Invasive ambrosia and bark beetles (further BB) represent a threat to biodiversity, functional ecosystems, and the economic productivity of forestry (Brockerhoff et al., 2006; Aukema et al., 2011; Gohli et al., 2016), as well as to parks and orchards (Francardi et al., 2017; Branco et al., 2019; Fiala et al., 2022). BB are important vectors of fungal diseases that cause massive tree death. The simultaneous effect of several invasive species, their symbiotic fungi, and the subsequent interaction with climate change creates a situation in which it is difficult to predict the future impact of ambrosia and bark beetles on the environment (Lovett et al., 2013). Early detection is key to controlling BB because only then can a real integrated pest management (IPM) strategy be developed (Brockerhoff et al., 2006, 2010; Douglas et al., 2009; Samons, 2022).

Bark beetles spread in several ways, the most common being the global trade in wood material (treated and untreated wood), wooden packaging, and fruits or live seedlings of various non-native trees (Mathew, 1987; Meissner et al., 2008; Pombo et al., 2010; Augustin et al., 2012; Brockerhoff and Liebhold, 2017; Meurisse et al., 2019). It has also been confirmed that they can be introduced with wooden material that has been treated according to the international standard ISPM 15 (Haack and Petrice, 2009; Haack et al., 2014). In Europe, ports on the Atlantic and Mediterranean coasts are most often the gateway (Hagedorn, 1910; Hoffmann, 1942; Schedl, 1962; Cola, 1971, 1973; Faccoli, 2008; Moraal, 2010; Inghilesi et al., 2013; Rassati et al., 2015; Binazzi et al., 2019; Branco et al., 2019; Barnouin et al., 2020). Another entry point is botanical gardens, where non-native ambrosia and bark beetles may be introduced when expanding collections of exotic trees (Chobaut, 1897; Merkl and Tusnádi, 1992; Schuler et al., 2023).

Due to climate change, the host tree species are spreading northwards into areas where they did not originally occur (Ge et al., 2017). Even ambrosia and bark beetles, which are only found in southern Europe, may spread north; e.g., the bark beetle Phloeosinus aubei Perris, 1855 has spread to colder areas in Central Europe (Fiala and Holuša, 2019). Ambrosia and bark beetles not only spread through global trade but also naturally, as some are good flyers (Nilssen, 1984; Jones et al., 2019). Dry summers contribute to the appearance of ambrosia and bark beetles in alpine locations, even though they do not normally ascend to high altitudes, also (Marini et al., 2012).

However, the influence of humans on the spread of BB is far greater than the influence of climate (Gohli et al., 2016; Ward et al., 2019). Establishing plantations of non-native trees increases the risk of introducing non-native ambrosia and bark beetles (Lantschner et al., 2017). In Central Europe, this mainly concerns the cultivation of black pine (Pinus nigra) and bark beetles, which feed on it; Pityogenes bistridentatus Eichhoff, 1878 and Orthotomicus robustus Knotek, 1899 are found in several areas in the Czech Republic (Pfeffer and Knížek, 1996; Urban, 2000; Knížek, 2006; Knížek and Mertelík, 2017; Fiala et al., 2022). Climate change may help the maintenance of populations of BB on continents (Rassati et al., 2016a).

Most ambrosia and bark beetles are native to temperate and subtropical forests, so they represent the greatest danger for southern Europe due to a similar climate; hence, damage is most concentrated here (Pennacchio et al., 2004, 2012; Alfaro et al., 2007; Francardi et al., 2017; Leza et al., 2020). In the more northern countries of Europe, only damage by the ambrosia beetle Xylosandrus germanus Blandford, 1894 has been recorded (Maksymov, 1987; Graf and Manser, 2000; Galko et al., 2019).

Due to the economic and ecological damage caused by ambrosia and bark beetles, some governments perform regular monitoring of BB in their territory. This is helpful for identifying risk in a timely manner. There have been several monitoring attempts, of which baited traps are the most effective and least expensive method (Poland and Rassati, 2019).

Since BB are spreading increasingly around the world, there have also been efforts to introduce global monitoring. Observations were made on several continents at the same time to determine the abundance of ambrosia and bark beetles in the affected regions. The following semiochemicals were used in the traps: α-pinene + ethanol and α-pinene + ethanol + ipsdienol + ipsenol + Z-verbenol. The study is the first step toward the development of an international monitoring protocol based on trapping in traps baited with different types of substances (Faccoli et al., 2020).

There are six species of BB in the Czech Republic with a stable population in the wild (Knížek, 1988; Procházka et al., 2018; Fiala and Holuša, 2019; Fiala et al., 2020, 2021), and other species can be expected to occur in this territory (Gebhardt, 2014; Gebhardt and Doerfler, 2018). In the Czech Republic, there are no guidelines or methods for the early detection of BB. In addition, approximately half of the records of new species of ambrosia and bark beetles for the Czech Republic were accidental; the species were caught by amateur entomologists, and there was a delay of approximately 1–3 years between detection and publication (cf. Knížek, 2009a,b, 2011; Knížek and Kopecký, 2021). An extreme example is a report published 18 years after the species Pityophthorus balcanicus Pfeffer, 1940 was captured (Knížek and Liška, 2015). Therefore, it is necessary to create a stable network of traps for monitoring invasive species of ambrosia and bark beetles. To determine the methodology, several experiments were carried out in the Czech Republic, providing basic knowledge about the spread of BB and their bionomics in the Czech Republic (Fiala and Holuša, 2019, 2020; Fiala et al., 2020; Holuša et al., 2021; Fiala et al., 2023).

The aim of this work is to propose a methodology for monitoring BB based on the following:

In the Czech Republic, there are six species of BB with a stable natural population: C. bodoanum Reitter, 1913, Dryocoetes himalayensis Strohmeyer, 1908, G. materiarius Fitch, 1858, P. aubei, Xyleborinus attenuatus Blandford, 1894, and X. germanus (Knížek, 1988; Procházka et al., 2018; Fiala and Holuša, 2019; Fiala et al., 2020, 2021, 2023). Furthermore, several introduced species that could not form a stable population due to an unfavorable climate or absence of host plants were found in the territory of the Czech Republic: Coccotrypes dactyliperda Fabricius, 1801, Hypothenemus areccae Hornung, 1842, Hypothenemus hampei Ferrari, 1867, Hypothenemus setosus Eichhoff, 1868, Xyleborus affinis Eichhoff, 1868, Xyleborus volvulus Fabricius, 1794, and Xylosandrus morigerus Blandford, 1894 (Reitter, 1913; Fleischer, 1927–1930; Pfeffer and Knížek, 1989).

New invasive species of ambrosia and bark beetles which are already present in Germany may be expected to invade the Czech Republic. These include, Xyloterinus politus Say, 1826, which was detected in Bavaria in 2014 (Gebhardt and Doerfler, 2018), and Cyclorhipidion pelliculosum Eichhoff, 1878, which was found in Baden-Württemberg in 2013 (Gebhardt, 2014). The greatest economic danger to tree species in the Czech Republic is the bark beetle Pityophthorus juglandis Blackman, 1928, which has been spreading in Italy since 2013 and is a carrier of the serious fungal disease, thousand cankers disease (Montecchio and Faccoli, 2014). From the east, we can expect an invasion of the bark beetle Polygraphus proximus Blandford, 1894, which spreads from Siberia toward the west, and its harmfulness is comparable to that of I. typographus (Peña et al., 2020). Therefore, a pest risk analysis was developed for both species (EPPO, 2014, 2015).

The MaxEnt algorithm can be used to model the spread of invasive species around the world. For the invasive ambrosia beetle Xylosandrus compactus Eichhoff, 1876, which occurs in southern Europe (Pennacchio et al., 2012; Barnouin et al., 2020; Leza et al., 2020; Riba-Flinch et al., 2021), with the continuation of average climatic values from 1970 to 2000, X. compactus is predicted to find suitable ecological conditions for development in southern Moravia (which is the warmest region of the Czech Republic) by 2050 (Urvois et al., 2021).

The selection of locations is based on possible entry points such as border crossings, border river valleys, international airports, large timber warehouses, and botanical gardens; at the same time, these points will be used to monitor already established species whose abundance is still very low (Procházka et al., 2018; Fiala and Holuša, 2019, 2020; Fiala et al., 2020, 2021, 2022; Holuša et al., 2021). For the purposes of regular and permanent monitoring of BB, we therefore propose the following locations (Table 1 and Figure 1). A quarter of the locations are in protected areas; there is sufficient dead wood, and there are overgrown stands that provide a suitable environment for the development of ambrosia and bark beetles (Lee et al., 2019; Fiala et al., 2021).

Some invasive bark beetles are polyphagous, such as X. germanus (Weber and McPherson, 1983) and X. politus (MacLean and Giese, 1967), and can attack both coniferous and deciduous trees; some attack only deciduous trees, such as X. attenuatus (Kvamme et al., 2020), or only conifers, such as G. materiarius (Kamp, 1970). The representation of tree species is not significant for ambrosia and bark beetle monitoring because the type of forest has no effect on the abundance of beetles (Bouget et al., 2008). Therefore, the type of forest in which the trap is placed is not important, although a mixed forest with different tree species is preferable. We prefer oak forests, in the vicinity of which there are also conifers. In the Czech Republic, almost all forests are cultural, and conifers grow even at low altitudes. Therefore, choosing a combination of forests at the different locations was straightforward (Table 1).

Most BB in Europe are ambrosia species (Alonso-Zarazaga et al., 2023), and in our study in oak forests in western Bohemia, we found that ambrosia beetles had a higher abundance with a greater canopy cover, due to the wetter microclimate and greater amount of dead wood (Holuša et al., 2021). The influence of the close canopy on the abundance of ambrosia and bark beetles was also confirmed by Menocal et al. (2022). Therefore, forests with close canopy is generally preferred, although we are aware that C. bodoanum seems to prefer open forests (Fiala et al., 2021).

We also tested substances suitable for trapping BB. Factory-produced pheromones were suitable for trapping ambrosia and bark beetles of the genus Trypodendron; we found one specimen of X. germanus (Fiala and Holuša, 2020). Among volatile substances, we found the best combination of ethanol and juniper twigs suitable for trapping bark beetles P. aubei (Fiala et al., 2023). We found ethanol to be the most suitable for G. materiarius (Fiala et al., 2023). Likewise, C. bodoanum was captured in ethanol (Fiala et al., 2021), and although D. himalayensis and X. germanus were captured in impact traps as such, they were also captured in ethanol (Procházka et al., 2018; Hauptman et al., 2019a; Fiala et al., 2020; Appendix Table 2). X. attenuatus, like the ambrosia bark beetle, was attracted to ethanol (Galko et al., 2014).

Although sulcatol, which is considered a potential aggregation pheromone of G. materiarius, was expected to be successful (Flechtmann and Berisford, 2003), it was not the best lure tested in Central European conditions. The combination of sulcatol and ethanol resulted in the capture of a significantly greater number of beetles of Gnathotrichus sp. (McLean and Borden, 1977). However, in our case, ethanol alone captured more beetles than the combination of baits. Ethanol also significantly attracted other invasive ambrosia beetles, C. bodoanum, X. germanus, X. attenuatus, and other species of native ambrosia and bark beetles. Ethanol attracts both ambrosia and bark beetles X. politus and C. pelliculosum, which are already present in Germany (Ranger et al., 2011, 2014). Ethanol generally has a better capture ratio of invasive ambrosia beetles than the other substances (Fiala et al., 2023). Ethanol has long been known to be the main volatile substance on ambrosia and bark beetles (Kelsey and Joseph, 2003; Ranger et al., 2013, 2019).

For capturing and monitoring the dangerous invasive species P. juglandis, ethanol is also a suitable substance (Roling and Kearby, 1975). However, in acute situations, the monitoring network can be extended by adding a trap with the aggregation pheromone prenol, which was detected in this bark beetle (Seybold et al., 2015). Ethanol can also be used to detect P. proximus, although the beetles will most likely be caught in small quantities, as it reacts mainly to cis-verbenol, ipsdienol, and ipsenol (EPPO, 2014), like I. typographus (Schlyter et al., 1987). If the occurrence of P. proximus in the vicinity of the Czech Republic has already been predicted, the monitoring network can be expanded by adding another trap to the monitoring location with one of the industrial attractants containing cis-verbenol.

We propose total of 24 monitoring locations. Most of them are located at the border crossings of the Czech Republic and in river valleys, which are probably the entrance gates to the Czech Republic of BB (Figure 1). In addition, two large timber warehouses in which international trade takes place were selected (Žemlička, 2012), along with all international airports and three botanical gardens with tropical greenhouses. The latter locations cover a variety of modes of invasion by ambrosia and bark beetles: natural dispersal by the flight abilities of ambrosia and bark beetles and spread by global trade (Table 1).

We designed specific locations so that they were easily accessible in forests and were warmer locations of southern exposures. We selected overgrown forests near state borders or places that represent a “steppingstone,” as in the case of point 22, NPR Děvín (a woven area in an agricultural landscape), and point 23, NPR Jezerka (located on the migration route along the Ohøe River valley). From airports and large timber warehouses, we assume that bark beetles will fly to the nearest forest stands. Botanical gardens have the character of open forests and are mostly surrounded by forests, so localities in the territory of the garden have been suggested.

Three traps at each location is sufficient (Rassati et al., 2015; Thurston et al., 2022). In the Czech Republic, two types of impact traps are used; both are inexpensive and commonly available. They are easy to install and do not catch large numbers of non-target insects (Lubojacký and Holuša, 2014; Galko et al., 2016). The traps can be a Theysohn slot type, which is the most widely used in forestry in the Czech Republic (Zahradník and Zahradníková, 2016), or impact type Ecotrap, from which it is easier to extract the caught beetles. They can be disassembled after each season and stored in a much smaller space than the Theysohn traps.

These types of traps are primarily intended for catching economically important bark beetles that are attracted by specific pheromones (Flechtmann et al., 2000; Šramel et al., 2021); however, they can also be used to capture invasive species without any problems (Holuša et al., 2021; Fiala et al., 2023). Different species of ambrosia and bark beetles are found to prefer different types of traps. Dryoxylon onoharaense Murayama, 1934, an invasive species also found in Europe (Marchioro et al., 2022), or G. materiarius prefer the Ecotrap type. In contrast, bark beetles X. affinis and Premnobius cavipennis Eichhoff, 1878 prefer the Theysohn type (Flechtmann et al., 2000; Dodds et al., 2010; Miller and Crowe, 2011).

Each trap is baited with ethanol, which is universal for catching ambrosia and bark beetles (Rassati et al., 2016b; Chen et al., 2021). Traps should be placed between 30 and 50 m apart (Niemeyer, 1997; Rassati et al., 2014). Ethanol is also partly attractive to common species of ambrosia and bark beetles that live on conifers (Fiala et al., 2023). Traps should be operated from mid-April to the end of July, as the flight activity of ambrosia and bark beetles decreases in August (Fiala et al., 2023). Traps are checked once every 14 days, and the collected samples are then stored in the freezer for later determination. Ethanol should be changed in early June since the evaporators are active for approximately 60 days.³

In total, there are only 72 traps (e.g., three traps at 24 locations), which represent 144 ethanol lures per year (Appendix 4). Given that the Czech Republic is a small country, the number of locations is small, and monitoring should be carried out annually. Since most of the locations are forested, we suggest, if agreeable, partnering with the local forest administration of Forest of the Czech Republic (LČR, s.p., in Czech), a company that manages more than 50% of the Czech Republic’s forest stands and has cooperation with the Forest Advisory Service (Lesní ochranná služba in Czech) of Forestry and Game Management Research Institute (FGMRI, VÚLHM in Czech) Jíloviště at Prague, capital of the Czech Republic. In total, the LČR manages thousands of trappers throughout the country every year. The traps that we suggest, slightly more than 70 traps, are not difficult to manage because foresters move around the forests every day. Similarly, workers at the botanical gardens and timber warehouses move around daily and can send samples for determination. The average catch per trap in the world varies between 200 and 500 specimens, similarly in the Czech Republic it is between 50 and 500 specimens (Appendix Table 5).

The entire organization of monitoring corresponds to the activity and assignment of the Forest Advisory Service. The Forest Advisory Service deals with research, expert, and monitoring activities in forest protection against biotic pests. It monitors the occurrence of the bark beetle Ips duplicatus Sahlberg, 1836, every year. This monitoring has been ongoing for a total of 25 years, and during this period, a total of approximately 400 traps baited with I. duplicatus were placed around the country (Holuša et al., 2010; Knížek and Liška, 2022). The traps were checked by foresters, and beetles were collected and sent to FGMRI for determination. In Central Europe, other forest research institutes have also been involved in monitoring BB, e.g., in Slovenia (see Hauptman et al., 2019a), Slovakia (see Galko et al., 2014), and Latvia (see Ostrauskas and Tamutis, 2012); however, these were one-time events.

Our proposed monitoring of BB can be easily merged with the existing monitoring of I. duplicatus. It involves incorporating only 72 traps. The Forest Advisory Service would purchase ethanol vaporizers for cooperating entities and provide basic operator training; however, it is also possible to use a recorded instructional video. The total volume of all samples from the three traps does not exceed 1 dm³, so workers can place it in closed cans in any freezer where the insects will be frozen. It is necessary to determine the entire material of beetles into species by a specialist because data will be obtained on several species of ambrosia and bark beetles, especially rare ones (Fiala, 2021; Holuša et al., 2021; Fiala and Nakládal, 2022; Fiala et al., 2023).

Due to the importance of early detection of invasive species of ambrosia and bark beetles, the economic costs are minimal (Table 2) compared to the damage that can occur. In the US, the annual loss associated with all invasive species is estimated at $120 billion (Pimentel et al., 2005). In Europe, the loss caused by all invasive species is estimated to be hundreds of millions of € per year (Vilà et al., 2010); e.g., for invasive longhorned beetles of the genus Anoplophora, the cost of eliminating one infested hectare of vegetation is $25,000 (Anonymus, 2014). Estimated economic loss to landowners exceeded hundreds of dollars per hectare for invasive pests in Pinus taeda Linnaeus, 1753 stands in the southern US when no monitoring was performed (Susaeta et al., 2016). When carrying out integrated protection, the cost is less than the loss of value of the wood (Franjević 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).

The proposed monitoring method based on commonly used traps in selected locations (entrance gates at borders, wood warehouses, tropical greenhouses, and airports) is necessary because we BB have already been detected in the Czech Republic. Therefore, it is necessary to monitor these species and be able to detect new ones. Ethanol is effective for capturing the species that have already been detected, and the method is inexpensive. The method can be implemented by the research institute for monitoring pests. The monitoring results can inform the professional actions of the Central Institute for Supervising and Testing in Agriculture and for the targeted eradication of invasive species, as required by EU regulations.

TF and JH contributed to the conception and design of the study and wrote the first draft of the manuscript. Both authors contributed to manuscript revision, read, and approved the submitted version.