Pityogenes chalcographus (Coleoptera: Curculionidae): biology, pest status, and current management options

Abstract

Bark beetle Pityogenes chalcographus is a common species that can impact coniferous forests throughout Europe, especially Norway spruce. Under typical conditions, standard forestry management practices do not lead to increased population densities or pose significant threats to forest stands. This beetle multiplies after abiotic disturbances like windthrow, drought, or snow damage, leading to localized outbreaks. P. chalcographus poses a significant threat to young spruce stands and infests the upper, thin-barked sections of older trees, often alongside Ips typographus. To effectively manage P. chalcographus, it is necessary to implement both preventive and direct control measures. Integrated pest management strategies emphasize the importance of maintaining cleanliness in logging areas, promptly removing infested trees, and reducing available breeding material, particularly fresh logging debris. Pheromone traps are primarily used to monitor flight activity. Cultural control measures involve carefully managing logging debris. This includes piling branches in shaded areas to reduce breeding opportunities for beetles. Timely logging and thinning operations are also important for reducing population growth by making trees less susceptible to attacks. In smaller areas, pheromone traps can be used to capture and to concentrate emerging beetles effectively. This comprehensive review underscores the importance of enhancing current management practices to address the rising challenges posed by P. chalcographus in spruce forests. A deeper understanding of its ecological interactions and adaptive strategies will be key to developing more effective control measures.

Bark beetles are commonly found in coniferous forests of the Northern Hemisphere. The Ips and Dendroctonus bark beetles are ecologically and socioeconomically the most significant forest pests in Europe and North America (Schelhaas et al. 2003, Gregoire and Evans 2004, Raffa et al. 2008, Six and Bracewell 2015). In some regions of Europe, forests dominated by Norway spruce (Picea abies (L.) Karst.) have been affected by the spruce bark beetle Ips typographus (Linnaeus, 1758), accompanied by the secondary pest Pityogenes chalcographus (Linnaeus, 1761). Outbreaks typically follow abiotic disturbances such as windthrow, snow breakage, or drought, which provide a large quantity of suitable breeding material for these insect pests. This results in rapid population growth and subsequent colonization of less-stressed trees (Schroeder and Eidmann 1993, Schroeder and Lindelöw 2002, Hedgren et al. 2003, Hedgren 2004, Grégoire et al. 2015, Netherer et al. 2015, 2021, Biedermann et al. 2019).

P. chalcographus is considered a significant pest in Europe due to its responsibility for damaging coniferous forests over the past century (Eidmann 1992, Wood and Bright 1992, Göthlin et al. 2000, Hedgren et al. 2003). P. chalcographus has been classified as a forest pest in 6 countries: Austria, the Czech Republic, Germany, Hungary, Romania, and Slovakia (Gregoire and Evans 2004). It is even listed as one of the most important forest pests in the Czech Republic under national legislation (Decree No. 101/1996 Coll.). Although its impact and the volume of infested timber are not as high as those caused by I. typographus, the significance of P. chalcographus appears to be increasing in spruce stands where the I. typographus population is in a retrograde phase (Grodzki 2004). Currently, it is also recognized as a pest on exotic trees in parks (Fiala et al. 2022).

The primary host tree, P. abies, may face increasing threats from P. chalcographus in the future. Likely, climate change will indirectly support the expansion of the pest’s range (Cao and Feng 2023). Unlike I. typographus, P. chalcographus also poses a threat to young trees and has a wider range of host trees (Berthelot et al. 2021).

Foresters, managers, and stakeholders have expressed concerns about further damage and have sought appropriate measures to prevent additional losses. The integrated pest management (IPM) strategies for P. chalcographus are not as well developed compared to those for I. typographus (Marković and Stojanović 2010). In this review, we summarize the bionomics, damage, significance, and current knowledge on bionomics, as well as IPM options for this species. Although there is a relatively extensive body of literature on aspects of P. chalcographus, many areas remain understudied. Therefore, this work is supplemented by our experiences and case studies. A substantial portion is also dedicated to the reproductive potential of P. chalcographus, as understanding its variability is crucial for developing defense strategies.

Taxonomic Description

The adult beetles of P. chalcographus measure between 1.6 and 2.9 mm in length and can be either black or bicolored (with a black head and thorax and red-brown elytra) (Novák et al. 1976, Cavey et al. 1994) (Fig. 1). In males, the frons is flat, while in females it is rounded with a depression between the eyes (Novák et al. 1976, Pfeffer 1995). The adults have a moderately excavated elytral declivity, with 3 widely spaced conical teeth per elytron in males and 3 smaller, rounded teeth in females (Novák et al. 1976, Cavey et al. 1994) (Fig. 1). Although females are slightly heavier than males, the difference is not significant (Kacprzyk and Bednarz 2015a).

In Europe, there are 12 species and subspecies of the genus Pityogenes (Alonso-Zarazaga et al. 2023). The species most like P. chalcographus is Pityogenes trepanatus (Nördlinger, 1848). Males of P. trepanatus differ in that the distance between the second and third teeth is 1.5 times longer than that between the first and second, whereas P. chalcographus has evenly spaced teeth. Females of P. trepanatus lack the glossy elytra seen in females of P. chalcographus, and the depression on the vertex of the head is deep and circular at the center, while in P. chalcographus it is broadly oval (Pfeffer 1955). Another similar species is Pityogenes saalasi Eggers, 1914. In this species, the males have the second tooth closer to the first than the third, and females have a shallow, circular depression on the upper part of the vertex (Kvamme et al. 2015).

Distribution and Host Species

P. chalcographus is an oligophagous species that feeds on conifers (Pfeffer 1995). Its host tree species include Abies spp., Larix spp., Picea spp., Pinus spp., Pseudotsuga menziesii, and Tsuga heterophylla (Kleine 1935, Galoux 1947, Pfeffer 1995, Fiala et al. 2022). Despite not showing any significant diversification of lineages related to its host (Schwerdtfeger 1929, Postner 1974, Bertheau et al. 2012), P. abies is the preferred tree species. This tree is the most suitable host for larval development (Bertheau et al. 2009). The oligophagous behavior of P. chalcographus may provide an ecological and evolutionary advantage by allowing the species to shift to alternative hosts when primary hosts are scarce or absent, such as during the invasion of new habitats (Schebeck et al. 2023).

P. chalcographus is distributed in alignment with its primary host tree, P. abies (Avtzis et al. 2008), and can be found throughout spruce stands from Scandinavia to the Balkans (Pfeffer 1995, Avtzis et al. 2010, Novák et al. 1976, Wood and Bright 1992).

P. chalcographus survived the glacial periods in three major refugia: the Russian Plain, the Carpathian Mountains, and the Italian-Dinaric region followed by postglacial secondary contact (Avtzis et al. 2008, Schebeck et al. 2018). However, due to partial cross-incompatibility, the divergent lineages have maintained their genetic identity in the postglacial period (Avtzis et al. 2008). As a result, one lineage occupies northeastern Europe, another covers the Italian-Dinaric region up to southern Austria, and the third is found in Central and Southeastern Europe (Schebeck et al. 2018). These lineages still retain partial reproductive isolation (Führer 1977, 1978). Its preferred food source, Norway spruce, is one of Europe’s most common and economically significant tree species (Spiecker 2003). Today, Norway spruce is cultivated and grows extensively outside its natural range. The greatest coverage of Norway spruce is found in Sweden, the Czech Republic, and Austria, where it occupies over 40% of forested areas. In Switzerland and Germany, it covers more than 30% of forest land (Spiecker 2000). The species also has substantial coverage in Finland, Norway, and Slovakia, comprising over 25% of forested areas.

General Biology

Niche

Smaller bark beetles tend to colonize the upper, thin-barked parts of trees, while larger beetles are found in the lower, thick-barked sections. This separation in ecological niches reduces competition between different species and is influenced by body size (Grünwald 1986, Amezaga and Rodriguez 1998). Due to its larger body size of I. typographus, is more prevalent in thicker host material/stem sections of mature host trees compared to P. chalcographus, although there are instances of overlap (Grünwald 1986, Göthlin et al. 2000). On younger trees, it attacks the entire tree (Grodzki 1997, Kula and Zabecki 2001, Kacprzyk and Bednarz 2015b, Fiala and Holuša 2022) (Fig. 2).

P. chalcographus is known to prefer sun-exposed wood with thin bark (Harding et al. 1986, Peltonen and Heliövaara 1999, Göthlin et al. 2000, Hedgren et al. 2003, Jurc et al. 2006, Kacprzyk and Bednarz 2021, Špoula and Kula 2024), although some authors suggest that this species prefers sites with only slight exposure to sunlight (Kolk and Starzyk 1996).

Gallery Formation

The galleries consist of a mating chamber located in the tree’s phloem and initiated by the male. Despite the male’s resistance, several (usually 5 to 6, but sometimes 3 to 9) females force their way into the mating chamber. The females copulate with the male and start excavating maternal galleries with niches for the eggs (Schwerdtfeger 1929, Klauser 1954). Each female typically lays 10 to 26 eggs and a maximum of up to 40 (Schwerdtfeger 1929, Führer and Mühlenbrock 1983). The larvae establish individual tunnels in the phloem layer. The larval galleries are dense and measure 2 to 4 cm in length. Numerous short (0.5 to 4 cm) larval galleries run parallel to each other but remain well separated and perpendicular to the egg galleries. The larvae feed for 4 to 6 wk before finally pupating in pupal chambers located in the bark. After pupation, the young adults perform maturation feeding in the surrounding phloem to develop gonads and flight muscles. Once the adults completed maturation feeding, they bore through the outer bark and disperse to establish the next generation (Schwerdtfeger 1929, Postner 1974) (Fig. 3).

The mating system slightly impacts the sapwood surface, except for the mating chamber, which is always located in the phloem. The larval galleries are almost entirely excavated in the bark of medium to large branches without affecting the wood (Faccoli 2015) (Fig. 4).

Life History

Phenology and Flight Activity

After hibernation, adults of P. chalcographus leave their overwintering habitat and start swarming in the spring. In Scandinavia, there is usually one generation per year, and the new generation overwinters as either adults or larvae. The flight period in Scandinavia typically lasts from May to August (Eidmann 1974, Harding et al. 1986).

In Central Europe, P. chalcographus typically has two generations from lowlands to mountainous areas, with continuous flight activity from April to August (Isaia and Paraschiv 2011, Galko et al. 2012, Ogris et al. 2020) or flight periods in April/May and, for the offspring generation (F1), in July/August (Fig. 5). Under favorable conditions, F2 generation may appear by late August or September (Pfeffer 1955, Zahradník 2007). Parental beetles can re-emerge after establishing the first brood and initiate another offspring generation, ie, a sister brood (Postner 1974, Jurc et al. 2006, Kacprzyk and Bednarz 2021). There are no detailed data on the proportion of individuals that enter the sister brood, though weather conditions will influence this phenomenon (Ogris et al. 2020). In our simple experiment, approximately 10% of the males re-emerged and established new galleries. While the first generation had 4.5 ± 0.1 maternal galleries per mating chamber, the sister brood had 3.6 ± 2.3 (Supplementary Material Case study 1).

P. chalcographus has a flight activity threshold at around 16.8 to 17 °C (Lobinger 1994). The development takes 652.8 ± 22.7 degree-days with a critical temperature of 7.4 °C (Ogris et al. 2020). It seems that P. chalcographus also enters a photoperiodically regulated diapause in the adult stage (Führer and Chen 1979). Diapause begins at the end of August when daylight falls below 13.6 h (Ogris et al. 2020). At lower temperatures, day length has a distinct influence on development, suggesting a photoperiodically mediated diapause modified by warm temperatures (Führer and Chen 1979). The adults overwinter in the leaf litter or under the bark of trees (Simionescu 2000). The supercooling point in Central Europe is −25 °C, while in northern countries, it is −32 °C (Košťál et al. 2011, 2014). In P. chalcographus, larvae, pupae, and adults can survive low winter temperatures (Košťál et al. 2014). However, preimaginal stages have a higher supercooling point than adults, and pupae are more cold-resistant than larvae. Feeding behavior influences cold tolerance, and to increase the chances of overwintering, both adults and larvae empty their gut before hibernation to eliminate potential ice nucleators. In Central Europe, adults overwinter under the bark, with the proportion of beetles overwintering outside the tree increasing with altitude. In northern countries, many more beetles overwinter outside the tree (Schebeck et al. 2017).

Pheromones

The male initiates the breeding process by choosing a host based on visual, olfactory, and gustatory signals cues. After overcoming the tree’s initial defense mechanisms, such as resin flow, the male bores through the bark and creates a mating chamber in the phloem (Postner 1974). Male P. chalcographus beetles are the exclusive producers of key pheromone components, such as chalcogran and methyl (E,Z)-2,4-decadienoate (E,Z-MD), which are released only after the males begin feeding on the host tree. These components are crucial for the aggregation and colonization of trees. The pheromones attract other individuals of both sexes and help successfully attack the host tree (Francke et al. 1977, Byers et al. 1988, Birgersson et al. 1990). E,Z-MD works synergistically with chalcogran to attract individuals to colonization sites, but E,Z-MD alone is not attractive enough (Byers et al. 1989). Additionally, the release of monoterpenes, including (+)-alpha-pinene and (−)-alpha-pinene, from the host trees enhances the attractiveness of the pheromones (Byers et al. 1988).

During feeding, 1-hexanol is found in the gut of male beetles. This compound likely facilitates communication between individuals during the infestation of trees, potentially signaling the presence of competition or a nonhost tree. Chemical signaling also involves monoterpenes (trans-verbenol, myrtenol, and trans-myrtanol), which affect communication between bark beetles and help coordinate their activities to avoid overloading the tree and compromising their ability to colonize the host (Birgersson et al. 1990). Yeasts produce trans-verbenol or verbenone, which are anti-aggregating for P. chalcographus (Leufvén et al. 1984).

Females of P. chalcographus produce very limited or no pheromones. Experimental studies have shown that females have low or undetectable amounts of pheromones such as chalcogran or E,Z-MD after feeding on host trees. This confirms that males are the primary producers of these pheromonal signals (Byers et al. 1988, Birgersson et al. 1990).

Flight Capacity

The maximum flight range of P. chalcographus, aided by wind, is likely 10 km (Nilssen 1984). However, P. chalcographus is about the same size as the invasive quarantine species Pityophthorus juglandis Blackman, 1928. The flight capabilities of P. juglandis have been carefully studied, revealing a flight range of a few hundred meters. The maximum total observed flight distance for P. juglandis was 3.6 km in 24 h. However, the average and median distances flown by beetles that initiated flight were 372 and 158 m, respectively (Kees et al. 2017). The flight distances of P. chalcographus may be similar (Supplementary Material Case study 2), although according to Schroeder (2013), P. chalcographus likely has a shorter flight range than I. typographus.

Reproductive Success of Pityogenes Chalcographus

The average reproductive output of female bark beetles varies significantly under natural conditions (Thalenhorst 1958) and is influenced by several factors, including the quality of the breeding material and both inter- and intraspecific competition (Grünwald 1986, Johansson et al. 2006, Jurc et al. 2006, Hedgren and Schroeder 2004, Kula and Zabecki 2006).

While P. chalcographus often coexists with I. typographus on trees (Faccoli 2015), it primarily inhabits areas where the bark is too thin for I. typographus (Göthlin et al. 2000). However, when I. typographus populations are low, P. chalcographus can thrive on stumps and trunks with thicker bark (Johansson et al. 2006), with densities ranging from 480 to 650 galleries per m² (Schebeck et al. 2023) (see Fig. 4). Clearly, P. chalcographus benefits from coexisting with I. typographus. When I. typographus occupies the lower sections of trees, and P. chalcographus inhabits the upper crown (Kolk and Starzyk 1996), reproductive success increases significantly—reaching 1.20 offspring per female—compared to a reproductive success of only 0.75 offspring per female when P. chalcographus colonizes the crown alone (Hedgren 2004).

The frequency and density of P. chalcographus infestations on spruce branches is significantly higher on scattered branches compared to branches in piles. For instance, there are almost 20 individuals per dm² on scattered branches, while there are about 10 individuals per dm² on branches in piles (Ząbecki and Kacprzyk 2007, Kacprzyk 2012, Kacprzyk and Bednarz 2015b). However, reproductive success is greater in surface branch piles, with averages of around 2.0 to 2.5 offspring per beetle, compared to randomly scattered branches, which yield only about 1.3 to 1.4 offspring (Kacprzyk and Bednarz 2015b). This leads to a similar total number of beetles emerging from both scenarios, approximately 40 to 46 (Kacprzyk 2012). This phenomenon supports the established understanding that higher densities of parent beetles can negatively impact the number of offspring emerging from branches (eg, Kacprzyk 2012, Holuša and Lukášová 2017). In addition to the density of colonization by parental beetles, the microenvironmental conditions at the slash storage site play a crucial role. The microclimate surrounding the slash left after removal influences its physiological state and, consequently, its susceptibility to insect colonization (Kacprzyk 2012).

The highest density of colonization was found in the upper sections of the piles, while the average offspring-to-parent ratio in branch piles was greater in the middle layer (Kacprzyk 2012). This suggests that a considerable number of offspring beetles emerge from branch piles. In contrast, lower reproductive success is anticipated for Pinus sylvestris L. compared to the residual material from spruce logging operations (Bertheau et al. 2009).

Symbiotic and Auxiliary Fungi

Pityogenes chalcographus carries a variety of species from the order Ophiostomatales (Supplementary Material Table 1). P. chalcographus lacks mycangia, and dry fungal spores are carried on its body, ie, external spore-carrying pockets (Davydenko et al. 2017, Schebeck et al. 2023). Most of these associated fungal species are probably commensals or antagonists. However, bark beetles typically maintain a few beneficial fungal associates as well. These mutualistic fungi provide nutritional benefits, assist in detoxifying tree defense compounds and pheromone production, and/or protect their hosts from antagonistic microbes (Birkemoe et al. 2018, Biedermann and Vega 2020). Potential mutualistic candidates in P. chalcographus include fungal species within the ascomycete order Ophiostomatales, specifically the genera Ophiostoma, Grosmannia, and Ceratocystiopsis, as well as species from the Microascales order, particularly the genus Endoconidiophora (Harrington 2005, Six 2013). The roles of fungi, yeasts, and bacteria in the life cycles of most bark beetles remain largely unknown (eg, Six 2013, Davis 2015, Zhao et al. 2019). Ophiostoma macroclavatum s. l., has a significant ecological relationship with P. chalcographus. This fungus is categorized as a “blue-stain fungus”, which colonizes tree cambium and causes discoloration (Linnakoski et al. 2016).

Filamentous fungi are found more commonly in the galleries of I. typographus compared to those of P. chalcographus (Grosmann 1930, Mathiesen-Käärik 1960, Krokene and Solheim 1996, Kirisits 2004). A likely explanation for this difference is that thinner phloem dries out more quickly, creating less favorable conditions for fungal growth (Grosmann 1930, Mathiesen-Käärik 1953). This notion is further supported by the observation that relatively dry and heat-resistant fungi, such as those in the order Hypocreales, particularly in the genus Geosmithia, are more frequently found in the galleries of P. chalcographus (Kolařík and Jankowiak 2013, Jankowiak et al. 2014). For example, Grosmann (1930) demonstrated that yeasts are abundant in young breeding systems, particularly around the eggs, where larvae pick them up and can later be found as gut symbionts. If these early observations are accurate, P. chalcographus may benefit from internal fungi that helps detoxify tree defense compounds. Still, it typically lacks external fungal symbionts, which assist I. typographus in overcoming living trees.

So far, no fungus associated with P. chalcographus has been identified as causing widespread tree die-offs, such as Dutch elm disease (Ophiostoma novo-ulmi Brasier, 1991) (Bernier et al. 2015) or Thousand Cankers Disease (Vito et al. 2016). Unlike I. typographus, P. chalcographus is not consistently associated with destructive tree fungi (Krokene and Solheim 1996, see detailed discussion in Schebeck et al. 2023).

Natural Enemies

Bark beetles host a diverse and ecologically important community of natural enemies. This community includes predators such as birds, beetles, flies, true bugs, and mites, as well as parasitoids like wasps and flies. It also encompasses pathogens, including viruses, fungi, microsporidia, and protozoa, along with nematodes (Wegensteiner et al. 2014).

Bark Beetles–Pathogen Interactions

So far, 3 species of entomopathogenic fungi have been identified in P. chalcographus, ie, Beauveria bassiana (Bals.-Criv.) Vuill., 1912, Beauveria caledonica Bissett and Widden, 1988, and Metapochonia bulbillosa (W. Gams and Malla) Kepler, S.A. Rehner and Humber, 2014. B. bassiana is the most found pathogen in P. chalcographus. However, the overall prevalence of these fungi, including B. bassiana, is low and is unlikely to significantly regulate bark beetle populations during outbreaks (Hyblerová et al. 2021) (Supplementary Material Table 1).

Three species of gregarines and possibly 3 species of microsporidia have been recorded in P. chalcographus, found in various organs of the beetles (Supplementary Material Table 2). Generally, gregarines cause mechanical and physiological damage to the intestinal epithelium, affecting metabolites and toxins’ excretion during pinocytosis. The development of trophozoites damages epithelial cells, creating entry points for other pathogens into the body cavity (Lipa 1967).

The neogregarine Mattesia schwenkei (Purrini, 1977) can influence the flight activity of I. typographus. Infection by this pathogen may reduce the beetles’ tendency to leave their galleries, potentially leading to overestimating infection levels. This effect has been particularly observed at the end of the growing season when it is hypothesized that infected individuals remain longer in their galleries due to reduced flight capability (Holuša and Lukášová 2017). It is highly likely that this also applies to P. chalcographus. Additionally, Chytridiopsis typographi Weiser, 1970 is a nonspecific pathogen that infects the intestinal epithelium of several members of the subfamily Scolytinae (Wegensteiner et al. 2014).

Bark beetles live beneath the bark in gallery systems, significantly hindering pathogen transmission. While horizontal transmission among beetles of the same generation is relatively easy in the nuptial chamber (Lukášová and Holuša 2011), most pathogens can only be transmitted to a new generation at very high population densities unless there is ovarian transmission. In these cases, a callow beetle chews through the maternal gallery, which may contain sources of infection. As a result, pathogen infection rates in bark beetles are generally very low (Supplementary Material Table 2), making it unlikely that pathogens will significantly influence the population dynamics of P. chalcographus.

Nematodes

The relationships between nematodes and bark beetles can be phoretic, meaning the nematode uses the bark beetle for transport to a new environment. One example is Bursaphelenchus piceaeTomalak and Pomorski, 2015, which has been found in the larval galleries of P. chalcographus on Norway spruce. This nematode damages the Malpighian tubules of adult beetles and their larvae (Tomalak and Pomorski 2015). However, its impact on population dynamics is unknown. Endoparasitic nematodes, such as Cryptaphelenchus diversispicularis Korentchenko, 1987 and Parasitorhabditis sp. (Takov et al. 2019), enter the beetle to obtain nutrients from the host and rely on the beetle to complete their life cycle (Rühm 1956).

Parasitoids

The parasitoids of P. chalcographus belong to the order Hymenoptera, primarily within the families Pteromalidae, Eulophidae, Encyrtidae, Eupelmidae, Eurotomidae, and Chalcididae, which are the most associated with parasitism. Other families are represented less frequently (Loftalizadeh 2012). There is currently no data available on the impact of these parasitoids on beetle populations; only species lists of parasitoids have been documented (Supplementary Material Table 3).

Predators

The impact of mites on bark beetle population dynamics is largely unexplored, yet it is often considered significant. Mortality rates caused by Pyemotes spp. and Iponemus spp. cunpan reach as high as 90% (Gäbler 1947, Kielczewski et al. 1983, Moser et al. 1989). Adult Pyemotes mites are free-living, while their juveniles develop within the mother’s body (Gerson et al. 2003). Additionally, the effect of Trichouropoda polytricha (Vitzthum, 1923) on P. chalcographus remains unknown (Hofstetter et al. 2015).

A diverse range of predators from the orders Diptera and Coleoptera have been identified (Supplementary Material Table 4). While Thanasimus formicarius (Linnaeus, 1758) also preys on P. chalcographus (Wigger 1996a), the primary predator is Nemozoma elongatum (Linnaeus, 1761) (Dippel 1996). N. elongatum is attracted to kairomonal cues and the boring dust produced by bark beetles. Its long-term abundance, with a time lag, is closely linked to P. chalcographus (Kopf and Funke 1998); however, its seasonal phenology varies significantly (cf. Baier 1991, Wigger 1996b).

The larvae of N. elongatum (across 3 instars) consume about 30 bark beetle larvae or pupae each. On average, adult N. elongatum live for 3 to 5 mo, and females typically lay around 50 to 60 eggs at varying temperatures. Adult N. elongatum consumes approximately one adult bark beetle per day, although some individuals can eat up to 3 prey items daily (Wigger 1994a, Dippel 1996). N. elongatum shows a positive correlation with the abundance of P. chalcographus, yet its predation does not appear to significantly impact the population density of this prey. Despite its high abundance, this predator likely cannot effectively suppress the population dynamics of P. chalcographus during outbreaks (Holuša et al. 2025).

Alongside N. elongatum, the most significant predators of P. chalcographus include dipterans from the genus Medetera and beetles from the genus Rhizophagus (Hulcr et al. 2005, Fora et al. 2012). Other predators play a minor role in the predation of P. chalcographus (Fora et al. 2012). Nicolai (1995) observed that up to 45% of larvae from I. typographus were killed by Medetera dendrobaena Kowarz, 1877, using infested logs. Based on prey consumption rates of M. dendrobaena larvae, Dippel et al. (1997) projected similar mortality rates for P. chalcographus.

Species of the genus Rhizophagus are likely only partially predatory, although there is documented evidence of predation by the larvae of Rhizophagus ferrugineus (Paykull, 1800), Rhizophagus depressus (Fabricius, 1792), and Rhizophagus dispar (Paykull, 1800) on various bark beetle species (Hanson 1937, Schroeder 1996).

While the impact of certain predators, such as T. formicarius and N. elongatum, is well documented, further research is needed to understand the dynamics of less-studied predator–prey interactions.

Damage and Outbreaks

The species is commonly found in large quantities within logging residues and debris on clear-cuts (Harding et al. 1986, Hedgren et al. 2003, Foit 2012). They are also present on tall stumps (Schroeder et al. 1999) and on wind-felled trees in gaps (Göthlin et al. 2000). The availability of suitable breeding material in clear-cuts makes P. chalcographus more prevalent in these clear-cuts compared to mature managed stands or old-growth forests (Johansson et al. 2006). Additionally, P. chalcographus can exploit damage caused by wind and snow in both young and older stands (Persson 1972, Rottmann 1985, Kula and Zabecki 2006, Kula et al. 2007). However, when given a choice, it tends to favor wind-felled trees, particularly older ones, while often ignoring breaks caused by other factors (Modlinger et al. 2009).

Furthermore, this species primarily attacks weakened hosts, such as trees damaged by wind or snow, as well as trees injured during logging operations (Starzyk et al. 2008) or weakened by defoliators (Kolk and Starzyk 1996). Additionally, P. chalcographus can detect and colonize trees weakened by drought (Escherich 1923, Harding et al. 1986, Chlodny et al. 1987, Byers et al. 1988).

P. chalcographus can independently kill thinner spruce trees when trees are damaged and weakened by abiotic factors and insect outbreaks (Christiansen 1989, Grodzki 1997). At high population densities, it can also attack healthy trees (Harding et al. 1986, Kolk and Starzyk 1996, Grodzki 1997, Lieutier 2004). Chararas (1960) noted that the critical number for tree mortality is 30 galleries per 1 m².

It is unclear to what extent P. chalcographus can overcome the defenses of large, mature spruce trees. However, it seems unlikely that this beetle could lead to tree mortality without the presence of other bark beetle species (Hedgren et al. 2003, Hedgren 2004). The older trees can only be killed if the lower part of their trunk is also colonized by I. typographus (Ehnström et al. 1974, Jurc at al. 2006, Zeniauskas and Gedminas 2010, Krams et al. 2012). This suggests that I. typographus is primarily responsible for killing those trees (Hedgren et al. 2003). Additionally, it has been observed that P. chalcographus prefers branches left after logging rather than those on standing Picea pungensEngelm. trees (Kula et al. 2009).

On smaller trees (eg, 2 to 4 m tall), P. chalcographus infests the entire tree length. This results in symptoms in the crown, such as red needles, which become fully visible (Harding et al. 1986). In larger trees, infestations initially occur high on the trunk, and needle discoloration is not usually noticeable at first. Over time, subsequent attacks gradually lower the trunk (Novák et al. 1976) (Fig. 2).

Outbreaks of P. chalcographus are typically associated with forests of Norway spruce, as this bark beetle species primarily targets spruce trees. While bark beetle outbreaks may affect various tree species, such as pines, P. chalcographus is generally not considered a pest of pine forests. P. chalcographus occasionally causes high tree mortality in young stands of Norway spruce (Trägårdh and Butovitsch 1935, Thomsen 1939, Klauser 1954, Ehnström 1985, Eidmann 1992). For example, in 1970–1971, many young spruce trees in southern Sweden were attacked and killed (Ehnström et al. 1974, Eidmann 1992). The primary effects of the bark beetle outbreak in Poland from 1981 to 1986, along with a previously applied selective pest control strategy, mainly involved the disruption of competition among insect communities and an increased availability of breeding materials, which benefited P. chalcographus (Grodzki 1997). It seems that harmful infestations have become more frequent since the late 20th century. In Slovenia, P. chalcographus caused damage for only 3 yr during the 1980s, while in the 1990s, the damage persisted for 8 yr (Jurc et al. 2006).

In the summer of 2015, a widespread infestation of bark beetles affected spruce stands across all age categories in the Czech Republic due to unusually warm and dry weather (Knížek et al. 2016). High beetle populations continued to pose a threat to forests in 2016. However, only a few trees in young spruce stands were affected. In certain locations, P. chalcographus attacked the youngest stressed trees as well as entire planted forest cultures. By 2016, population densities had significantly declined, but the affected areas remained relatively small, ranging from 0.01 to 0.1 hectares. Infestations were mainly concentrated on open young stands created after salvage logging, while closed young stands generally remained unaffected (Fig. 6).

Additionally, since P. chalcographus often accompanies I. typographus during outbreaks (Kunca 2007, Zúbrik et al. 2008), it is challenging to estimate the total volume of damaged timber (Galko et al. 2017).

Management and Control Options

A significant amount of freshly logged debris, primarily tops and branches left behind in the forest stands, created ideal breeding conditions, especially for smaller bark beetles like P. chalcographus. The rapid increase in this insect population seems to result from recent human mistakes in insect control and forest management practices during outbreak conditions (Grodzki 1997).

A comprehensive set of prevention, control, and monitoring methods has been implemented as part of IPM in various European countries (Gregoire and Evans 2004). However, few studies assess the actual impact of these practices or examine the differences in colonization rates or reproductive potential.

The reduction of P. chalcographus populations and other secondary bark beetle species primarily depends on preventive measures. These measures include following forest health principles, such as promptly removing infested trees before the beetle swarm (Wermelinger 2004) and minimizing the amount of wood that the beetles could target.

Monitoring

Predictive models, such as CHAPY, and various pheromone traps are available for monitoring the swarming of P. chalcographus. While pheromone traps can help track beetle activity, they might not accurately indicate the risk of localized outbreaks in vulnerable areas. Regular field inspections of young spruce stands allow for early detection and support more effective pest management.

Considering the long-term use of pheromone traps and their relatively straightforward operation, their application principles, advantages, and disadvantages are further described. Pheromone traps are beneficial for monitoring beetle populations over large areas and require minimal maintenance. However, their limitations include inconsistent population control and the potential to attract nontarget species, which can reduce their overall effectiveness in managing P. chalcographus outbreaks.

The CHAPY model was developed in Slovenia to simulate the phenology of P. chalcographus. The development of P. chalcographus is influenced by temperature, which affects crucial life stages such as swarming, tree infestation, and the emergence of new generations. The model incorporates critical temperature thresholds, including lower (7.4 °C) and upper (39.4 °C) limits, along with an optimal development temperature of 30 °C. It is calibrated using data from field studies. It can accurately predict phenological events, such as spring swarming (with an average deviation of 5.6 d) and the emergence of new beetle generations (with a deviation of 2.1 d) (Ogris et al. 2020, Ogris 2024).

The CHAPY model can be applied in various forest management scenarios to predict beetle outbreaks and assist in the timely deployment of monitoring tools, such as pheromone traps. The model output provides essential information to guide decisions on implementing control measures, including mass trapping or sanitation logging. Moreover, the model has been integrated into publicly available web applications, allowing forest managers to access predictions for beetle development in any location within Slovenia (Ogris 2024).

In Central European countries, synthetic pheromones for the P. chalcographus beetle were introduced into forestry practices using artificial pheromone traps as early as the 1990s (Brutovský 1996). Numerous studies on their practical use have focused on techniques for trap deployment and assessing their effectiveness (Babuder et al. 1996, Brutovský 1996, Dubbel and Vaupel 1996). These studies also examined the suitability of pheromone traps for monitoring and prediction purposes (Niemeyer 1992, Zuber and Benz 1992) and assessed the selectivity of the traps themselves (Pavlin 1991). Recently, 2 types of panel black traps have been commonly used: Theysohn traps (eg, Galko et al. 2012, Kasumović et al. 2016, Fora and Balog 2021), and Ecotraps (eg, Pernek 2002, Marković and Stojanović 2010, Galko et al. 2012).

A variety of pheromone dispensers, primarily containing chalcogran and E,Z-MD, have been found to attract adult P. chalcographus effectively when placed in traps. However, the number of beetles captured varies among different types of pheromone dispensers (Zuber and Benz 1992, Pernek 2002, Brutovský et al. 2011, Isaia and Paraschiv 2011, Kasumović et al. 2016). Additionally, the sex ratio of the captured beetles varies based on the type of dispenser used. For example, Chalcoprax-baited (the main active ingredient is chalcogran) traps tend to catch 60% to 75% of females, while Pheroprax-baited (the main active ingredient is (S)-cis-verbenol) traps capture only 20% to 30% of females from the total P. chalcographus caught. The sex ratio also changes throughout the flight activity period: more males are captured during the first flight peak, but after 2 to 3 wk, the number of males captured begins to decline (Zuber and Benz 1992).

Some pheromone dispensers include a combination of volatile compounds that target other bark beetle species, primarily I. typographus. Nevertheless, the capture rates for P. chalcographus in such setups are likely to be lower (Zahradník and Zahradníková 2015), although one study suggests otherwise (Brutovský et al. 2011). Additionally, the type of trap used also can influence these capture rates (Pernek 2002). A significant challenge is that separating the two species, P. chalcographus and I. typographus, is difficult, making it unclear how many individuals of P. chalcographus are captured.

It is not advisable to place Pheroprax and Chalcoprax in the same trap; however, it is possible to set up multiple traps in a star-shaped arrangement, with each trap equipped with a separate dispenser (Pavlin 1991, Dubbel and Vaupel 1996). The interaction between the response of the two species to each other’s pheromones is quite significant. The pheromone components of P. chalcographus (chalcogran and E,Z-MD) actually inhibit I. typographus from responding to its own pheromones. In contrast, the pheromones released by I. typographus to its own attractants. As a result, I. typographus tends to avoid tree trunks that have already been colonized by P. chalcographus, while P. chalcographus shows no avoidance behavior toward I. typographus (Byers 1993). This interaction provides a distinct evolutionary advantage for P. chalcographus. It can colonize the upper parts of trees where the lower trunk is already occupied by I. typographus, which means the tree’s defenses have already been compromised. Additionally, P. chalcographus can exploit any available space on thicker sections of the tree trunk between the galleries of I. typographus.

The distance between the trap and the nearest tree should be at least 10 to 15 m in young stands and 5 m in older stands. Hanging traps as high as possible is recommended (Hrašovec 1995). Niemeyer (1992) reported significant variations in trap distances, ranging from 30 to 50 m apart, with traps placed 10 to 15 m away from the nearest young spruce or 5 to 10 m from mature trees. Reducing these distances increases the risk of infestation in standing spruces (Hrašovec 1995).

Traps should be checked regularly, as dead bark beetles release 1-hexanol and verbenone, reducing pheromone trap effectiveness. (Zhang et al. 2003).

While a relationship has been established between the number of P. chalcographus caught in pheromone traps, and the percentage of spruce fell during the creation of mature stands used as trapping sites (Schroeder 2013), it is important to recognize that trap catches do not necessarily reflect the actual population densities in the surrounding area. Even when traps capture many beetles, these catches may not significantly impact the population dynamics of P. chalcographus (Kacprzyk and Bednarz 2021). Data on reproductive success in branches (Kacprzyk 2012) indicate that an average branch, measuring 4 m in length and 10 cm in circumference (equivalent to 40 dm²) can produce 1,600 beetles. In contrast, a single mature spruce tree can produce up to 160,000 beetles. Therefore, estimates like those provided by Jurc et al. (2006), which are based on only 5 trees or 1 to 2 mature spruces (Kasimović et al. 2016), should be interpreted with caution. Thus, it would be unreasonable to conclude that trap catches of 30,000 or 60,000 P. chalcographus individuals per year indicate a high population density of this bark beetle (Simionescu et al. 2003, Zúbrik et al. 2008).

The effectiveness of pheromone traps in controlling P. chalcographus populations is generally limited. The number of beetles emerging from infested trees can often equal or exceed the number of individuals captured in a single pheromone trap throughout the entire active period. Therefore, using pheromone traps primarily for monitoring bark beetle populations rather is recommend than as the main control method. The number of beetles can be estimated by measuring the volume of those caught; for example, 1 ml of trapped beetles corresponds to approximately 400 individuals of P. chalcographus (Hrašovec 1995).

Thus, traps can effectively indicate the beginning of flight activity (Kacprzyk and Bednarz 2021). Furthermore, pheromone traps may also be used in smaller areas to capture and aggregate emerging beetles (see Supplementary Material Case study 3). Notably, the capture of adult P. chalcographus using traps baited with industrially produced dispensers follows a similar trend in regions that experience 2 generations per year (Zuber and Benz 1992, Zúbrik et al. 2008, Galko et al. 2012, Kacprzyk and Bednarz 2021). Although the number of overwintering beetles caught in the spring is relatively low, the quantity of offspring beetles that fly in the summer is significantly higher (Fig. 5). This increase is not due to reproduction, which typically results in 2 to 3, and at most 4 offspring per female (Hedgren 2004, Kacprzyk 2012, Kacprzyk and Bednarz 2021), but rather because the traps strongly attract beetles from the surrounding area. This attraction is linked to their strong response to chalcogran combined with E,Z-MD (Francke et al. 1977, Byers et al. 1988, Birgersson et al. 1990).

Pheromone traps play a crucial role in detecting the introduction of nonnative species into new countries (Sweeney et al. 2007). The species P. chalcographus has also been introduced to several regions, including New Zealand, where there have been more than 60 recorded instances of its introduction before 2000 (Brockerhoff et al. 2003). In the United States, P. chalcographus has been intercepted multiple times, with nearly all detections occurring on wood products such as solid wood packaging materials, crates, and pallets (Haack 2001, 2006, Rabaglia et al. 2019).

One significant drawback of using pheromone traps is that they tend to capture large numbers of non-target insects (Aukema et al. 2000). Specifically, in the case of pheromone dispensers containing chalcogran, N. elongatum is particularly abundant. According to various studies (Baader and Vité 1986, Wegensteiner and Führer 1991, Skatulla and Feicht 1992, Zahradník and Zahradníková 2020), this species can make up to 20% of the total catch (Wigger 1996a).

Cultural Control

Managed forests can provide a substantial amount of breeding materials for bark beetles due to the abundance of logging debris and the increased likelihood of windfalls at forest edges (Schlyter and Lundgren 1993). Among these beetles, P. chalcographus is the most prevalent species found on logging debris of Norway spruce (Kula and Kajfosz 2006, 2007, Za̧becki and Kacprzyk 2007). This species also occurs on pine (Pinus spp.), where it is more commonly found on thicker materials compared to branches that are only 1 cm thick (Foit 2012, 2015).

The number of P. chalcographus beetles attracted to logging debris after regeneration felling does not depend on the volume of these residues. However, the highest number of beetles tends to fly into clear-cut areas, remaining elevated up to 50 m from the edge of the clear-cuts. The population stabilizes at 75 m, equal to the numbers found in the surrounding areas. While regeneration operations do increase the abundance of P. chalcographus, there is no significant risk of these beetles spreading into nearby healthy forests (Toivanen et al. 2009).

Several methods are recommended for removing logging debris (eg, dragging, burning, chipping), to prevent the proliferation of pests (DeGomez et al. 2008). However, in situations where pest populations are high, and forest stands are weakened, the primary objective is to reduce the amount of available material or to make the material less attractive to pests. This can be achieved by using the following methods.

Logging Residues in Piles

After mature spruce trees are felled, the leftover branches provide excellent breeding material for P. chalcographus beetles, resulting in many offspring (Kacprzyk 2012). Under weather conditions that do not deviate from the long-term average and in the absence of bark beetle outbreaks, leaving logging debris after clear-cutting of Norway spruce stands as part of standard forest management practices does not appear to affect the mortality of surrounding trees. However, the impact of severe drought conditions remains unclear (Hedgren et al. 2003). Consequently, some researchers suggest that the overall effect of logging residue piles on the proliferation of P. chalcographus is still uncertain, particularly in the case of debris found in pine stands (Foit 2015).

The use of branches collected in piles as natural traps for small bark beetles has been suggested in previous research (Wegensteiner et al. 1989, Za̧becki and Kacprzyk 2007, Schroeder 2008). Some authors argue that spruce branches and tops should be left in sunny areas after logging to dry more quickly, which could reduce their attractiveness to bark beetles (Harding et al. 1986). However, this practice might complicate other forestry operations. It is noteworthy that dispersed branches attract twice as many beetles and have a significantly higher infestation density of P. chalcographus compared to those collected in piles. Nevertheless, the number of offspring beetles emerging from the branches remains the same (Za̧becki and Kacprzyk 2007, Kacprzyk 2012). The piles increased the risk of P. chalcographus attack, but almost no attacked tree was killed. Attacks were associated with pile colonization, whereas emergence by the new generation beetles did not increase the risk of attack (Hedgren 2002, 2004).

The difference often stems from how the branches are colonized from various pile sections. Colonization tends to be highest in the top layers to the bottom. Interestingly, reproductive success is like, or even higher in the middle layers of the piles. However, even under conditions of severe weakening of spruce stands, the location and method of logging residue disposal play a secondary role and do not significantly affect the number of new-generation individuals emerging from the material (Kacprzyk 2012).

When comparing dispersed branches to piles of P. sylvestris branches, it was found that the piles had lower colonization rates than the dispersed branches; the lower layers were less attractive to most species. Piling branches helps to reduce the reproduction of specific pest species, such as Ips acuminatus (Gyllenhal, 1827) and P. chalcographus, because the microclimatic conditions in the lower layers of the piles are less favorable for them (Foit 2015).

Timing of Thinning and Final Felling

Leaving material from thinning operations on-site is a common practice, especially when removal is not economically feasible. This is particularly true for precommercial thinning, where the removed trees are not large enough to be sold. As a result, clearing this material incurs costs without the benefit of timber sales. This material is often left to decompose on-site, contributing to improved soil quality and supporting forest ecosystems. Research shows that, in the case of precommercial thinning, leaving smaller trees on the forest floor is considered an investment in the future growth of the forest (Simon and Ameztegui 2023).

In Central Europe, where P. chalcographus produces two generations per year, significantly higher average infestation densities are observed in P. abies saplings during the spring, particularly in hilly areas, with 80% of the trees affected. In contrast, timber fell later in the summer and autumn shows much lower infestation levels, around 3%. There is a slight increase in infestation density in material collected during autumn (Hochmut 1977).

Also logging debris from Scots pine final felling provides a substrate that supports significant reproduction of pests such as P. chalcographus, as well as other potentially significant species like I. acuminatus and Pityophthorus pityographus (Ratzeburg, 1837) (Foit 2015). The risk of reproducing these pests can be reduced by scheduling of tree felling for August (and likely in September and October as well), as these months typically have the lowest levels of infestation (Foit 2015). The highest populations of bark beetles were also observed on debris produced during the winter and spring months thinning, while infestations were minimal in summer and autumn (August to October). This trend may be attributed to the lack of beetle swarming during these months, as well as the fact that older logging debris is less attractive to the beetles (Foit 2012).

The frequent colonization of logging debris in August, September, and to some extent in October can likely be attributed to 2 main factors: (i) the debris from these months is not colonized during the current year, likely due to a lack of beetle swarming; and (ii) by the following growing season, the residues have aged enough that they are no longer attractive for colonization (Foit 2015).

Based on these findings, it is recommended to focus felling operations in August and September (October) to reduce the risk of P. chalcographus outbreaks without the need to remove logging debris, which could be environmentally unsustainable. This method could effectively manage bark beetle risks, especially in Central Europe’s lower and mid-elevation areas (Foit 2012, 2015). Research indicates that pine logging debris can provide a substrate for bark beetle development; however, removing these residues from the forest could harm the biodiversity of saproxylic species, which is considered environmentally unsustainable (Foit 2012, 2015).

Thinning Intensity

The abundance of P. abies saplings infested by P. chalcographus is influenced by the intensity of the thinning operation and the distance from a suitable material to the forest edge. Infestation rates at the edge of the stand are twice as high (Hochmut 1977). Additionally, the intensity of the thinning intervention affects the frequency of individual tree infestations. In cases of high thinning intensity, where every second row of trees is removed, the infestation rate can reach 100%. At a lower intensity, where every fourth row is removed, the infestation rate decreases to 89%. In individual interventions, with an average attack rate of one nuptial chamber per dm², infestation rates drop to 60%, 20%, and 10%, respectively (Hochmut 1977).

Shortening of Trees during Thinning Operations

When thinning, there are 3 options for handling the felled saplings: leaving them in place, dragging them to an open area, or cutting them into shorter pieces. Dragging the saplings out is labor-intensive and costly while cutting them is less demanding. Research shows that the highest rates of infestation occur in uncut saplings left standing, while the lowest rates are found in sections shorter than 2 m (Kula and Kajfosz 2006, 2007). Therefore, cutting the remaining saplings can significantly reduce the population buildup of P. chalcographus.

Mechanical and Physical Control

Trapping Bark Beetles Using Pheromone Traps

The importance of pheromone traps should not be overstated, as mass trapping of I. typographus over large areas using these traps may not be fully effective. This is because mass trapping can only capture a limited proportion of the overall bark beetle population (Weslien and Lindelöw 1990, Dimitri et al. 1992, Heber et al. 2021). Therefore, mass trapping should not be the only method employed for forest protection; it should be combined with other strategies, such as the prompt removal of infested trees, to reduce the damage caused by bark beetles effectively (Faccoli and Stergulc 2008).

In the case of P. chalcographus, pheromone traps can be effectively used to capture local populations, particularly the offspring generation (see Supplementary Material Case study 3), in limited areas. The efficiency of the traps is high, allowing them to capture beetles from the surrounding environment (see chapter “Importance of Traps”).

To capture the offspring generation of beetles, use of pheromone traps should be preferred instead of other methods like insecticide-sprayed logs, which can be harmful to various nontarget insects. However, a significant challenge arises from the “spillover effect”. This occurs when pheromone traps attract more beetles than they can capture, causing the excess beetles to infest nearby trees (Kuhn et al. 2022).

Trap Trees and Baited Trees

P. chalcographus naturally colonizes suitable materials from its preferred host trees, which include spruce, pine, and larch. These trees are often used as trap trees or logs for other bark beetle species, such as spruce or pine bark beetles. P. chalcographus typically infests the upper regions of trees and areas not occupied by I. typographus. Interestingly, trap trees (even those set for I. typographus) also effectively attract P. chalcographus, with its population density increasing toward the top of the tree (Grodzki 2003, Holuša et al. 2017). However, it is impractical to prepare trap trees from smaller-diameter spruce, as sometimes suggested (Sedlaczek 1922, Zahradník 2007), because a significant number of residual trees already remain in the forest, rendering additional trap trees unnecessary and economically inefficient.

Trees baited with pheromones tend to have significantly higher infestation levels, especially in areas with thinner bark, which is more conducive to P. chalcographus (Grodzki 2003). This strategy can be used to “clean up” areas affected by bark beetles if the use of traps is to be avoided for cost-saving reasons. Infested trap trees should be either removed from the forest or debarked, and the bark, along with debris material (tops and branches), should be burned or otherwise destroyed (Grodzki 2003).

Lure-baited Insecticide-treated Trap Logs

Insecticide-treated trap logs, designed as tripods or quadpods, have been commonly used by foresters since the 1990s (Lubojacký and Holuša 2013) to control I. typographus (Lubojacký and Holuša 2011, 2014, Hurling and Stetter 2012) and Ips duplicatus (Lubojacký and Holuša 2013). These trap logs consist of 3 or 4 logs, each approximately 2 m long with a minimum diameter of 15 cm. The surfaces of the logs are coated with a contact insecticide mixture. While this method has been widely effective against I. typographus (Lubojacký and Holuša 2011, Hurling and Stetter 2012) and I. duplicatus (Lubojacký and Holuša 2013), there have been only a few documented instances of its use for P. chalcographus (Tomiczek 2009, Galko et al. 2012, Holuša personal observation in 2006).

Pheromone traps generally capture more beetles than insecticide-treated quadpod trap logs. However, the quadpods (referred to as “Fangtipi” in the original sources) tend to result in a higher number of nontarget catches, which can include beneficial organisms such as ants and beetles. Additionally, there is a risk of insecticide runoff into the soil; however, this can be minimized by placing trap logs on geotextile fabric (Tomiczek 2009).

Although this method is convenient because it does not require collecting trapped insects (only periodic reapplication of insecticide and replacing pheromone dispensers) it is not recommended. The method kills a significant number of nontarget insects, and its effect on bark beetle population dynamics is uncertain and likely minimal (eg, Lubojacký and Holuša 2014).

Treatment of Infested Host Tree Parts

General measures against bark beetles include the removal of freshly infested spruce trees from the forest (infested material should be either chipped or burned) (Schroeder 2008, Fora and Balog 2021). Chipping is an effective method for killing large numbers of beetles, but it is important that the size of the wood chips, including bark, does not exceed 100 cm² (Haack and Petrice 2009). Unfortunately, chipping standing P. pungens trees has proven insufficient, as larger pieces (greater than 10 cm) were left behind, allowing P. chalcographus to survive. Therefore, chipping alone is not fully effective (Fiala and Holuša 2022).

One alternative method for treating logs before the flight of P. chalcographus is to use an aqueous solution of Ca(OH)₂ combined with milk and vegetable oil. Although treated logs showed lower infestation levels, the difference between treating entire piles of logs and individual logs was insignificant (Grodzki et al. 2007).

To minimize the risk of bark- and wood-boring beetle pests, forests often implement extensive removal of logging debris. However, this practice can result in a decline in the diversity of saproxylic insects (Jonsell 2008a, 2008b, Bouget et al. 2012, Jonsell and Schroder 2014).

Push–(Pull) Strategy

The push–pull strategy combines repellents (“push”) that repel pests from trees and attractants (“pull”) that entice them into traps (Afzal et al. 2023). However, using repellents, such as nonhost volatiles (NHVs) derived from trees that pests do not prefer, along with trapping on logs, has not effectively reduced populations of I. typographus. Studies show no statistically significant impact on lowering bark beetle populations or diverting them from forest edges into traps (Lindmark et al. 2022). While anti-attractants can partially repel beetles, they may also lead to a “switch effect”, redirecting beetle attacks to adjacent trees. This can potentially cause higher infestations in those areas than in the protected ones (Jakuš et al. 2023). The push-and-pull method is especially ineffective during extreme drought conditions and when beetle populations are high. Further optimization is recommended, including placing antiattractants lower in the trees (Jakuš et al. 2023). Similar challenges are likely to occur when applying antiattractants for P. chalcographus.

Since P. chalcographus is a species that prefers coniferous trees, it is probable that repellent volatile compounds emitted by broadleaf trees (Zhang et al. 1999a, Poland and Haack 2000, Schlyter et al. 2000, Peverieri et al. 2004), especially birch, would have a repelling effect. Research has shown that these compounds can reduce the effectiveness of aggregation pheromones (Dickens et al. 1992). In the case of I. typographus, these NHVs have been found to reduce attacks by up to 70% (Zhang et al. 1999b). Therefore, it is likely that spruce trees would be better protected from colonization in mixed forests with a higher proportion of birch trees.

The flight activity of P. chalcographus toward wilting trees or logging debris from thinnings in spruce and pine stands may be significantly reduced by increasing the proportion of birch trees. Volatile compounds released by birch, such as (Z)-3-hexen-1-ol and 1-hexanol, inhibit the beetles’ ability to respond to aggregation pheromones. When combined, verbenone and (Z)-3-hexen-1-ol have a synergistic effect, and males are more strongly inhibited than females. Additionally, ethanol has been shown to reduce beetle attraction, as it reflects the activity of microorganisms on decaying wood (Byers et al. 1998). Since this approach only repels P. chalcographus, it is the Push-(pull) strategy. However, whether this method can be effectively applied in IPM or combination with pheromone traps for the “pull” component remains to be seen.

Biological Control

Currently, there is no known biological control method for P. chalcographus. Direct application of entomopathogenic fungi, such as B. bassiana, Metarhizium anisopliae (Metsch.) Sorokin, 1883, and Cordyceps fumosorosea (Wize) Kepler, B. Shrestha and Spatafora, 2017, has resulted in 100% mortality of P. chalcographus beetles within 10 d. However, this approach is impractical in natural settings as beetles are only outside the bark during their flight period. Preventive spraying of entire logs is not cost-effective and only causes mortality in a small percentage of I. typographus beetles (Jakuš and Blaženec 2011). Introducing entomopathogenic fungi by using beetles caught in traps is misguided because fungi from adult beetles do not transfer to larvae, meaning it will not lead to an epizootic outbreak (Weiser 1966, Vakula et al. 2019). Consequently, this method is ineffective, and re-releasing trapped beetles serves no purpose.

Recent studies indicate that using antiattractants for I. typographus, along with attractants for its natural predator, T. formicarius, could effectively prevent tree infestation by bark beetles. This combination of substances enhances tree protection (Korolyova et al. 2024). Additionally, this strategy might increase pressure on the boring beetles of P. chalcographus. Conversely, the pheromone dispensers containing bicolorin could be used to attract N. elongatum to areas with high densities of P. chalcographus, potentially boosting their populations (Holuša et al. 2025).

Sterilization has not been studied or used as a control strategy for bark beetles, including P. chalcographus. This may be due to practical challenges in breeding, sterilizing, and releasing enough of males for the method to be effective. Additionally, bark beetles’ complex biology and life cycles complicate the process further.

Conclusion

P. chalcographus is typically regarded as a secondary pest, often accompanying I. typographus. It benefits from outbreak in 2 primary ways. First, during I. typographus outbreaks, many infested trees are cut down, leaving behind an abundance of branches and tops that provide ideal breeding material for P. chalcographus. Second, when I. typographus attacks mature trees, P. chalcographus can successfully reproduce in the thinner, upper parts of those trees. Although P. chalcographus has limited ability to kill healthy trees on its own, it can cause significant damage when its populations are high, particularly in young spruce stands. The pest becomes especially problematic following abiotic disturbances, such as drought or windthrow, as it can exploit weakened trees and logging debris. While its impact is generally less severe than that of I. typographus, P. chalcographus can still result in considerable mortality among young spruce trees under the right conditions.

IPM should focus on practical, targeted approaches instead of relying solely on extensive monitoring, as population densities can still increase despite consistent surveillance. When high population densities are confirmed through visual inspection of wilting trees and the infestation spreads, removing the most affected areas is advisable by cutting down and destroying the infested trees. Additionally, using pheromone traps can help capture any remaining individuals. Reducing available breeding materials by burning or chipping logging debris is essential to control outbreaks effectively.

For effective cultural control, logging operations, and thinning should ideally be conducted in August and September. Thinning, along with cutting trunks into sections shorter than 2 m, has proven to be one of the most effective methods for managing P. chalcographus. Although piles of logging debris do not significantly affect beetle populations, they provide several practical benefits. Piling branches not only facilitates forest operations, such as planting, but also simplifies overall management. It is recommended to place the logging debris, whether from spruce or pine, into piles. While branches on the surface are often heavily infested with high reproductive potential, those buried deeper in the piles are less colonized. This reduces the amount of breeding material for P. chalcographus.

The use of chemicals should be limited because broad treatments, including chemical spraying, can harm nontarget species. Therefore, targeted mechanical and cultural controls are the most sustainable strategies for managing P. chalcographus outbreaks.

As climate change progresses, several factors may influence the population dynamics and management of P. chalcographus. Warmer temperatures and prolonged droughts may increase host susceptibility, making trees more vulnerable to infestation. Changes in flight activity, including earlier or prolonged emergence periods, could impact monitoring efforts, necessitating adjustments in pheromone trap deployment. Additionally, rising temperatures might accelerate development and lead to an increase in the number of generations per year, potentially intensifying outbreaks. Such shifts could alter its pest status, either by increasing its impact due to higher reproduction rates or, conversely, reducing its influence if competition with I. typographus or other factors become limiting. Effective long-term monitoring and adaptive management will be crucial to address these potential changes and maintain control over P. chalcographus populations in the evolving climate.

Author contributions

Jaroslav Holusa (Conceptualization [lead], Investigation [equal], Methodology [equal], Visualization [equal], Writing—original draft [lead], Writing—review & editing [equal]), and Tomas Fiala (Conceptualization [equal], Investigation [equal], Visualization [equal], Writing—original draft [equal], Writing—review & editing [equal])

Funding

This work was supported by the grant of the Ministry of Agriculture of the Czech Republic, NAZV QL24010235.

Conflicts of interest. None declared.

References