Evolutionary ecology of the bark beetles Ips typographus and Pityogenes chalcographus

Abstract Ips typographus (L.) and Pityogenes chalcographus (L.) (Coleoptera: Curculionidae) are two common bark beetle species on Norway spruce in Eurasia. Multiple biotic and abiotic factors affect the life cycles of these two beetles, shaping their ecology and evolution. In this article, we provide a comprehensive and comparative summary of selected life-history traits. We highlight similarities and differences in biotic factors, like host range, interspecific competition, host colonization, reproductive behaviour and fungal symbioses. Moreover, we focus on the species' responses to abiotic factors and compare their temperature-dependent development and flight behaviour, cold adaptations and diapause strategies. Differences in biotic and abiotic traits might be the result of recent, species-specific evolutionary histories, particularly during the Pleistocene, with differences in glacial survival and postglacial recolonization. Finally, we discuss future research directions to understand ecological and evolutionary pathways of the two bark beetle species, for both basic research and applied forest management.


Introduction
Bark beetles (Coleoptera: Curculionidae: Scolytinae) are among the most destructive forest pests worldwide . This group of weevils harbours highly diverse species that spend the majority of their life cycles in various tissues of woody and herbal plants (Kirkendall et al., 2015). Bark beetles are of high ecological importance, as the majority of species lives in dead or dying plants, thus being early decomposers in forest ecosystems . Only a small number of species can successfully breed in living hosts (Kirkendall et al., 2015). These species, however, make bark beetles major forest or agricultural pests Raffa et al., 2015). In addition, bark beetles are interesting study systems in evolutionary research, as several traits, e.g. feeding modes, reproductive systems, sex determination systems and levels of sociality, often originated multiple times independently (Kirkendall et al., 2015).
Here we compare life-history traits of I. typographus and P. chalcographus, discuss them in the light of their evolutionary past and propose future research directions for a better understanding of the biology and ecology of these bark beetles.
Ips typographus and P. chalcographusan overview Both I. typographus and P. chalcographus have a similar geographic range and are distributed from the Mediterranean region to Northern Scandinavia and from Western Europe to East Asia (Knizek, 2011). The life cycles of I. typographus and P. chalcographus correspond to that of 'standard' polygynous phloem breeders (Sauvard, 2004). In brief, after hibernation adults leave their overwintering habitat and start to swarm in spring. In both species the male initiates a breeding system on a host that was selected on the basis of visual, olfactory and gustatory cues. After overcoming initial tree defence mechanisms, e.g. resin flow, the male bores through the bark and establishes a mating chamber in the phloem (Postner, 1974;Wermelinger, 2004). By releasing aggregation pheromones, other male and female conspecifics are subsequently attracted (Francke et al., 1977;Schlyter et al., 1987aSchlyter et al., , 1987bByers et al., 1988Byers et al., , 1989Birgersson et al., 1990). In both species usually multiple females enter the mating chamber, copulate with a single male and start to excavate individual mother tunnels. On either side of these tunnels eggs are deposited in small niches. Male and female parents express subsocial behaviour, as they perform brood care, e.g. by grooming eggs, providing a well-protected environment and a highly disposable food source for their offspring. Larvae establish individual tunnels in the phloem layer and finally pupate in pupal chambers. Afterwards young adults perform a maturation feeding in the surrounding phloem for gonad and flight muscle development, bore through the outer bark and disperse to establish the next generation (Postner, 1974). Additionally, parental beetles of both species can re-emerge after a first brood establishment and initiate another offspring generation, i.e. a sister brood (Annila, 1969;Postner, 1974;Anderbrant and Lofqvist, 1988;Anderbrant, 1989). Under favourable environmental conditions I. typographus and P. chalcographus can establish up to three generations (plus sister broods) per year (Postner, 1974;Wermelinger, 2004;Wermelinger et al., 2012), even though the discrimination of different generations and sister broods in the field is difficult.

Biotic factors
Host range Both I. typographus and P. chalcographus can utilize different conifer species of the family Pinaceae. However, they differ in their potential host range, their preference for and their performance on different tree species (table 1, fig. 1) (Postner, 1974;Pfeffer, 1995;Bertheau et al., 2009b).
Ips typographus is generally classified as a monophagous bark beetle, whereas P. chalcographus is regarded to be oligophagous (Eidmann, 1987;Mayer et al., 2015). The primary host of both beetles is Norway spruce, as it was proven by presence-abundance data under natural conditions or by preference-performance studies in the laboratory (Postner, 1974;Pfeffer, 1995;Bertheau et al., 2009aBertheau et al., , 2009bCognato, 2015;Schroeder and Cocos, 2018). Secondary, less commonly used hosts of both species are from the genera Pinus, Larix, Abies or Pseudotsuga, both native and non-native species (Postner, 1974;Bertheau et al., 2009b;Schroeder and Cocos, 2018). Although the two beetles can colonize and breed in similar host species, P. chalcographus is more abundant on all secondary hosts (Bertheau et al., 2009b;Schroeder and Cocos, 2018). The oligophagous behaviour can confer an ecological and evolutionary advantage to P. chalcographus, as it can shift to other plants when primary hosts are scarce or absent, e.g. when new habitats are invaded.

Interspecific competition
Resource partitioning and competition have been described from various bark beetles (Paine et al., 1981;Light et al., 1983;Bouhot et al., 1988;Rankin and Borden, 1991). Smaller-sized species colonize upper, thin-barked parts of trees, whereas larger-sized beetles are found in lower, thick-barked sections. This separation in ecological niches reduces interspecific competition and is driven by the diameter of the body (Grünwald, 1986;Schlyter and Anderbrant, 1993;Amezaga and Rodriguez, 1998). Hence, given the larger body size of I. typographus, it prevails in thicker host material/stem sections than P. chalcographus, although overlaps occur (table 1, fig. 1) (Grünwald, 1986;Göthlin et al., 2000). The latter one can utilize a broader range of bark sections, including those preferred by I. typographus. In these thicker parts, however, P. chalcographus is apparently outcompeted (Grünwald, 1986).
Mating system, reproductive performance and gallery morphology Ips typographus and P. chalcographus are polygynous bark beetles, however, they differ markedly in the number of females attracted per male (table 1, fig. 1). After building a mating chamber in the phloem, P. chalcographus males can mate with up to nine females (Schwerdtfeger, 1929), whereas I. typographus has harem sizes of generally two or three, sometimes four, females (Wermelinger, 2004).
Both species differ in the architecture of their galleries (table 1, fig. 1). After mating I. typographus females build their mother tunnels in a vertical direction, following the fibre direction of the host. In contrast, females of P. chalcographus construct their mother tunnels in different directions starting from a central mating chamber, resulting in a stellar breeding system (Postner, 1974). Obviously, these galleries are shaped by the number of females per male but underlying evolutionary drivers for these differences in the reproductive biology are unknown. It is also unclear if there is an optimal direction for maternal or larval tunnels, which might be influenced by factors, like the spatial structure of nutrients and tree defences as well as growth properties of associated fungi.
Pityogenes chalcographus usually shows higher attack densities and higher egg gallery densities on hosts and tends to be more ubiquitous than I. typographus (Hedgren et al., 2003;Hedgren, 2004). The latter one, however, has a much higher tree-killing ability on living hosts (Hedgren, 2004). Moreover, P. chalcographus is often found on trees that have already been attacked by I. typographus (Hedgren, 2004). In habitats with plenty of suitable breeding material, e.g. storm-felled trees, P. chalcographus is more abundant in later phases of a post-storm period (Hedgren, 2004;Schroeder and Cocos, 2018), underlining different preferences for host quality.
Frequency differences of fungal associates, however, might result in different tree-killing abilities of the two bark beetle species (Krokene and Solheim, 1996). Ips typographus is commonly associated with the highly virulent fungus Endoconidiophora polonica that may help beetles to overcome trees (Horntvedt et al., 1983; but see Six and Wingfield, 2011) by over-stimulation of host defence (Lieutier et al., 2009). Frequencies of E. polonica are locally highly variable and appear to differ between epidemic and non-epidemic populations (Harding, 1989;Solheim, 1993;Krokene and Solheim, 1996). Likely this fungus is a good colonizer of well-defended phloem of trees during epidemics (Solheim, 1992;Gibbs, 1993). Like other symbionts, it can cope with tree defensive compounds, e.g. terpenes or phenolics, and can even use those substances as a carbon source for growth (Krokene and Solheim, 1998;Hammerbacher et al., 2013;Krokene, 2015;Wadke et al., 2016).
The second important fungal associate of I. typographus is Grosmannia penicillata. Like E. polonica, it is transmitted by dispersing beetles on the body surface via sticky spores (Furniss et al., 1990) and is virulent to Norway spruce (Horntvedt et al., 1983). Recently, it has also been shown that it can synthesize components of the aggregation pheromone of I. typographus (Zhao et al., 2015) and together with another associate Grosmannia europhioides it is even more efficient in degrading beetle-toxic phenolics than E. polonica (Zhao et al., 2019). Furthermore, adult beetles prefer to feed on artificial spruce substrate colonized and thus detoxified by these fungi (Zhao et al., 2019). These three fungi (E. polonica, G. penicillata, G. europhioides) were also the only ones emitting volatiles that attracted adult I. typographus in behavioural assays on an artificial spruce substrate and may thus be regarded as mutualists . Other common associates, like Ophiostoma piceae and Ophiostoma bicolor, are neither attractive nor repellent to I. typographus and may thus be regarded as commensals . Interestingly, bark beetle-infesting parasitoids can use volatiles emitted by certain microorganisms to detect their hosts, adding another part to these multi-trophic systems (Wegensteiner et al., 2015).
Assays like those for I. typographus are lacking for P. chalcographus but given the lower abundance of fungi within their galleries beneficial effects might be smaller. Several studies, however, indicate that Geosmithia and yeasts that are transmitted by P. chalcographus are less dry-sensitive than Ophiostomatales and Microascales associated with I. typographus (Grosmann, 1930;Kolarik and Jankowiak, 2013;Jankowiak et al., 2014). Grosmann (1930), for example, showed that yeasts are abundant in young breeding systems, especially surrounding eggs, from where they are picked up by larvae and can later be found as gut symbionts. If these early observations are true, P. chalcographus may profit from internal fungi in detoxification of defensive tree compounds but typically lacks external fungal symbionts that can help I. typographus to overwhelm living trees.
Yeasts are generally understudied in bark beetles (Davis, 2015) and their role has only been partly elucidated for I. typographus (Leufven, 1991). Main I. typographus pheromone components are synthesized by beetles de novo (Blomquist et al., 2010). However, several gut yeasts of I. typographus, the most common ones are Pichia holstii and Candida diddensii, oxygenate tree-defensive monoterpenes like α-pinene that beetles themselves oxidize to cis-verbenol, to either trans-verbenol or verbenone (Leufven et al., 1984(Leufven et al., , 1988Leufven and Nehls, 1986). Interestingly, cis-verbenol acts as an aggregation pheromone for I. typographus but not for P. chalcographus, whereas yeastproduced trans-verbenol or verbenone have anti-aggregation effects for both species (Leufven, 1991;Davis, 2015). The yeasts of P. chalcographus have not been studied for their semiochemical role and it needs to be determined if both species harbour different yeasts and how frequently they get exchanged.

Overwintering and diapause
Adverse conditions during cold winters can cause high mortality in bark beetle populations and thus drive population dynamics (Faccoli, 2002;Kostal et al., 2011Kostal et al., , 2014Wang et al., 2017). Numerous insects enter diapause to overcome unfavourable periods, like harsh winters in temperate regions. During diapause, development and reproduction are arrested, the metabolic rate is reduced and resistance against environmental stressors is increased (Kostal, 2006). Other adaptations of insects to cold are directly related to mitigating the lethal effects of low temperatures (Lee, 2010). The diapause and overwintering behaviour of I. typographus has recently been reviewed in a comprehensive manner (Schebeck et al., 2017). Therefore, we will cover this species just briefly, rather focus on P. chalcographus and especially highlight similarities and differences between the two bark beetles (table 1, fig. 1).

Cold tolerance
Major adaptations of insects to survive cold winters are strategies to cope with the freezing of body fluids. Ips typographus and P. chalcographus are both freeze-avoidant species (Kostal et al., 2011(Kostal et al., , 2014, as they die when ice formation in body fluids occurs the temperature when ice is built is defined as the supercooling point (SCP) (Bale, 1993;Lee, 2010;Sinclair et al., 2015). Both species evolved a high supercooling capacity to survive cold conditions, by using a set of sugars and polyols to decrease the freezing point of the haemolymph (Kostal et al., 2007(Kostal et al., , 2011(Kostal et al., , 2014. Adults of P. chalcographus from Central European populations survived cold conditions below average temperatures of −26°C in mid-winter, with some individuals surviving even −36°C (Kostal et al., 2011(Kostal et al., , 2014. Adult beetles of Central European I. typographus had average SCPs in mid-winter of about −20/−22°C, with single individuals reaching SCPs of almost −27°C (Kostal et al., 2011). In addition, both beetles have also a high chilling potential to survive sub-zero temperatures above the SCP, even over long periods (Kostal et al., 2011(Kostal et al., , 2014. Cold tolerance and survival is a plastic response, as insects can acclimatize to ambient conditions. In Northern Europe where winter conditions can be harsher than in more southern latitudes I. typographus can cope with temperatures as low as −32°C, with an average of about −29°C in mid-winter (Annila, 1969). Moreover, I. typographus and P. chalcographus show similar patterns in their seasonal supercooling capacity, with the lowest SCP values in December/January (Kostal et al., 2011(Kostal et al., , 2014, reflecting modifications depending on prevailing temperature conditions. As the two bark beetles often overwinter under the bark of host trees, they might get in contact with ice in the moist, frozen phloem layer. This can result in inoculative freezing, a process when environmental ice enters the body via orifices or through the cuticle and results in freezing of body fluids (Lee, 2010). Both species evolved adaptations to avoid inoculative freezing (Kostal et al., 2011(Kostal et al., , 2014. Experiments with adult P. chalcographus, however, suggest that it can survive harsher conditions in the presence of external ice (Kostal et al., 2014) than adult I. typographus (Kostal et al., 2011). The overwintering performance of the two beetle species was also tested under natural field conditions. In a Czech study, adult P. chalcographus survival rates over a winter ranged in most cases between ∼30 and ∼75% (Kostal et al., 2014). Considering also the survival rates under laboratory conditions and the SCP data obtained by Kostal et al. (2014), high proportions of P. chalcographus populations might survive cold temperatures during Central European winters. Low temperatures, however, are not the only limiting factors for overwintering survival. Biotic and abiotic parameters that can vary on a very small, local scale contribute to a complex picture that determine mortality during cold.
Ips typographus overwinters either under the bark of hosts or in the forest litter (Biermann, 1977). In Central Europe, overwintering survival rates of I. typographus (across all developmental stages) hibernating under the bark of Norway spruce of about 50% were reported (Faccoli, 2002). In Northern Europe, however, mortality of adult I. typographus overwintering under the bark ranges from ∼60 to 100%, whereas adult survival in the litter or moss layerwith an additional insulating snow coveris over 90% (Annila, 1969). Unfortunately, studies on the overwintering behaviour and survival of P. chalcographus outside of host trees, e.g. in the forest litter, are currently lacking. Such data would be important to get a comprehensive picture of the beetle's life history.
One major difference in the overwintering biology of the two bark beetles concerns the developmental stages surviving low sub-zero conditions. In I. typographus only the adult stage can survive low sub-zero temperatures (Annila, 1969;Faccoli, 2002;Dworschak et al., 2014), whereas pre-imaginal stages survive only mild but still sub-zero conditions (Stefkova et al., 2017). In P. chalcographus larvae, pupae and adults are able to survive low winter temperatures (Kostal et al., 2014). Ips typographus might face a decrease of population levels when beetles do not reach the adult stage before winterwhich can happen when cold autumn temperatures slow down development or when warm conditions hamper the induction of diapause (details see below). In P. chalcographus, however, beetles that started their individual development late in the season or whose development was slowed down due to thermal conditions might still be able to survive winter although not completing their entire life cycle. This could relax selection on the timing of oviposition and thus increase the length of the egg-laying period, although the effects of a photoperiodically regulated reproductive diapause are not entirely clear (see below). Moreover, I. typographus overwintering only as adults could result in a synchronization of populations which might be advantageous during host colonization in the following season.

Diapause
Diapause expression is a major strategy for I. typographus to increase stress resistance during winter and additionally regulates development, reproduction and voltinism (Schopf, 1985(Schopf, , 1989Dolezal and Sehnal, 2007;Schebeck et al., 2017;Schroeder and Dalin, 2017;Schebeck et al., 2022). The induction of the facultative reproductive diapause of the adult in the larval/pupal instars is driven by short-day photoperiods and the critical day length for Central European beetles is about 15 h (Schopf, 1989;Dolezal and Sehnal, 2007). Diapause induction is not regulated by photoperiod alone, as the effect of day length is overridden by high temperatures (Dolezal and Sehnal, 2007). Moreover, the critical day length increases with latitude, to respond to local environmental conditions (Schroeder and Dalin, 2017). Diapause termination occurs in mid-winter after experiencing a chilling period. Afterwards I. typographus remains in a post-diapause quiescence and resumes development and reproduction when conditions are favourable again (Dolezal and Sehnal, 2007). In addition, several lines of evidence suggested that this species expresses a second diapause phenotype. Observations on suppressed development, reproduction and dispersaldespite permissive environmental conditionsindicated the presence of an obligate diapause, with both diapause phenotypes in one population (Schopf, 1985(Schopf, , 1989Dolezal and Sehnal, 2007;Schroeder and Dalin, 2017). Schebeck et al. (2022) proved the existence of facultative (photoperiod-dependent) and obligate (independent of photoperiod) diapause phenotypes in I. typographus, showing varying portions in Central and Northern European populations with implications for seasonality and voltinism.
Pityogenes chalcographus seems to enter a photoperiodically regulated diapause in the adult stage as well (Führer and Chen, 1979). Developmental studies under long-day and short-day conditions at different temperatures showed that all individuals of a generation emerge from their breeding systems at warmer conditions with only a minor influence of photoperiod. At lower temperatures, however, a distinct influence of day length on development was described, suggesting a photoperiodically mediated diapause which is modified by warm temperatures (Führer and Chen, 1979). This would be a similar mechanism Bulletin of Entomological Research as described in I. typographus (Schopf, 1985(Schopf, , 1989Dolezal and Sehnal, 2007), however, additional studies are necessary to get a comprehensive picture.
Although bark beetles spend the majority of their live cycles in subcortical environments, they can process photoperiodic signals in various developmental stages. Even pre-imaginal stages, like larvae that lack eyes (stemmata) were reported to respond to photoperiodic cues, as it was shown in the mountain pine beetle, Dendroctonus ponderosae, by opsin gene expression and negatively phototactic behaviour (Wertman et al., 2018).

Thermal effects on life history
The temperature-dependent development of I. typographus has been studied extensively under laboratory and field conditions (Annila, 1969;Lobinger, 1994;Coeln et al., 1996;Wermelinger andSeifert, 1998, 1999;Baier et al., 2007). Data on developmental times, developmental thresholds or optimum temperature differ considerably among studies (Annila, 1969;Coeln et al., 1996;Wermelinger and Seifert, 1998). These differences are likely related to the experimental design or population-related factors. Values for the lower developmental threshold for the whole life cycle range from 5°C (Annila, 1969), to 8.3°C (Wermelinger and Seifert, 1998) and even to 12.3°C (Coeln et al., 1996). Data from the most recent study by Wermelinger and Seifert (1998), however, are commonly used to model the phenology and voltinism of I. typographus and they reliably predict its development under natural conditions (Baier et al., 2007;Jönsson et al., 2009). The optimum temperatures for development and reproduction are about 30°C (Wermelinger and Seifert, 1998) and around 29°C (Wermelinger and Seifert, 1999), respectively. The upper developmental threshold is about 39°C (Wermelinger and Seifert, 1998) and Annila (1969) reported a strong increase of I. typographus mortality of all developmental stages at temperatures above 45°C.
Data on the thermal performance of P. chalcographus are scarce. The temperature-dependent development was studied by Coeln et al. (1996). These data on developmental thresholds are probably higher than under natural field conditions. Additional comprehensive and comparative studies on all temperaturerelated parameters have to be conducted to get a better understanding of its thermal performance.
In addition, temperature affects the flight activity of I. typographus and P. chalcographus. For both species, the lower limit for swarming initiation is relatively similar, about 16.5°C/17°C. The upper limit, however, is 30°C in I. typographus, while P. chalcographus was still observed swarming at 35°C (Lobinger, 1994).
Although developmental parameters, like lower thermal thresholds or developmental times, of I. typographus and P. chalcographus seem to differ from each other, some similar patterns have been observed. For example, the developmental time of the larval stage is about threefold longer than the embryonic or pupal development, respectively. Furthermore, the maturation period of young adults comprises roughly 40% of the whole development in both beetles (Coeln et al., 1996;Wermelinger and Seifert, 1998;Baier et al., 2007).

Biogeography
Differences in certain life-history traits of I. typographus and P. chalcographus could be related to differences in their evolutionary histories. Previous studies focused on processes during the last ice ages and assessed the effects of climate-driven range changes during glacial-interglacial periods (Stauffer et al., 1999;Salle et al., 2007;Avtzis et al., 2008;Bertheau et al., 2013;Mayer et al., 2015;Schebeck et al., 2018Schebeck et al., , 2019. Studies on various European and North American bark beetles revealed that Pleistocene climatic oscillations shaped the genetic structure of several species. Moreover, these events are related to certain life-history traits, like the evolution of pheromone races, developmental variations, reproductive incompatibilities and the formation of novel sex chromosomes (Cognato et al., 1999(Cognato et al., , 2003Mock et al., 2007;Bracewell et al., 2011Bracewell et al., , 2017Dowle et al., 2017). Both I. typographus and P. chalcographus survived Pleistocene glaciation events in multiple, geographically isolated European refugia, shared with their main host plant Norway spruce (Stauffer et al., 1999;Salle et al., 2007;Avtzis et al., 2008;Bertheau et al., 2013;Mayer et al., 2015;Schebeck et al., 2018). Apart from this general pattern, the two species exhibit important differences in their Pleistocene histories. Pityogenes chalcographus survived glacial periods in three major refugia: in the Russian plain, in the Carpathian Mountains and in the Italian-Dinaric region, followed by postglacial secondary contact (Avtzis et al., 2008;Bertheau et al., 2013;Schebeck et al., 2018). In addition to these main refugia, P. chalcographus might also have survived cold events in smaller areas, like in the Apennine Mountains (Schebeck et al., 2019) and other regions of Norway spruce Pleistocene survival (Tollefsrud et al., 2008).
In contrast, the Pleistocene history of I. typographus is less clearly resolved; specifically, the number and locality of refugia is still under debate. Recent studies propose that European populations are generally structured in two major genetic groups with an overall pattern of north-south clustering Mayer et al., 2015) with slight signals of sub-structure (Krascsenitsova et al., 2013;Mayer et al., 2014Mayer et al., , 2015. Irrespective of the exact number of refugial areas, the locality of one or several regions is also unknown but was very likely shared with one refugium of Norway spruce (Schmidt-Vogt, 1977;Tollefsrud et al., 2008) and additional minor areas were Norway spruce was present (Tollefsrud et al., 2008). A genome-wide survey as well as a thorough population sampling across the species' range might elucidate this open question in bark beetle biogeography.

Evolutionary history
Differences in certain life-history traits of I. typographus and P. chalcographus could additionally be the result of the species' different evolutionary age. Present mitochondrial P. chalcographus lineages diverged about 100,000 years ago, whereas the evolutionary history of I. typographus is about five times younger . The genetic structure of both beetles is characterized by high levels of gene flow, however, P. chalcographus has a deeper structure than I. typographus (Stauffer et al., 1999;Avtzis et al., 2008;Bertheau et al., 2012Bertheau et al., , 2013. Differences in age, genetic structure and glacial refugia could be related to varying patterns in host range, overwintering biology or thermal performance, however, detailed future studies have to test these hypotheses on the evolution of these two bark beetles.

Summary and outlook
Ips typographus and P. chalcographus are two common and widespread bark beetle species with diverse life histories. Ips typographus is clearly a stronger tree killer than P. chalcographus. Given in particular its narrower host range and higher host-quality demands, we hypothesize that selection for treekilling is stronger in I. typographus. This could explain the evolution of I. typographus as a facultative tree-killer. If this hypothesis is correct and the lower adaptability could indeed be linked to the lower genetic diversity as a result of the last ice ages, tree-killing might be a relatively recent trait in I. typographus.
Although these two species are most likely the best-studied bark beetles in Europe, many facets of their evolutionary ecology remain unknown. Based on our comparative review, we propose the following future research directions that should be approached ideally using not only I. typographus and P. chalcographus, but several representatives of the species-rich group of Scolytinae: 1. Host usage: Studying physiological mechanisms, e.g. detoxification of defence-related tree metabolites, and the influence of different phloem properties, like moisture or nutrients, on performance in trees would help to understand host colonization by bark beetles (Krokene, 2015). 2. Reproduction: Bark beetle mating systems are highly diverse, from monogamy to inbreeding and harem polygyny (Kirkendall, 1983). Also within harem-polygynous species there is variation in the number of females per gallery, as seen for the two species reviewed here. Whether this is the consequence of higher mortality of males during dispersal flights, host search and gallery establishment and whether this is affected by host species, host quality or quality of male beetles are only some research directions to shed light on the evolution of bark beetle mating behaviour (Kirkendall, 1983). 3. Symbioses: The roles of fungi, yeasts and bacteria in the life histories of most bark beetles are largely unknown (for example, Six, 2013;Davis, 2015;Zhao et al., 2019). Elucidating their nutritional values, detoxification potential, influence on feeding habits or effects on pheromone production are only some examples for future research directions. Moreover, unravelling the evolutionary history of these associations will be crucial for understanding these bark beetle symbioses. 4. Life-cycle regulation: Effects of abiotic factors on the life histories of bark beetles are mainly known for economically important species. Data on the influence of temperature and photoperiod on ontogenetic development, diapause expression, survival or phenology from a wide range of species are essential to understand their ecology and evolution.
The rapid increase in modern 'omic' tools and the increasing availability of whole-genome data for bark beetles (Keeling et al., 2013;Bracewell et al., 2018;Powell et al., 2021) gives us the opportunity to tackle these questions with more sophisticated approaches than in the past. Such research on these and other bark beetle species (i) can help to understand basic evolutionary processes, e.g. population dynamics, niche construction, treekilling ability, mating system evolution and the evolution of symbioses, and (ii) are the basis for developing novel control measures against these major forest pests.