The flat bug fauna of Switzerland is still insufficiently studied. We describe the first records of Aradus angularis J. Sahlberg, 1886 and Quilnus marcosi Heiss & Baena, 2006 (Heteroptera: Aradidae) in Switzerland from a burnt forest area in the canton Valais. These are also the first records for Central Europe. With these new records, the number of Aradidae species recorded in Switzerland increases to 21, and in the canton Valais to 16. A cantonal record list of aradid species in Switzerland and photographs of the two recently new recorded species are provided. The pyrophilous A. angularis shows a disjunct distribution pattern, disconnected from its known distributional range from Fennoscandia to Northeast China, which suggests that the species is a relict of the Ice Age in Central Europe. In contrast, the sub-Mediterranean Q. marcosi seems to have expanded its distributional range to the north-east or has previously gone unnoticed in Switzerland. The host plants of Q. marcosi include Pinus nigra and P. sylvestris.

Die Rindenwanzen der Schweiz sind noch ungenügend erforscht. Wir beschreiben den ersten Nachweis von Aradus angularis J. Sahlberg, 1886 und Quilnus marcosi Heiss & Baena, 2006 (Heteroptera: Aradidae) für die Schweiz, von einer Waldbrandfläche im Kanton Wallis. Diese beiden Nachweise sind zugleich die ersten für Mitteleuropa. Dadurch erhöht sich die Zahl der für die Schweiz bekannten Aradidenarten auf 21, für das Wallis auf 16. Eine kantonale Liste der Aradidenarten der Schweiz und Photos der neu gemeldeten Arten werden vorgestellt. Der pyrophile A. angularis zeigt ein, von seinem bekannten Verbreitungsgebiet von Fennoskandinavien bis nach Nordost-China, disjunktes Vorkommen. Dies lässt vermuten, dass es sich bei der Art in Mitteleuropa um ein Eiszeitrelikt handelt. Im Kontrast dazu sieht es so aus, als dass die sub-mediterrane Q. marcosi ihr Verbreitungsgebiet nach Nordosten ausdehnt oder sie bisher in der Schweiz übersehen wurde. Das Wirtsbaumspektrum von Q. marcosi umfasst Pinus nigra und P. sylvestris.

first record, true bugs, pyrophilous, forest fire, Pinus , relict species, fungi, biogeography, Switzerland

Flat bugs (Heteroptera: Aradidae) represent the most species-rich family of saproxylic Hemiptera, besides Achilidae and Derbidae (both Fulgoromorpha) (Gossner and Damken 2018). All aradids feed on the hyphae of wood-decomposing fungi, with only a few exceptions, such as Aradus cinnamomeus Panzer, 1806, which sucks on living pines. The bodies of juveniles as well as of adults are strongly widened and flattened, with the upper side featuring many pointed excrescences, humps and warts. Most species are highly specialized, living on specific decomposing fungi of certain woody plants, hidden under the bark or in the wood (Heiss and Péricart 2007). Some rather black-coloured species show an adaptation to forest fires, being able to rapidly detect burnt deadwood infested by a particular host fungi, e.g. Aradus lugubris Fallén, 1807 on Daldinia loculata (Lév.) Sacc. (Xylariaceae) (Wikars 2001). Their hidden way of life and the low amount of deadwood, in particular large dimensions, in Central European forests, explains the relatively few records of most flat bug species in this region (Gossner et al. 2007).

In the Palaearctic region, 241 species and nine subspecies from 28 genera of flat bugs are known to occur (Aukema and Rieger 2001, Aukema et al. 2013). Of these, 31 species, of which one is divided in two subspecies, of the four genera Aneurus, Aradus, Calisius and Mezira have been recorded in Central Europe (Günther and Schuster 2000, Heiss and Péricart 2007, Aukema et al. 2013), and 18 species, one comprising two subspecies, of this subset have been observed in Switzerland (Table 1). Due to losses of older collections, in particular that of Frey-Gessner (Anonymous 1866), and to unclear author names, historical records sometimes cannot be verified. Furthermore, some records are very questionable, such as Aradus annulicornis Fabricius, 1803 commented as the “most common Aradidae species in Switzerland” (Frey-Gessner 1864). In addition, a qualitative investigation on aradids in Switzerland has not been conducted so far. Based on that, the knowledge on the Aradidae of Switzerland is still very limited irrespective of the canton. However, 17 of the 19 known aradid species were also found after 1990 (Table 1). The record list for the canton Valais alone comprises 14 species and thus can be considered a hotspot of Aradidae diversity in Switzerland.

In this study, we present records of two species new to Central Europe, sampled in a forest stand in the canton Valais that was heavily affected by wild fire on 300 ha in 2003 (Wohlgemuth et al. 2005). The species Aradus angularis J. Sahlberg, 1886 was sampled during a post-fire invertebrate succession survey ten years after the fire. Quilnus marcosi Heiss & Baena, 2006 was sampled during a field excursion in 2017, 14 years after the fire.

During the post-fire invertebrate succession surveys by the WSL, two aradid species were recorded for the first time in Central Europe. Aradus angularis J. Sahlberg, 1886 occurred at a distance of 2177 km from the closest known record in Fennoscandia, and Quilnus marcosi Heiss & Baena, 2006 occurred 278 km from the closest record in southern France.

First detections and diagnoses

Aradus angularis J. Sahlberg, 1886

VS, Leuk, Thel, forest fire area, 1730 m a.s.l., 46°20’05”N, 7°39’28”E, 1 ♂ in Combi-trap, 19 June 2013. Voucher specimen is stored in the collection of R. Heckmann, Konstanz.

The species differs from other smaller, very dark species of the genus by its elongated body, almost-parallel outer margins of the wings and very thin antennas (Fig. 2A). To confirm identification of the specimen, the genitalia were dissected: The paramere has a typical distal basal tooth, distinct for angularis. Furthermore the 9^th tergite shows a pointed shape, as shown in Heiss and Péricart (2007).

Figure 2. 

Two flat bug (Aradidae) species new to Central Europe, sampled in the burnt forest site in Leuk, Valais, Switzerland. (A) Male of Aradus angularis J. Sahlberg, 1886, sampled with a “Combi-trap” at 1730 m a.s.l. in June 2013 (sampling period 21 May to 23 June), leg. MM, det. coll. RH. Characteristic for the species are the size, the very thin antennae and the elongated body with the almost parallel outer margins of the wings and the form of the male genital structures. (B) Stenoptere male and juvenile of Quilnus marcosi Heiss & Baena, 2006, sampled under the bark of a burnt pine (Pinus sylvestris) tree at 1480 m a.s.l. on 1 June 2017, leg. det. coll. MMG. Characteristic for the species within the genus are the small size, the rounded apical corners of the pronotum and the form of the male genital structures. Photo credits: Gerhard Strauß.

Quilnus marcosi Heiss & Baena, 2006

VS, Leuk, Thel, forest fire area, 1480 m a.s.l., 46°19’56”N, 7°38’51”E, 1 ♂, 1 juv., below one of a few remaining bark pieces on a sun-exposed burnt Pinus sylvestris snag, with a diameter at breast height of 35 cm (Fig. 1D, E). 1 June 2017, leg., det. M.M. Gossner, 19 June 2013. Voucher specimens are stored in the private collection of MMG, Fronreute, Germany.

The species (Fig. 2B) differs from other larger Quilnus species by rounded apical corners of the pronotum, the shorter second antennal segment and the structure of the genitalia (paramere, parandria, tergit IX), as shown in Heiss and Péricart (2007) and Heiss (2010).

The records of the two new species increase the number of flat bug species in Switzerland to 21 species, with canton Valais being the most species-rich canton with 16 species. The frequent fires in south-facing regions, such as Valais and Grisons, as well as Ticino, which contribute to a large amount of deadwood, combined with warm and dry climatic conditions, might be a main reason for such high diversity of Aradidae in Switzerland. Although the aradids are not well studied across cantons in Switzerland, data on saproxylic beetles also support the idea of Valais and Ticino being hotspots of saproxylic species diversity (Chittaro and Sanchez 2016, Sanchez et al. 2016). In the German federal states of Baden-Württemberg and Bavaria, 18 and 16 aradid species have been identified, respectively, which means that Switzerland, which is the same size but is not as well studied, already exceeds the number of species in Baden-Württemberg and Bavaria (Simon et al. 2018).

The record of Aradus angularis J. Sahlberg, 1886 in Switzerland is remarkable. In Europe, the species is distributed in Fennoscandia (Finland, Sweden) and European Russia (Fig. 3), but also occurs further east in Siberia and Mongolia (Heiss and Péricart 2007). More recently, it was also found in Northeast China (Bai et al. 2010). The vastly disconnected, rather disjunct occurrence in Switzerland at high altitude (Figs 3, 4) suggests that the species might be a relict of the last Ice Age. These relicts are cold-adapted species originating from the arctic or high alpine regions, which dispersed to lower elevation and to the south during cold periods and survived in higher elevations or “cooling holes” during later warmer periods (Hewitt 1999, Habel and Assmann 2010). Other examples of such Ice Age relicts among Heteroptera are the CorixidaeArctocorisa carinata (C. Sahlberg, 1819) (Jansson 1986), the LygaeidaeGeocoris lapponicus Zetterstedt, 1838 (Péricart 1998), and the SaldidaeMicracanthia fennica (Reuter, 1848) (Günther and Strauß 2006).

Figure 3. 

Records of Aradus angularis J. Sahlberg, 1886 (blue, n=46) and Quilnus marcosi Heiss & Baena, 2006 (red, n=7) in Europe, and new records of both species in Switzerland (star). Data are based on Heiss and Péricart (2007), Brustel (2009), Heiss (2010), Esser (Mallorca, unpubl., leg. Esser, det. coll. MMG) and the website of the Muséum national d’histoire naturelle Paris (https://science.mnhn.fr/institution/mnhn/collection/eh/item/eh18974?lang=en_US) for Q. marcosi and Reuter (1900), Helioevaara and Vaisanen (1983), Pettersson and Nilsson (1986), Lammes and Rinne (1990), Roth and Coulianos (2014), Hagglund et al. (2015), Heikkala et al. (2017), Heikkala (pers. com.), Martikainen (pers. com.), Artportalen Sveden (https://artfakta.artdatabanken.se/taxon/100379) and Kirishenko (1951) for A. angularis. Please note that data points in the European part of Russia represent the centre of each Oblast in which the species has been recorded.

Figure 4. 

Frequency of occurrences of Aradus angularis J. Sahlberg, 1886 with respect to time since fire (left) and elevation (right), based on previous records in Fennoscandia (grey) and the new record in Switzerland (red). Please note that data were available for a subset of records only.

Aradus angularis lives on dead burnt conifer trees, in particular Picea (Helioevaara and Vaisanen 1983), a tree species also found in the sampling area. It is listed as vulnerable in the Red Lists of Sweden and Finland (Gärdenfors 2000, Rassi et al. 2001). In Sweden and Finland the species has been found on charred tree trunks up to three years after forest fires (Helioevaara and Vaisanen 1983, Pettersson and Nilsson 1986, Heiss and Péricart 2007, Hagglund et al. 2015, Heikkala et al. 2017) (Fig. 4) and therefore is classified as a pyrophilous species by Wikars (1992, 1997). It is assumed that the pyrophilous Aradidae depend on pioneer fire-related ascomycete fungi that arrive early on competition-free substrates but are rapidly outcompeted by other fungi (Wikars 1997). The host fungi of A. angularis remains unknown. Surprisingly, we found only a single specimen of this species, 10 years after the fire, although sampling started in the year after the fire and continued two, three and five years after the fire occurred. This finding does not support the classification as a pyrophilous species, at least not as an obligatory one. In contrast, several individuals of another pyrophilous species, Aradus lugubris Fallén, 1807, and two pyrophilous longhorn beetle species (Pradella et al. 2010) were caught in the year after fire, in the same trap where A. angularis was captured, and at several other sampling sites. This suggests that the trapping method is suitable for catching pyrophilous species. It is still unclear how pyrophilous species survive during periods without forest fires. They might persist hidden in old burnt forests or in matrix forests with a large amount of deadwood (Saint-Germain et al. 2008), or they might extend their diet to other fungi that are not dependent on fire. Alternatively, being generally highly mobile species, the pyrophilous specimen reported here might have dispersed from a nearby burnt forest stand, such as that in Visp (21 km distance), where a large intensive forest fire occurred in 2011, and become caught in the trap. Similar to our observations regarding flat bug species, tens of individuals of one carabid (ground beetle) species (Pterostichus quadrifoveolatus Letzner, 1852), which is pyrophilous according to Wikars (1997), was found in the burnt forest in Leuk in 2013, while other pyrophilous beetles were only found in the first years after fire (Pradella et al. 2010). This suggests that some species are obligatory pyrophilous, while others might have a higher flexibility in terms of feeding or habitat requirements.

The record of Quilnus marcosi Heiss & Baena, 2006 is less surprising, as it has already been detected 279 km away in southern France in the West Alps (Heiss 2010). Quilnus marcosi has a sub-Mediterranean distributional range, expanding to the mountain range of the Alps. The species seems to be rare, as it was discovered only eleven years ago and not more than seven sites of occurrences of this species are known so far (Fig. 3). All previous records have described occurrences in southern Europe, including Spain and France. Thus, the present record might show its range expansion to Central Europe or a general wider distribution for the first time. Heiss (2010) already suggested that this species might further spread to the north and east, in particular with climate warming. However, Heiss (2010) predicted only colonisation of dead Pinus nigra, within its disjunct distributional range from the Iberian Peninsula south of the Alpine mountain range up to the relict site at the eastern border of the Alps in Austria. The present record is the second one from Pinus sylvestris, following a record from Vallée du Rioumajou (Hautes-Pyrénées) (Brustel 2009). As a larva was also found in the tree in the present study, it is likely that this tree species can also be used as a host, which would allow further spread within the distributional range of P. sylvestris. The host fungi of Q. marcosi is the polyporous Antrodia xantha (Fr.) Ryvarden, 1973, which occurs on several Pinus species (Brustel 2009). There are no indications that Q. marcosi is associated with burnt deadwood. Thus, the large supply of deadwood resources after forest fires most likely promotes the occurrence of this species, as has been shown for other Aradidae species (Seibold et al. 2014).

The overall high number of flat bug species that have been recorded previously in Valais, Switzerland (Heiss and Péricart 2007) and the two recent species records suggests that this region may be a hot spot of flat bug diversity in Central Europe. This is also supported by other taxa that depend on deadwood such as saproxylic beetles (Chittaro and Sanchez 2016, Sanchez et al. 2016). The canton Valais has an unexpected long fire history (see Gimmi et al. 2004) and burnt areas and hotspots similar to the study area Leuk might exist in the region. The large amount of deadwood created by frequent forest fires most likely generates suitable habitats for pyrophilous as well as non-pyrophilous species. We strongly encourage entomologists to conduct a qualitative and even standardized quantitative assessment (see Gossner et al. 2007, Marchal et al. 2013, Morkel 2017) of aradids across Switzerland to evaluate the diversity distribution of this highly specialised saproxylic family with many endangered species.

We are grateful to Gerhard Strauß (Germany) for providing high-quality photographs of the sampled specimens, to Beat Wermelinger (BW, Switzerland) and Martin Obrist (MO, Switzerland) for contributing photographs of the study site, helping installing traps (BW, MO) and managing the data base (MO), to Ernst Heiss (Austria) for verifying the determination of A. angularis, Osmo Heikkala (Finland) and Petri Martikainen (Finland) for providing information on unpublished records of A. angularis from Fennoscandia, Jens Esser (Germany) for providing specimens of Q. marcosi from Mallorca, and to Melissa Dawes (Switzerland) for linguistic editing. The valuable suggestions of the reviewers Denise Wyniger (Switzerland) and Wolfgang Rabitsch (Austria) further improved the manuscript.