Morimus asper reproduces mainly in freshly dead wood and, as a consequence, populations are affected by modern forestry practices. The taxon M. funereus, now incorporated into the species M. asper, is protected by the Habitats Directive (Council Directive 92/43/EEC) and its monitoring has received attention in recent years. Larvae of M. asper are polyphagous, but some studies indicate that adults prefer the wood of some tree species. Freshly cut log piles, which attract adults, have been proposed as a monitoring tool. For monitoring programmes, it is essential to select the most appropriate wood and therefore the attraction of different types of wood for adults of M. asper was investigated in two sites in northern Italian, using freshly cut log piles. The first experiment was carried out in the Nature Reserve Bosco della Fontana (a lowland forest, Mantua province), testing two autochthonous species (Carpinus betulus and Fraxinus ornus) and two allochthonous species (Juglans nigra and Quercus rubra). The second study was conducted in the Parco Naturale Regionale delle Prealpi Giulie (a mountain area, Udine province), employing Fagus sylvatica, Fraxinus excelsior and Picea abies (all autochthonous species). The population of this area belongs to the taxon formerly named M. funereus. In both sites, adults clearly preferred the wood of some particular species: J. nigra at Bosco della Fontana and F. sylvatica in the mountain area. This is the first study which compared the attraction of several tree species and it showed that this selection is important for the monitoring of M. asper.

Longhorn beetles, deadwood, monitoring, biodiversity, conservation, Northern Italy, Bosco della Fontana, Julian Prealps

Biodiversity loss is altering the functioning of ecosystems (Cardinale et al. 2012) and also continues to be a concern in Europe (de Heer et al. 2005, Henle et al. 2007, Nieto and Alexander 2010). Modern forestry has substantially changed the species and age composition of forests over the last centuries (Bengtsson et al. 2000, Gossner et al. 2013) and these changes have resulted in loss of dead wood in terms of quantity, quality and dynamics with obvious consequences for forest biodiversity in general and particularly for those species associated with dead wood (Siitonen 2001, Stokland et al. 2012, Seibold et al. 2015). Saproxylic beetles, i.e. species that depend on dead wood material at some stage of their life cycle (Speight 1989, Stokland et al. 2012), are reliable indicators of many aspects of dead-wood ecology (Stokland et al. 2012, Gossner et al. 2013). The presence of dead wood and saproxylic Coleoptera in forests, woodlands and parklands is indicative of high quality mature habitats (Davies et al. 2008). Many saproxylic beetles, occurring in Europe, are threatened (Nieto and Alexander 2010) due to continued habitat loss caused by logging and wood harvesting as well as the decline in veteran trees (Speight 1989, Nieto and Alexander 2010) and these concerns have led to 17 species of saproxylic beetles being listed in the Annexes of the Habitats Directive (Council Directive 92/43/EEC). Article 11 of this Directive specifies that monitoring of the conservation status of these species is obligatory for all member States. For Italy, Trizzino et al. (2013) were the first to propose standard monitoring protocols for all arthropods listed in the Habitats Directive and, recently, a manual for the monitoring of all species and habitats of community interest was published for Italy. This manual provides methods and protocols recommended for the monitoring required by the Habitats Directive (Stoch and Genovesi 2016).

The Cerambycidae is one of the largest families of all Coleoptera and the largest family of saproxylic beetles (Grimaldi and Engel 2005, New 2010); it comprises primary saproxylics, hence initiating the decomposition process and preparing the substrate for colonisation by the secondary and tertiary saproxylics (Hanks 1999, Stokland et al. 2012). Many authors have suggested that Cerambycidae might be particularly suited as indicators in forest ecosystems (Holland 2007, Ohsawa 2010, Hardersen et al. 2014). One species of this family, Morimus asper (Sulzer, 1776), has recently received particular attention. In the past the European populations of the genus Morimus were divided into five species (Reitter 1894, Dajoz 1976), but a recent study, based on COI and ITS2 gene sequences, found that all European and Turkish populations studied should be referred to as a single species, M. asper (Solano et al. 2013). This study confirmed the recent classification by several authors that M. asper and M. funereus belong to the same species (e.g., Sama and Löbl 2010, Sama and Rapuzzi 2011, Danilevsky 2015) and here the two taxa asper and funereus Mulsant, 1863 have been considered as subspecies of M. asper. This classification has implications for the conservation of this taxon, as M. funereus is listed in Annex II of the Habitats Directive and its status needs to be re-evaluated in the light of the new taxonomic results. For that purpose, a standard monitoring method for M. asper is urgently needed.

Probably the first method proposed for the monitoring of M. asper has been published by Vrezec et al. (2009), based on pitfall traps, placed in groups around fresh stumps. In contrast, Campanaro et al. (2011) suggested the use of freshly cut log piles as bait for monitoring and this method has also been proposed by Trizzino et al. (2013) and Bologna et al. (2016) for the monitoring of M. asper. Chiari et al. (2013) investigated this method in more detail and reported that it is reliable for detecting the presence and abundance of the target species and that occupancy probabilities increased with increasing volume of log piles. Hardersen et al. (2017) found that dead wood with a diameter of more than 12 cm was more attractive. Most authors concur that M. asper is a polyphagous species (e.g., Sturani 1981, Luce 1996, Sama 2002, Polak 2012) and this suggests that many tree species would be suitable for the monitoring. However, the data from Hardersen et al. (2017) suggested that the wood of two tree species tested did not have the same attraction for the adults of M. asper. Bărbuceanu et al. (2015) also reported that M. asper preferred certain tree species over others. As the freshly cut log piles are meant to function as bait for adults of M. asper, it was important to investigate the power of attraction of different types of wood and to analyse if the wood from different tree species were equally suitable for monitoring. The aim of this study was to test whether the choice of tree species affects the power of attraction of woodpiles for the adults of M. asper, investigating also allochthonous species. Two forests in northern Italy were investigated in two independent case studies.

At Bosco della Fontana, a total of 385 individuals of M. asper were counted and, in the Prealpi Giulie, a total of 51 adults were observed (Table 1). This corresponded to an average of 1.7 Morimus adults on each FCLP during one survey for Bosco della Fontana and to an average of 0.19 Morimus adults on each FCLP during one survey for the Prealpi Giulie. Over the sampling period, the sex ratio was highly significantly in favour of males in both study areas (Bosco della Fontana: 318 males, 67 females, P=0.008, Wilcoxon matched-pairs signed-ranks test; Prealpi Giulie: 42 males, 9 females, P=0.002, Wilcoxon matched-pairs signed-ranks test).

At Bosco della Fontana, the number of adults, both males and females observed on the different types of wood, was significantly different (Friedman’s Test, all adults, Fr=22.1, P <0.0001; males, Fr=21.9, P<0.0001; females, Fr=12.2, P=0.007). The wood of J. nigra and Q. rubra was significantly more attractive than that of F. ornus, with C. betulus being intermediary, considering all specimens, males and females, separately (Figure 2).

Figure 2.

Bosco della Fontana. Average number (±SE) of adults of M. asper observed per sampling on the seven wood piles built from wood of different tree species. Different capital and small letters above columns (total adults) and on the right (males and females) indicate significant differences among wood types at the 0.01 and 0.05 levels, respectively (Dunn’s Multiple Comparisons Test).

In the Prealpi Giulie, the number of individuals observed on the different types of wood was significantly different for all adults and for males (Friedman’s Test: all adults, Fr=10.6, P=0.005); males, Fr=10.2, P<0.006). The number of female individuals observed followed the same pattern as for all adults and for males, but was not significant (Friedman’s Test: females, Fr=3.7, P=0.16). The wood of F. sylvatica was significantly more attractive than that of P. abies with F. excelsior being intermediary, considering all adults and males (Figure 3).

Figure 3.

Prealpi Giulie. Average number (±SE) of adults of M. asper observed per sampling on the seven wood piles built from wood of different tree species. Different capital letters above columns (total adults) and on the right (males and females) indicate significant differences among wood types at the 0.01 level (Dunn’s Multiple Comparisons Test).

On the FCLPs of both sites investigated, adults of M. asper were observed regularly and permitted the monitoring of the local populations. These data are in line with the findings by Chiari et al. (2013) and Hardersen et al. (2017) that FCLPs are reliable for detecting the presence and abundance of the target species. In both sites, M. asper males were recorded more frequently, as is commonly observed when surveying Cerambycidae (Lopez-Pantoja et al. 2008, Drag et al. 2011) and this also applies to M. asper (Chiari et al. 2013, Hardersen et al. 2017).

At Bosco della Fontana, the average number of M. asper observed on each wood pile was 1.7 during each sampling whereas, in the Prealpi Giulie, this number was only 0.19. Even though the two studies are not directly comparable, the large difference in the number of observed adults is indicative of different population densities. The high figure observed at Bosco della Fontana revealed a large population, probably as a direct consequence of the management actions which are aimed at the protection of dead wood in all its forms (Campanaro et al. 2014). In contrast, for the Prealpi Giulie, the data gathered indicated the presence of a small and patchy population of M. asper, in line with the current management of the forest investigated which is almost devoid of dead wood. The unfavourable conservation status of this protected beetle species is directly linked to the paucity of dead wood in this site of the Natura 2000 network and it would therefore be important to modify the management of these beech forests to increase the amount of dead wood, as indicated by the plan for the conservation and development of the Park (Piano di conservazione e sviluppo del Parco Naturale delle Prealpi Giulie, Decreto del Presidente della Regione Friuli Venezia Giulia 20 marzo 2015 n. 062/Pres).

This is the first study which compared the power of attraction of several tree species and it showed that the selection of the species is important for the monitoring of M. asper. This study is also the first showing that some tree species attracted hardly any adults. These results were also consistent when males and females were considered separately. In the Prealpi Giulie, the results for females followed the same pattern as in all adults and in males, but they were not significant. This may have been caused by the low number of female individuals observed. The results confirmed that the wood of some tree species is more attractive to M. asper (Bărbuceanu et al. 2015, Hardersen et al. 2017). It was surprising that the two allochthonous species (J. nigra, Q. rubra) tested at Bosco della Fontana were most attractive for the adults of M. asper, as a preference for these non-native species cannot have evolved by natural selection. In contrast, the wood of the dominant species of Bosco della Fontana, C. betulus, attracted only 53% of adults when compared to J. nigra and the wood of the other autochthonous tree species was not attractive at all; F. ornus attracted only 3.7% of adults. These data are in line with the fact that the genus Fraxinus (or the family Oleaceae) is not mentioned as a host plant for this longhorn beetle (e.g., Romero-Samper and Bahillo 1993, Sama 2002, Dojnov et al. 2012).

In the Prealpi Giulie, the dominant tree species, F. sylvatica, attracted the largest number of adults of M. asper and these findings are in agreement with numerous authors who indicated that beech is one of the main host species (Romero-Samper and Bahillo 1993, Sama 2002, Polak 2012 , Bărbuceanu et al. 2015) and with Fusu et al. (2015) who stated that beech is one of the preferred wood types by M. asper. In this case study, the second most attractive species was F. excelsior, a congener of F. ornus, which was the least attractive species at Bosco della Fontana. Picea abies was the least attractive species, even though M. asper was reported to complete its life-cycle in trunks of this tree species (Sturani 1981).

Adults of M. asper must be able to locate freshly dead wood, often a scarce resource, to reproduce. Once a tree has died, the decay process results in an extensive release of volatile organic compounds (Holighaus 2012) and host-plant recognition of insects depends on ratios of plant volatiles (Bruce et al. 2005). This has also been shown for adults of Cerambycidae (Paschen et al. 2012). For example, the composition of volatiles clearly separate hardwood from softwood species and, to some extent, individual species (Holighaus 2012). Even though no specific information is available on host location by M. asper, it seems likely that they seek out the preferred wood-types by means of volatile organic compounds.

The study revealed a further important aspect for the monitoring of M. asper, i.e. the weather. When a cold spell arrived in mid-late April in the province of Mantua, the number of adults observed almost halved. Similarly, in the Prealpi Giulie, late May and early June were characterised by particularly cold weather with frequent rain and the number of adults observed was low. These observations are in accordance with Stanić et al. (1985) and Polak (2012) who reported that, at low temperatures (respectively below 12°C or 17°C), activity of M. asper is much reduced.

In summary, this study is the first which compared the attraction of numerous types of wood to adults of M. asper and showed that the wood of the tree species tested was not equally attractive. This finding is in line with Bărbuceanu et al. (2015) and Hardersen et al. (2017) who found that certain tree species are more attractive. Thus, when selecting wood for FCLPs for the monitoring of M. asper, it is important to choose wood which is attractive. Currently it can be said that the wood of F. ornus and P. abies should not be used for building wood piles. In contrast, the wood of the following trees attracted large numbers of adults: J. nigra, Q. rubra, Q. robur and F. sylvatica (Hardersen et al. 2017 and current study). While the wood of C. betulus and F. excelsior was moderately attractive for M. asper, monitoring could still be carried out. (Hardersen et al. 2017 and current study). For long-term monitoring programmes, it is essential to assess the long-term availability of the wood. It is also important that the wood of the selected tree species is freshly dead and has a diameter larger than 12 cm (Hardersen et al. 2017). This study, together with those by Chiari et al. (2013) and Hardersen et al. (2017), demonstrated that by investigating the factors which influence the selection of dead wood by the target species, the monitoring method for this elusive, localised and protected saproxylic species can be optimised.

We would like to thank Agnese Zauli and Massimo Pugnetti who helped during the surveys. We are also indebted to Marco Ascari, Andrea Pusca and Roberto Cortellazzo who helped building the log piles. We are grateful to all the staff of the Parco Naturale Regionale delle Prealpi Giulie, in particular: Stefano Santi, Giulio Goi, Marco Micelli and Marco Di Lenardo. Thanks to Francesco Pavan who helped with data analysis. The present work was developed within the EU project LIFE11 NAT/IT/000252, with the contribution of the LIFE financial instrument of the European Union.

Special issue published with the contribution of the LIFE financial instrument of the European Union.