Bark and ambrosia beetles are among the most serious pests that threaten the stability of forest stands worldwide (Hulcr and Dunn, 2011; Hlásny et al., 2021). While bark beetles feed on the phloem, interrupting the transport of substances produced by photosynthesis and utilizing transferred fungi to overcome the defensive reactions of the host tree, ambrosia beetles feed on fungi carried in their mycangia and actively cultivate them on gallery walls within the wood (Hulcr et al., 2007). Invasive bark and ambrosia beetles pose threats not only to native biodiversity and functional ecosystems but also to the economic productivity of forest management (Brockerhoff et al., 2006a; Aukema et al., 2011; Gohli et al., 2016). Early identification of pests is a fundamental step in invasive bark and ambrosia beetle control, as it aids in the design of pest management strategies (Douglas et al., 2009).

Implementing integrated control measures proves more cost-effective than the loss of timber (Franjević et al., 2016). For example, failure to monitor the invasive ambrosia beetle in Pinus taeda L. forests in the southern USA has resulted in an estimated economic loss amounting to several hundred dollars per hectare for landowners (Susaeta et al., 2016). At the same time, lures require smaller financial expenditure than the human labor associated with the control of traps (Šramel et al., 2021).

Bark and ambrosia beetles use a complex chemical communication system (utilizing pheromones, allomones, kairomones, synomones) to locate and infest new host trees for feeding, mating, and reproduction (Wood, 1982; Hulcr et al., 2007; Seybold et al., 2018). Trees play a crucial role as producers of kairomones. When trees experience stress due to factors like drought, frost, floods, fire, or human-induced damage, they release ethanol, which is the most important volatile compound attractive to ambrosia beetles (Kühnholz et al., 2001; Kelsey and Joseph, 2003; Ranger et al., 2013, 2019, 2021). Different combinations of volatile compounds have been employed to monitor and detect bark and ambrosia beetles at an early stage. One of the most used approaches involves the simultaneous use of ethanol and α-pinene (Borden et al., 1980; Schroeder and Lindelöw, 1989; Miller and Rabaglia, 2009; Burbano et al., 2012; Flaherty et al., 2019).

In Europe, most invasive ambrosia beetles originate from temperate or subtropical regions. When these invasive beetles colonize new territories with suitable conditions, their population numbers can increase dramatically over time (Galko et al., 2014; Boland, 2016). Conversely, under unsuitable conditions, the abundance of ambrosia beetles remains low, or the population will even disappear (Fiala et al., 2020). Gnathotrichus materiarius Fitch, 1858 a polyphagous invasive ambrosia beetle that primarily infests conifers, has successfully spread throughout Europe (Kamp, 1970). Despite its presence in European forests for nearly a century, no significant damage has been reported from G. materiarius infestations (Mazur et al., 2018). Although we can lure this ambrosia beetle to host tree logs (Fiala et al. in prep.), it is often caught in traps baited with lures designed for capturing bark beetles of the Ips genus (Valkama et al., 1997; Fiala, 2019). Commonly used lures for this purpose include ID Ecolure^®, IT Ecolure^®, Cembräwit^®, Amitinuswit^®, Hostowit^®, and ethanol combined with other lures (Schneider, 1985; Knížek, 2009; Mazur et al., 2018). It is worth noting that sulcatol, a potential aggregation pheromone, has also been identified (Flechtmann and Berisford, 2003) and for other member of Corthylini ambrosia beetles, Monarthrum mali Fitch, 1856 (Miller and Crowe, 2020). All mentioned lures are alcohol-based, aligning with the use of ethanol for capturing ambrosia beetles (Lingren and Fraser, 1994; Rassati et al., 2015; Rabaglia et al., 2019).

We aimed to: (i) Analyze the effectiveness of various lure types on capturing invasive ambrosia beetles in endemic populations in central Europe; (ii) Analyze the impact of various lure types on species richness and abundance of both bark and ambrosia beetles, identifying their distinct responses to lure treatments; (iii) Examine the proportions of invasive ambrosia beetles and native coniferous bark beetles in relation to lure attractiveness, emphasizing the lure preferences of each group; (iv) Assess the potential interaction effects between season and lure type, elucidating how these factors jointly influence beetle behavior and their distribution across the study period.

This work aimed to compare the attractiveness of different lures and determine which one is most effective in attracting G. materiarius. The lures tested included Wood Stainers Lure, containing the pheromone sulcatol (Flechtmann and Berisford, 2003); α-pinene due to the preference of coniferous bark beetles (Schroeder and Lindelöw, 1989); ethanol as G. materiarius is an ambrosia beetle known to respond to ethanol (Kelsey and Joseph, 2003), and Cembräwit since G. materiarius has shown a positive response also to this lure (Schneider, 1985). We predicted greater catches of invasive ambrosia beetles, coniferous bark beetles, and Ips spp., in traps baited, respectively, with ethanol, α-pinene, and the Cembräwit lure.

In total, 46 species of bark and ambrosia beetleswere caught amounting to a total of 7,410 individuals (Supplementary Table S1).

Sulcatol has been reported as a potential aggregation pheromone of G. materiarius (Flechtmann and Berisford, 2003). This is likely because sulcatol has been detected in the guts of males that initiate attacks on the host tree and ceases production after mating with females (Flechtmann and Berisford, 2003). However, we captured more G. materiarius in ethanol-baited traps than in WSL-baited traps (Table 2). The details and chiral ratio of sulcatol in WSL are unknown to us, while the chiral ratio of the pheromone observed from the headspace of G. materiarius was 31% (S)-(+)- and 69% (R)-(−)-sulcatol (Flechtmann and Berisford, 2003). No G. materiarius has been caught on sulcatol in the USA (Miller and Crowe, 2020).

Ethanol alone significantly attracted all ambrosia beetles. It is well-established that ethanol is the primary volatile lure for ambrosia beetles (Kühnholz et al., 2001; Kelsey and Joseph, 2003; Ranger et al., 2013, 2019, 2021; Supplementary Table S2). Therefore, it was expected that ethanol would have a higher capture ratio of invasive ambrosia beetles compared to the other lures.

Among the 46 bark and ambrosia beetle species detected, C. bodoanum, G. materiarius, X. attenuatus, and X. germanus are the invasive ambrosia beetle species that were expected to be found. These species were identified in a total of 187 individuals, which accounts for less than 3 % of the total number of bark and ambrosia beetles collected. This suggests that the population densities of these beetles are still relatively low. In comparison, Slovakia and Slovenia have reported higher captures of the invasive ambrosia beetle X. germanus (Galko et al., 2018; Franjević et al., 2019), suggesting that the Czech Republic is on the edge of the range of the species’ range. Similar observations apply to C. bodoanum (Fiala et al., 2021) and G. materiarius (Fiala, observ.). X. attenuatus, on the other hand, is widespread throughout the Czech Republic but is not abundant (Fiala and Holuša, 2024), which explains its relatively low number of individuals in the current study.

Europe is home to approximately 30 species of invasive Scolytinae, with most of them belonging to the ambrosia beetle group (Marchioro et al., 2022; Alonso-Zarazaga et al., 2023). These ambrosia beetles are predominantly native to temperate or subtropical forests, making them particularly concerning for southern Europe regions due to similar climatic conditions (Francardi et al., 2017; Leza et al., 2020). In the Central and Northern European countries, damage has been recorded only for X. germanus, while other species have not caused significant harm (Maksymov, 1987; Galko et al., 2019). X. attenuatus is considered a secondary pest (Borowski et al., 2012; Skrylnik et al., 2019). As for C. bodoanum, no damage has been documented in Europe; and G. materiarius is also not known to cause damage in the region (Mazur et al., 2018). Nevertheless, it is important to note that all these ambrosia beetles can serve as vectors for fungi that can be detrimental to trees, so their occurrence should not be ignored (Batra, 1963; Nakashima et al., 1992; Kawasaki et al., 2010; McPherson et al., 2013; Moore et al., 2019).

The occurrence of all species of invasive ambrosia beetles, as well as most of the other bark and ambrosia beetles, was observed at a single locality (Kdyně). This can be probably attributed to the presence of diverse range of tree species, both deciduous and coniferous, in the surrounding area [e.g., Quercus robur L., 1753, Larix decidua Mill., 1768, Fagus sylvatica L., 1753, Abies alba Mill., 1768, Populus tremulae L., 1753, Sorbus aucuparia L., 1753, Acer pseudoplatanus L., 1753, Pinus sylvestris L., 1753, Corylus avellana L., 1753, Picea abies (L.) H. Karst, 1881].

In all conducted analyses, the day of season emerged as a significant factor, with the highest number of captured beetles observed in May during the study (as shown in Figures 2, 3). This pattern can be attributed to the increased flight activity of the most abundant beetle species during the period (Pfeffer, 1989). Notably, traps baited with Cembräwit exhibited a significant peak in summer, which can be attributed to the captures of Ips cembrae, with most often two generations per year in Central Europe (Holuša et al., 2014). The observed variation in species richness among the different localities can be attributed to the variations in tree species composition across the study area.

A total of 10 species of bark and ambrosia beetles were captured across all locations (Anisandrus dispar, Crypturgus cinereus, Dryocoetes autographus, Hylastes attenuatus, Hylastes cunicularius, Ips typographus, Pityogenes chalcographus, Pityophthorus pityographus, Taphrorychus bicolor, and Xyleborinus saxesenii). The species are commonly found in Central Europe (Pfeffer, 1989). Notably, two of these species, I. typographus and P. chalcographus, have recently experienced severe outbreaks and have become epidemic in the studied area (Fiala and Holuša, 2022). The occurrence of most of the species was observed in two or three localities, depending on the presence of their respective host trees. For instance, Pityokteines spinidens and Pityokteines vorontzowi were predominantly found in sites with Abies alba Mill. Another group of species was present in several localities but was captured in only a few specimens. An example is Phloeotribus spinulosus, which primarily develops on thin dead spruce twigs and is exclusively found on them (Pfeffer, 1989). In our experiment, this bark beetle was detected in three localities, with a total of 5 specimens caught. It can be speculated whether this occurrence is influenced by the I. typographus outbreak and the availability of abundant suitable breeding material, as observed in other species (Fiala and Holuša, 2021, 2022), or if it represents a long-term trend.

Relatively low catches of certain bark beetle species can be attributed to specific factors. Cryphalus piceae is generally a rare inhabitant of fir forests (Procházka and Schlaghamerský, 2019), and the occurrence of fir trees is limited in the Czech Republic (Kozáková et al., 2011). Dryocoetes hectographus primarily occurs in mountain spruce forests (Pfeffer, 1989), where traps were not placed. Only two specimens of the bark beetle Dendroctonus micans were caught in the Kladská locality because this species is not attracted to traps (Procházka et al., 2014). Although substances such as E-conophthorin, exo-brevicomin, and ipsdienol elicit a positive response from D. micans, they are most likely not its pheromones (Tømmerås et al., 1984; Zhang et al., 2002). Contrarily, the substance exo-brevicomin has shown effectiveness in trapping American and Asian species of the Dendroctonus genus (Barclay et al., 1998; Greenwood and Borden, 2000; Zhao et al., 2017). However, the fertilized females of Dendroctonus emerge from galleries and primarily rely on visual detection during host tree attacks, suggesting that pheromones may not play a significant role for them, except during the larval stage (Lukášová and Holuša, 2011).

An association between α-pinene and WSL had a significant effect on attracting conifer species of bark beetles that typically inhabit areas where pine bark meets the soil, such as Hylastes angustatus and Hylastes attenuatus (Schroeder and Lindelöw, 1989; Erasmus and Chown, 1994). While aggregation pheromones cis-verbenol and trans-verbenol have been reported for T. piniperda (Kangas et al., 1970), it has been suggested that T. piniperda relies on monoterpenes, including kairomones derived from its host, pine, for host and mate location (Lanne et al., 1987). This explains the high attractiveness of α-pinene observed in our experiments, as all the species live on pine trees (Byers et al., 1985; Schroeder and Lindelöw, 1989; Czokajlo and Teale, 1999; Poland et al., 2004).

In Cembräwit, the main component is ipsdienol, which serves as an aggregation pheromone for Ips typographus and Ips cembrae in low concentrations (Vité et al., 1972; Stoakley et al., 1978). Therefore, it is not surprising that most specimens of both bark beetles were caught in traps baited with Cembräwit (see also Grucmanová et al., 2014; Holuša et al., 2014). Pityokteines vorontzowi was also significantly attracted to Cembräwit, as it shares ipsdienol and ipsenol as its aggregation pheromones (Harring, 1978). The highest number of bark beetles caught was observed on Cembräwit^® compared to other lures, which can be attributed to the aforementioned outbreak of I. typographus in the study area (Fiala et al., 2022).

Ethanol and WSL significantly attracted two species of ambrosia beetles, Xyleborinus saxesenii and X. germanus as well as two species of bark beetles, Dryocoetes autographus and Hylurgops palliatus. These species have been frequently observed in traps baited with ethanol (Schroeder and Lindelöw, 1989; Lindelöw et al., 1993). More species of bark and ambrosia beetles responded to these lures compared to any other lure.

The ambrosia beetle Trypodendron lineatum was significantly associated with three lures, α-pinene, ethanol, and WSL. This is consistent with previous studies that have shown α-pinene and ethanol to be attractants for this species (McLean and Borden, 1977; Schroeder and Lindelöw, 1989). Conversely, this ambrosia beetle exhibits weak attraction to the substances used in Cembrawit, such as ipsdienol and methylbutenol (Gavyalis et al., 1981). The association of Cembräwit, ethanol, and WSL significantly attracted only the bark beetle Orthotomicus laricis. Bark beetles of the entire genus Orthotomicus are primarily attracted to ipsdienol, cis-verbenol, and 2-methyl-3-buten-2-ol (Giesen et al., 1984; Valkama et al., 1997). Ipsdienol is the main component of Cembräwit^®, which also contains ipsenol, methylbutenol, and amitinol (Zuhlke and Mueller, 2008). However, since O. laricis is found in the tops of uprooted spruces (Fiala, observ.), on pine stumps, and on felled spruce wood that is shaded (Holuša et al., 2017, 2019), it is evident that this bark beetle is also attracted to ethanol.

Overall, the results demonstrate that accounting for the influence of location and day of the season, the differences among lures were significant (see also Beaver and Löyttyniemi, 1991). Ethanol significantly attracted all ambrosia beetles in our experiments. In the USA and Italy, volatile lures such as ethanol, a combination of α-pinene and ethanol, and other combinations of ipsdienol+cis-verbenol+methylbutenol are used for monitoring invasive bark beetles (Rassati et al., 2015; Rabaglia et al., 2019). In Italy, these traps are placed in ports and adjacent forests; with higher capture success observed in deciduous forests compared to coniferous forests or ports (Rassati et al., 2015). Our experiment also shows the higher capture success of invasive ambrosia beetles was in deciduous forests, as evidenced by the capture of 162 specimens (e.g., 87%) at the Kdyně locality, probably due to the prevalence of deciduous trees in that area. A similar approach is employed in New Zealand, where traps are placed in ports, international airports, and adjacent forests using different lure combinations such as α-pinene+ethanol, β-pinene+ethanol, frontalin+ethanol, and ipsdienol. This monitoring model has proven successful in the early detection of invasive bark beetles, increasing the chances of eradication when infestations are still relatively small (Brockerhoff et al., 2006b). In Australia, the monitoring efforts have a broader focus and include the capture of Lepidoptera. Various lures such as ethanol, cineole, α-pinene, phellandrene, and a combination of pinene, phellandrene, cineole, terpene, and cymene as bait are used. Traps are positioned near ports and airports and within a 5 km radius of these areas (Bashford, 2012). In France, monitoring activities targeting invasive cerambycids include testing trapping methods using α-pinene+ethanol in Ecotrap traps. Traps were deployed in natural forests and later extended to ports, airports, and horticulture areas (Fan et al., 2019). In all these cases, the traps successfully captured invasive species.

There are several methods available for monitoring bark and ambrosia beetles, with baited traps being the most effective among them. Baited traps offer several advantages, including their ability to cover large areas, easy application in various locations, and cost-effectiveness. However, they also have some drawbacks, primarily related to the selection of bait types and the physical demands associated with checking numerous traps and subsequent determination in the laboratory (Poland and Rassati, 2019). While citizen science can help alleviate the physical demands of monitoring (Steininger et al., 2015), coordination is still necessary, and the expertise of bark beetle specialists will always be essential for accurate determination.

Ethanol serves as a universal lure that effectively attracts both bark and ambrosia beetles, with the majority of Scolytinae species being caught in traps baited with ethanol. Conversely, ethanol’s inability to attract economically important species in Central Europe, such as Ips typographus, is a significant advantage since these species are typically abundant. By not attracting them, the number of bark and ambrosia beetles captured is not needlessly increased, thus reducing the complexity and cost associated with sorting and determination. For the detection of invasive ambrosia beetles in Central Europe, it is recommended to use one bait of ethanol per year installed in April in any impact trap. Our preference lies with Ecotrap^® due to its disassemblability, storage capacity, and ability to catch beetles in a dry state without the need for preservative liquid. Moreover, the material remains intact by predators even after a two-week period.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

JH: Conceptualization, Methodology, Supervision, Validation, Writing – original draft, Writing – review & editing. TF: Conceptualization, Investigation, Methodology, Writing – original draft, Writing – review & editing. PP: Data curation, Formal analysis, Writing – original draft, Writing – review & editing.