The conservation of endangered insect species requires knowledge about their habitat (New, 2009; Samways et al., 2010). The saproxylic beetle Rosalia alpina (Linnaeus 1758) is legally protected in Europe (Council of European Communities, 1992) because it shows an overall decline of its continental distribution and population through the last century, despite of the recovery showed by some populations at regional scale (Luce, 1996; Lachat et al., 2013; Michalcewicz & Ciach, 2015). Rosalia alpina colonizes dead wood of dying and dead trees on open and sunlit conditions (Duelli & Wermelinger, 2005; Russo et al., 2010; Castro & Fernández, 2016). The aforementioned population trends are mainly linked to the availability of this particular habitat (Luce, 1996; Russo et al., 2010; Lachat et al., 2013). There are observations of individuals of R. alpina in many tree taxa (Čížek et al., 2009). However, the species seems to prefer the common beech (Fagus sylvatica L.) and beech forests, and successful colonizations have been confirmed at least in another nine broadleaf species (Čížek et al., 2009; Mazzei et al., 2013). Furthermore, recent research suggests the importance of trees as Acer pseudoplatanus L., Ulmus glabra Huds., and Fraxinus excelsior L. like habitat of R. alpina (Binner & Bussler, 2006; Michalcewicz et al., 2011; Michalcewicz & Ciach, 2012; Ciach & Michalcewicz, 2014). To our knowledge, this contribution shows the first observations of R. alpina in the genus Prunus.

All observations of R. alpina were done on a dying cherry tree, Prunus avium (L.), located in a plain surface of a valley bottom with the following UTM coordinates (WGS 84 Datum) and altitude: 30TWN7210482740 and 134 m. This location is included in the municipality of Asteasu (province of Gipuzkoa, the Autonomous Community of the Basque Country, Spain) (Fig. 1). According to the nearest weather station (Billabona, 172 m a. s. l.), annual values of rainfall and average temperature are 1551 mm and 13.6°C, respectively (Loidi et al., 2011). The cherry tree is in a garden of ornamental and fruit trees (owned by L. Martínez de Murguía), surrounded by one stand of temperate mixed forest on the south, and a countryside landscape on the north. The cherry tree has three main trunks from the base (Fig. 2), all partially stripped, with bracket fungi, and with the following diameters at breast height and aspects: 32 cm and N, 30 cm and SE, and 28 cm and SW. The tree died on 2016, and all branches above 4 m were cut on 19 March for safety reasons. On this date, cut branches were broken to check for presence of larvae, and exit holes assignable to R. alpina were counted. Exit holes showed the typical shapes and sizes described in beeches (Duelli & Wermelinger, 2005; Ciach & Michalcewicz, 2013), that also matched with those observed in Acer pseudoplatanus L. (Binner & Bussler, 2006; Michalcewicz et al., 2013), and Ulmus sp. (Čížek et al., 2009).

Individuals of R. alpina were photographed and distinguished among them by the pattern of their elytra (Luce, 1996). In this way, two males and one female were observed on 28 July 2013. One male was in a resting position and the other two individuals were mating (Fig. 2). On 17 July 2016 another three individuals were observed showing the same behavior. However, in this case, after separating from the male, the female started to explore the cracks of the stripped part of a cut branch looking for a place to lay eggs (Fig. 2). One last male was seen on 10 August 2016. Additionally, a larva of 20 mm in length was found inside a branch (Fig. 3) around 15 cm in diameter. This means an important development and growth of the larva inside the tree, considering that the eggs of the species average 3.6 mm (Ciach & Michalcewicz, 2009). The SW, SE, and N trunks showed 18, 17, and one exit holes assignable to R. alpina (Fig. 3), respectively. One adult male of the Cerambycidae Cerambyx scopolii Fuessly, 1775 (in a pupal chamber), and one larva of the Elateridae Ampedus pomonae (Stephens, 1820) were also found inside the cut branches. Larvae were stored and preserved in the collection of the Aranzadi Society of Sciences. Larvae were identified with the help of the works of Švácha & Danilevski (1987, 1988, 1989) for Cerambycidae, and Klausnitzer (1994) for Elateridae. C. scopolii was determined following the works of Bahillo & Iturrondobeitia (1996), and Vives (2000).

Regarding to these last two species, C. scopolii had already found in wood of P. avium close to the study area (Bahillo & Iturrondobeitia, 1996), while to our knowledge A. pomonae is recorded in the genus Prunus for the first time. Ampedus pomonae has been found in willows, poplars, birches, and pubescent oaks (Chatenet, 2000; Sautière & Calmont, 2014). According to the last authors, more information about the ecology of this beetle species, such as the one presented here, is needed.

So far, in the region of the study area (Basque Country) all R. alpina records of larvae or adults in trees with exit holes have been associated to wood of F. sylvatica (Bahillo & Iturrondobeitia, 1996; Martínez de Murguía et al., 2007; Castro et al., 2012). However, this contribution shows that in the absence of beeches, the species colonizes other tree species if other habitat preferences, such as thick trunks (Ø > 25 cm) with dry and sun-exposed dead wood (Duelli & Wermelinger, 2005; Russo et al., 2010; Castro et al., 2012; Castro & Fernández, 2016), are present. No trees meeting these habitat conditions were observed in the surroundings of the study area. The nearest records of R. alpina are from the Site of Community Interest of Hernio-Gazume (Castro et al., 2012), and located more than 2 km far away, exceeding the maximum dispersal distance known (=1628 m) of the species (Drag et al., 2011). However, recent genetic analysis suggests that R. alpina could have stronger dispersal abilities (Drag et al., 2015), allowing repeated colonizations of lowlands with less and more isolated available habitat (as it would be the case of the study area) from nearby mountains, where most part of the population would concentrate (case of Hernio-Gazume). Records of R. alpina in valley bottoms and lowlands close to the study area (Vega, 1981; Ugarte et al., 2002; Pagola, 2007; Ugarte & Salgueira, 2009–2010) could reflect this kind of dispersal movements. Rosalia alpina colonizes successfully other deciduous tree species than the beech in lowlands of central Europe (Čížek et al., 2009). In Spain the recommended measures of conservation of R. alpina are focused on beech woodlands, which are considered their preferred habitat (Viñolas & Vives, 2012). However, keeping in mind the presence of important populations in Quercus trees in the east of the country (Viñolas & Vives, 2012) and data from this contribution, it would be advisable to determine the role of other deciduous trees and forests as habitat of the species. If these are shown to be important, it will imply considering to include more woodland types in conservation actions for R. alpina, as habitat protection and enhancement, and design of net of corridors among populations.