, Thailand 156, Vietnam 105, Philippines 78, Indonesia 53, Burma 46, Laos 28, Singapore 6, Brunei 2, and Cambodia 1. These numbers apparently depend on the extensiveness of collection efforts taken up to the present. Chigger studies in Malaysia were mainly driven by Nadchatram; the fauna of Thai chiggers was studied by Lakshana (Tanskul); Vietnamese chiggers were studied by Schluger et al. in the 1960s and recent studies were summarized by Chau et al. (2007); new records and descriptions of many new species from the Philippines were published by Brown. Cambodia remains a white spot: there are no chigger species recorded in that country except for Leptotrombidium deliense . One erroneous record should be mentioned here: Goff (1989) assigned the distribution in Burma for his species Gahrliepia (Walchia) karinensis Goff, 1984 , which was really described from Birma in Egypt.

129 of 450 species (29%) were found in other territories, first of all, neighboring to the countries of Southeast Asia, namely, China (78 species), India (46), Papua New Guinea (27), Taiwan (17), Australia (16), and Ceylon (11). Lesser numbers of common species were observed between Southeast Asia and Japan (8 species), Nepal (5), Pakistan (5), Guam (4), Maldive Islands (4), Africa (3), Korea (3), Europe (3), Iran (2), Turkmenistan (2), US (1), Solomon Islands (1), Micronesia (1), and New Caledonia (1). We may conclude that the distribution of chiggers in that part of the world corresponds to the concept of the Oriental zoogeographic realm ( Rueda et al. 2013). This correspondence would have been more exact if the eastern part of Indonesia, situated on the New Guinea Island, was excluded from our analysis: it would obviously reduce the number of species common for the study area and Papua New Guinea with Australia.

The most speciose chigger genus in Southeast Asia is Leptotrombidium (95 species included in the checklist); other genera with more than ten species in the fauna of the study area are Walchia (42 species), Gahrliepia (39), Trombiculindus (26), Ascoschoengastia (23), Doloisia (17), Microtrombicula (16), Helenicula (14), Schoengastia (13), Schoengastiella (12), Neoschoengastia (12), and Walchiella (12). This distribution is rather similar with that for China ( Li et al. 1997): Leptotrombidium (122), Gahrliepia (including Intermedialia Yu, Yang and Wu, 1979 ) (42), Walchia (31), Neotrombicula (28), Trombiculindus (24), Doloisia (19), Herpetacarus (19), Ascoschoengastia (18), Helenicula (17), Microtrombicula (13), and Schoengastiella (12). In India ( Fernandes & Kulkarni 2003), Leptotrombidium is also the most speciose genus (43 species). Similarly with Southeast Asia and China, that country is characterized by the occurrence of many species of Schoengastiella (26), Microtrombicula (19), Gahrliepia (18), and Trombiculindus (11). Two more large genera characteristic for India are Ericotrombidium (12 species) and Neotrombicula (12). Australia has a highly specific chigger fauna ( Domrow & Lester 1985) with the prominent dominance of the genus Guntheria (52 species, i.e. almost a half of the chigger fauna).

Taking into account the above data on the distribution of species by genera, the Oriental region is first characterized by a high diversity of the two closely related genera, Leptotrombidium and Trombiculindus . The number of Palaearctic species in both genera is much lesser and only a few species of Leptotrombidium occur in other regions ( Kudryashova 1998; Stekolnikov 2013). The same is true for less speciose genera Ascoschoengastia , Doloisia , and Helenicula . The genera Gahrliepia and Schoengastiella are also diverse in Africa, but the third genus of Gahrliepiinae , Walchia , is much more diverse in the Oriental than in the Ethiopian region – 42 of its species were recorded in Southeast Asia and only five were described on the African continent – three in Rwanda and two in the Democratic Republic of the Congo ( Stekolnikov 2018). Microtrombicula is characteristic both for Ethiopian and Oriental regions, but with much higher number of species in the former: 87 its species are known from Africa ( Stekolnikov 2018) and only 16 from Southeast Asia.

The endemic chigger genera in the study area are as follows: 1) the monotypic genus Bishoplinia described from a bat in the West Papua Province of Indonesia, 2) the monotypic genus Octasternala described from a bat in Philippines, and 3) the genus Laotrombicula comprised of two sympatric species from the Laotian rock rat Laonastes aenigmamus . For the Oriental region, the list of endemics also includes Walchiella and Lorillatum .

Type deposition. The most important depository for the types from Southeast Asia is USNM: types of 152 species included in the current checklist are deposited there according to literature data (except those proved to be definitely absent in USNM in reality). However, I counted 41 species, which were described before 1989 with the declared deposition in USNM, but were not included in the catalogue of types published by Goff (1989). Previously, the absence of some types in the chigger collection of USNM was reported by Vercammen-Grandjean & Langston (1976) and Jacinavicius et al. (2018). One more important piece of information on this collection is that it repeatedly changed the place of its deposition. In last decades of the 20 th century it was housed in the Acarology Laboratory, Department of Entomology, University of Hawaii at Manoa ( Tanskul & Gingrich 1986; Brown 1990, 1999); at present, it is deposited at the Systematic Entomology Laboratory, United States Department of Agriculture, Agricultural Research Service, Beltsville, MD, US ( Jacinavicius et al. 2018, 2019; Bassini-Silva et al. 2020). The chigger collection of BPBM (now housed in the same laboratory) includes 47 species from the current checklist. The catalogue of types for that collection ( Tenorio 1976) is outdated, since many new species came to the collection after this publication.

The chigger collection at BMNH, according to my revision performed in 2019, includes types of 58 Southeast Asian species. In BMNH I have not found the holotypes of four species of Gahrliepia described by Nadchatram (1989), the holotype of Leptotrombidium fletcheri luzonense , and the holotype of Eutrombicula (Eltonella) eltoni (however, one paratype of the latter species is present in the collection). The absence of the holotype of Leptotrombidium imphalum in BMNH was already noted by Tanskul & Linthicum (1999). On the other hand, I found in the BMNH collection two specimens from SE Asia designated by Vercammen-Grandjean as holotypes of the species, which were not ever described – Leptotrombidium jalorense (nomen nudum) and L. tupaia (nomen nudum). Probably, these species were later found to be identical with some previously described chiggers, but this assumption needs verification by a thorough morphological study.

Deposition in SAM was reported for 27 types; other museums that include more than 10 types are ZMMU (25), IMR (24), ZSM (14), and ZIN (12). The material deposited in ZMMU includes the syntypes of 23 Vietnamese species described by Schluger et al. (1959, 1960a, b, c, 1961, 1963). These specimens were revised by Kudryashova (2004); re-descriptions of the species with designation of lectotypes are now preparing by me in collaboration with A.A. Antonovskaia (Moscow University, Moscow, Russia). Current deposition is unknown at present for 57 types. For 15 species described by Womersley, deposition in SAM can be assumed; types of 6 species described by Nadchatram can probably be found in IMR.

Hosts. According to our data, Southeast Asian chiggers were recorded on 649 valid species and subspecies of mammals, birds, reptiles, amphibians, and arthropods. The numbers of chigger species in the present checklist recorded from the host groups (except for Arthropoda) are as follows: Rodentia 312 ( Rattus rattus 103, R. tanezumi 98, Leopoldamys sabanus 72, Maxomys surifer 66, Berylmys bowersi 52, Menetes berdmorei 49, Sundamys muelleri 47, Bandicota indica 44, Niviventer niviventer 42), Scandentia 121, Aves 87, Chiroptera 64, Soricomorpha 64 ( Suncus murinus 32), Carnivora 48, Erinaceomorpha 42, Squamata 33, Primates 25, No data 24, Artiodactyla 17, Lagomorpha 12, Peramelemorphia 9, Diprotodontia 5, Afrosoricida 1, Anura 1, Dasyuromorphia 1, Dermoptera 1, and Perissodactyla 1. Noteworthy is the prevalence of species recorded on rodents, mainly rats—312 of 450, i.e. 69%. It can be partly explained by the fact that rats, due to their abundance and ease of capture, are more effective objects of parasitological collections than other animals. However, the much lesser number of Southeast Asian chigger species parasitizing birds and reptiles, as compared to the parasites of mammals, cannot be disputed.

Five species were recorded on Arthropoda. Microtrombicula eltoni was described from a scorpion Heterometrus longimanus and is still known only from its type series. Similarly, Neotrombicula scorpionis was described from an unidentified scorpion. Two species, Ascoschoengastia indica and Eutrombicula subrara , were recorded on specimens of the millipede Sphaeropaeus globusmagicus , and Leptotrombidium deliense was once found on a specimen of Tipulidae . Two of these cases (for A. indica and L. deliense ) were not included in the synopsis published by Felska et al. (2018); therefore, the full number of chigger species recorded from arthropods to the present is 12, but not 10, as was stated previously ( Stekolnikov et al. 2019a).

According to the data accumulated in the present checklist, 14 chigger species recorded in Southeast Asia can attack humans: Odontacarus australiensis , Helenicula kohlsi , Schoengastia pseudoschuffneri , S. psorakari (indirect evidence), S. schuffneri , S. vandersandei , Blankaartia acuscutellaris , Eutrombicula hirsti , E. rara , E. wichmanni , Leptotrombidium akamushi , L. deliense , L. keukenschrijveri , and L. scutellare . The most medically important chiggers in this list are the species of Leptotrombidium that can be vectors of scrub typhus. Currently, only species belonging to this genus are proven reservoirs of Orientia tsutsugamushi ( Santibáñez et al. 2015) .