INTRODUCTION

Superficial subterranean habitats have received increasing attention in recent decades (Culver & Pipan, 2014). The most researched of them is the “Milieu Souterrain Superficiel”, widely known as MSS (Mammola et al., 2016). This habitat comprises the network of voids between stony debris that lies beneath the surface. MSSs have different origins (colluvial, volcanic or alluvial) and different properties (among others, size of the stones, voids, and presence of soil layers) (Ortuño et al., 2013). MSSs are inhabited by species with different ecological requirements (epigean, edaphobionts, and troglobionts, among others) (e.g., Nitzu et al., 2014; Jiménez-Valverde et al., 2015; Mammola et al., 2016). Recently, new and rarely captured species have been frequently discovered in MSSs (e.g., Baquero et al., 2017; Gilgado et al., 2017; Ledesma et al., 2019; Jordana et al., 2020), allowing for a deeper understanding of the ecology of soil and subsoil habitats (Rendoš et al., 2012, 2016; Nitzu et al., 2014; Nae & Băncilă, 2017; Ledesma et al., 2020; Nae et al., 2021). MSS-research began in the Pyrenees (Juberthie et al., 1980, 1981) and has been largely conducted in Europe (Mammola et al., 2016). Some entomological faunistic records in MSSs of the Alps have been documented (Vailati, 1990; Christian et al., 1996; Růžička & Thaler, 2002; Zacharda & Kučera, 2006; Christian & Spötl, 2010), but there is no information on such habitats in Switzerland. Moreover, MSSs have never been sampled in rock glaciers.

Rock glaciers are defined as “lobate or tongue-shaped bodies of perennially frozen unconsolidated material supersaturated with interstitial ice and ice lenses that move downslope or downvalley by creep” (Haeberli, 1985). These landscape features are not uncommon in the Alps and other high mountains in temperate zones (Clark et al., 1998). Rock glaciers are formed by several processes (Clark et al., 1998), and the percentage of ice or permafrost in them is variable (Barsch, 1996). Rock glaciers have been studied from a biological perspective, including their plant and animal communities, and their role as climatic refugia (e.g., Cannone & Gerdol, 2003; Millar et al., 2015; Tampucci et al., 2017; Gobbi et al., 2020; Cannone & Piccinelli, 2021). So far, however, there have been no studies on the subterranean fauna of rock glaciers. We have collected hundreds of different specimens in a series of sampling efforts in the superficial subterranean environment of a rock glacier in the Swiss National Park. Among them, a parasitoid species the genus Aspilota Foerster, 1863 (Hymenoptera, Braconidae, Alysiinae) was identified.

The genus Aspilota is one of the most species-rich taxa of the Alysiinae with approximately 250 species described from almost all zoogeographical regions. Aspilota species are parasitoids of flies (Diptera), mainly of the families Phoridae and Drosophilidae; other families listed (Anthomyiidae, Lonchaeidae, Muscidae, Platypezidae, Sarcophagidae, Syrphidae and Tephritidae) (Yu et al., 2016) may have been erroneously included and need to be confirmed. This genus is well defined by the presence of large paraclypeal fovea, almost connected to the inner margin of the eye, and the presence of fore wing vein cuqu1 (2-SR) (van Achterberg, 1988; Peris-Felipo & Belokobylskij, 2016a).

In this study, we provide the first record of the Far Eastern species Aspilota umbrosa Belokobylskij, 2007 in Europe. We also present the first photographs of the species, and a key to identifying the eight known species of Aspilota from Switzerland.

MATERIAL AND METHODS

Sampling was carried at the rock glacier in Val Sassa, and the surrounding scree habitats, in the Swiss National Park (SNP) in the Eastern Alps, Grisons, Switzerland (46°39'N, 10°12' E)(Fig. 1). The SNP is a strict nature reserve (category Ia; IUCN/WCMC, 1994), with no habitat and wildlife management. Public access to the SNP is only permitted on marked paths during the summer months (Baur & Scheurer, 2014). The upper layer of this rock glacier consists mainly of dolomite rock and sediments (Trümpy et al., 1997). The lower end of the rock glacier is at an elevation of 2100 m, while its higher part reaches up to 2600 m.

We installed eight subterranean sampling devices (SSD) on 24.07.2019: four in a line on the surface of the rock glacier, two in its foreland and two in the lateral screes. The SSDs were modified from the design of previous research (López & Oromí, 2010; Mammola et al., 2016; Baquero et al., 2017) (Fig. 1). A SSD consisted of a 1 m long PVC tube, 11 cm in diameter, buried vertically in the ground with the upper end at surface level. The tubes had several lateral perforations (each 8 mm in diameter) below the first 40 cm, allowing invertebrates to enter the interior of the tube. A pitfall trap containing propylene glycol was placed at the bottom of each tube. The pitfall traps were recovered by means of a metal rod attached to it and emptied. This happened after two months (25.09.2019), 12 months (28.07.2020), and 14 months (23.09.2020). The captured animals were transported to the University of Basel, where they were sorted in the lab using a stereomicroscope.

The terminology of the morphological features, sculpture, and measurements, as well as the wing venation nomenclature followed Peris-Felipo et al. (2014), and in parenthesis van Achterberg (1993). To identify the Aspilota specimen we used the keys of Fischer (1976, 1978), Belokobylskij & Tobias (2007), and Papp (2008). The material was imaged using Digital Microscope Keyence^® VHX-2000 and Adobe Photoshop^® imaging system. The specimen is kept in the collection of the Bündner Naturmuseum Chur (BNM), Switzerland, in accordance with the guidelines of the Swiss National Park. The holotype of Aspilota umbrosa deposited in the Zoological Institute of the Russian Academy of Sciences in St. Petersburg, Russia (ZISP) was also studied and photographed.

TAXONOMIC PART

Fig. 1.

(A) Location of the sampling site on the rock glacier in Val Sassa in the Swiss National Park. (B) Surface of the rock glacier at the sampling site. (C) Subterranean Sampling Device (SSD) just before burying it. (D) Installation of the pitfall trap in the tube once it was buried.

Fig. 2.

Aspilota umbrosa Belokobylskij, 2007 (female, Swiss specimen). (A) Habitus, lateral view. (B) Head and mesosoma, lateral view. (C) Mandible. (D) Antenna. (E) Head, front view. (F) Head, dorsal view.

DISCUSSION

In our project we collected some parasitoid wasps in the subterranean traps. However, Aspilota umbrosa in our samples was a surprising finding for several reasons. Firstly, its disjunct distribution as the only known individual of this species to date has been found near the Klyuchevskaya Sopka volcano (Kamchatka, Russia), which is some 8600 km from Val Sassa in the Swiss National Park. It is possible that the species has a wide distribution, with many undiscovered populations between these locations. Alternatively, it is conceivable that the alpine population of A. umbrosa is a relict of past ice ages, remaining isolated from the north-eastern population (Schmitt et al., 2010). Furthermore, the alpine population could be a result of a modern dispersal event (Schmitt et al., 2010). Interestingly, the habitat at the type location is mixed forest, a completely different habitat than that of our record. However, the landscapes of both regions share some characteristics, such as mountain reliefs and snowfields. The currently known distribution of Aspilota umbrosa suggests that it is a boreo-alpine species.

Fig. 3.

Aspilota umbrosa Belokobylskij, 2007 (female, Swiss specimen). (A) Propodeum and first metasomal tergite, dorsal view. (B) Hind leg, metasoma and ovipositor, lateral view. (C) Fore and hind wings.

The identification of the captured specimen with the keys of Fischer (1976, 1978), Belokobylskij & Tobias (2007), and Papp (2008) led us to Aspilota umbrosa, but the significant geographical separation between the two locations made us consider that our specimen could belong to a closely related new species. However, further examination of their morphology showed only subtle differences between our specimen and the holotype of Aspilota umbrosa, consistent with a distance of 8600 km between collection sites, and not enough to justify the establishment of a new species. Moreover, minimal morphological differences are known to occur in other insects with boreo-alpine distributions (Paill et al., 2021). We contemplated the possibility of conducting a barcoding analysis as an additional source of information, but none of the authors involved had the possibility to perform it in the recently captured specimen, or the holotype of Aspilota umbrosa stored in Russia. In addition, we also considered that even if they belonged to the same species, the large distance between the two collecting sites would most surely show in the barcoding analysis, possibly not providing a conclusive result. On the other hand, even large differences in barcode sequences are not enough to justify species descriptions by themselves (Zamani et al., 2022). Thus, we consider that the morphological study is for the time being enough evidence to consider the identity of the newly captured specimen as Aspilota umbrosa, and the observed minor differences are consistent with a boreo-alpine distribution and a distance among collecting sites of 8600 km.

Another interesting feature of the present record is the microhabitat in which Aspilota umbrosa was found. This is not the first record of a braconid wasp in the subterranean realm (Peris-Felipo & Belokobylskij, 2016b), and specimens of the genus Aspilota have also been trapped in caves (Peris-Felipo et al., 2016). However, our record is the first of a braconid parasitoid in the MSS. At the same time, our finding is the first published record of an arthropod species inhabiting the scree layer (MSS) of a rock glacier in the Swiss Alps. The MSS acts as a shelter, reducing the diurnal and seasonal fluctuations in temperature and humidity at the surface (Jiménez-Valverde et al., 2015; Mammola et al., 2016; Ledesma et al., 2020). This can help preserve populations of high mountain species that are otherwise rare on the surface (e.g., Gilgado et al., 2014, 2015; Ledesma et al., 2019; Ortuño et al., 2019). Some of these species may be glacial relicts (Růžička et al., 2012), as could be the case with Aspilota umbrosa. This type of MSS on a rock glacier with permafrost underneath can sustain particularly cold temperatures in summer. It is worth noting that the SSDs only collected individuals below the first 40 cm of the scree layer to prevent accidental capture of epigean species. It is likely that A. umbrosa or its hosts use the MSS during at least a part of their life cycle. Further samplings can strengthen this idea. Finally, we do not have enough information to assess the conservation status of Aspilota umbrosa. However, the Alps are currently being affected by global warming, leading to melting glaciers, and changing the distribution ranges of many species, including arthropods (Vitasse et al., 2021; Gilgado et al., 2022a, b). Thus, the alpine population of A. umbrosa may be also affected.

Fig. 4.

Aspilota umbrosa Belokobylskij, 2007 (female, holotype). (A) Habitus, lateral view. (B) Head and mesosoma, lateral view. (C) Antenna. (D) Head, front view. (E) Propodeum and first metasomal tergite, dorsal view. (F) Hind leg, metasoma and ovipositor, lateral view.