Bark beetles (Coleoptera: Curculionidae: Scolytinae) live and reproduce in the phloem of trees. Often species specific, the relationships between beetle and host tree species have evolved over millions of years (Labandeira et al., 2001; Raffa, 2014). These relationships are apparent through the complex chemical signals and receptors by which bark beetles find host trees and mates (Byers, 1989; Francke et al., 1995), the presence of symbiotic fungi, and the physical structures by which some species carry fungal spores (Paine et al., 1997; Six and Wingfield, 2011), and the physical and chemical defenses that host trees have evolved to evade death by bark beetles (Raffa and Berryman, 1983, 1987; Krokene, 2015). In recent decades, anthropogenic-driven climate change has facilitated range expansions of bark beetles that have resulted in new relationships with novel and naïve host tree species. Insect life cycles are highly temperature dependent and rising temperatures have facilitated range expansions of several species of bark beetles northwards and/or to higher elevations (Bentz and Jönsson, 2015; Pureswaran et al., 2018). Naive host tree species along these expanding range margins may be at increased risk of mortality if they have not co-evolved defenses against beetle attack or if they have lost defensive capabilities over time (Logan and Powell, 2001; Raffa et al., 2013).

Range expansion of some species of Dendroctonus have occurred and has the potential to occur for three more species as a result of climate change (Pureswaran et al., 2022). The range of mountain pine beetle (Dendroctonus ponderosae Hopkins) has expanded northward and eastward over the continental divide (Safranyik et al., 2010; de la Giroday et al., 2012; Rosenberger et al., 2017a). The range of mountain pine beetle has also expanded higher in elevation, increasing exposure to high-elevation five-needle pines such as whitebark pine Pinus albicaulis Engelm (Logan and Powell, 2001; Raffa et al., 2013; Six et al., 2021; United States Fish and Wildlife Service, 2022). Southern pine beetle (Dendroctonus frontalis Zimmermann) has moved northwards as increasing winter temperatures have permitted survival in new habitats (Ungerer et al., 1999; Lesk et al., 2017). Another example is spruce beetle (Dendroctonus rufipennis Kirby), which will see an increase in climatically suitable habitat containing its host, Engelmann spruce (Picea engelmannii Parry ex Engelm.), as temperatures rise with climate change (DeRose et al., 2013). Moreover, rising temperatures and increasing drought negatively impact host tree defenses, making trees more susceptible to beetle attack (Bentz et al., 2010; Kolb et al., 2019).

Native to North America, the eastern larch beetle Dendroctonus simplex LeConte has a range sympatric to that of its host, the eastern larch (tamarack) Larix laricina (Du Roi) K. Koch. Eastern larch grows across northeastern North America down into the Great Lakes region and west into northwestern Canada with disjunct populations in Alaska (Figure 1) (Johnston, 1990; Seybold et al., 2002). Dendroctonus simplex was first recorded by A. D. Hopkins in 1909 as less damaging than other Dendroctonus species due to its preference for dead and dying larch (Hopkins, 1909). However, landscape-level outbreaks occurring in the 1970s and 1980s in northeastern North America and Alaska prompted reconsideration of D. simplex as a primary forest pest (Werner, 1986; Langor and Raske, 1989). These early outbreaks typically lasted 2–6 years before resolving and were often preceded by other biotic and abiotic host stressors such as defoliation by the larch sawfly Pristiphora erichsonii Hartig (Hymenoptera: Tenthredinidae), larch casebearer Coleophora laricella Hübner (Lepidoptera: Coleophoridae), larch budmoth Zeiraphera sp. (Lepidoptera: Tortricidae), or drought and flooding (Langor and Raske, 1989).

In Minnesota, an eastern larch beetle outbreak that began in 2000 continues to present. The outbreak has impacted just over four hundred thousand hectares of eastern larch forest type to date (MN DNR Forest Health Team, 2024). There are no apparent predisposing biotic external factors that can account for the spatial and temporal scale of the outbreak, with defoliation by larch casebearer only occurring in approximately 5% of impacted area between 2000 and 2017 (Ward and Aukema, 2019a). Eastern larch beetle was previously thought to be univoltine with an obligate, overwintering adult reproductive diapause (Hopkins, 1909; Langor and Raske, 1987a). Work conducted in the early 2010s, however, demonstrated that some adults in Minnesota were capable of reproducing without overwintering in laboratory conditions (McKee and Aukema, 2015b), and evidence for a second generation was observed in the field (McKee, 2015). Therefore, the ongoing outbreak is attributed to increased beetle populations as a result of an ameliorating climate (Venette and Walter, 2008; McKee, 2015).

Eastern larch beetle, to date, has never been recorded infesting allopatric species of North American Larix such as western larch, Larix occidentalis Nutt., or subalpine larch, Larix lyalli Parl. Both of these species are free of potential phloem-inhabiting specialist competitors, save for the larch engraver Scolytus laricis Blackman (Coleoptera: Curculionidae) that is typically restricted to pole-sized, distressed trees and slash (Furniss and Carolin, 1977). Western larch is found within western Montana, eastern and central Washington, northeastern Oregon, and southeastern British Columbia (Figure 1). It is a major timber species in western North America and provides ecological benefits (Schmidt and Shearer, 1990). A shade intolerant pioneer species, western larch rapidly regenerates in the bare soils of burned and logged areas. Root diseases are extremely prevalent tree mortality agents in the inland northwest, and western larch is considered to be one of the most tolerant native tree species to root diseases such as Armillaria ostoyae, Heterobasidion annosum, and laminated root rot Phellinus weirii (Shaw and Kile, 1991; Thies and Sturrock, 1995; Hagle et al., 2003). Due to its high commercial value, key role in forest restoration of root diseased areas, and limited insect and disease problems, land management agencies have prioritized planting of western larch (Tyler Nelson, Idaho Department of Lands Forest Management Program, personal communication). However, this emphasis on western larch has resulted in tens of thousands of hectares of monocultures, raising concerns about forest resilience in the face of a changing climate that could bring about novel insect and disease issues.

Subalpine larch (Larix lyalli) is found within the Northern Rockies and Cascades at high elevations, typically 2,300 m or greater, but it can be found as low as 1,520 m in the Cascades (Arno and Habeck, 1972; Arno, 1990). Although separated by an elevation gradient of 150–300 m, the ranges of western and subalpine larch overlap geographically and the two species sometimes hybridize in western Montana (Carlson and Blake, 1969; Arno and Habeck, 1972; Figure 1). Subalpine larch has occasionally been found growing in lower elevations with western larch on sites that have experienced disturbance (Arno and Habeck, 1972; Arno, 1990). As a high-elevation species, subalpine larch will likely experience increasing stress due to rising temperatures as a result of climate change.

The goal of this study was to compare the suitability of eastern, western and subalpine larches for reproduction of eastern larch beetle, motivated by the ongoing outbreak in the Great Lakes region. Spread of Larix pests from eastern to western hosts has occurred in the past, as seen with the invasive, needle-mining larch casebearer Coleophora laricella (Ryan et al., 1987; Ward and Aukema, 2019b). Work in the 1970s found that a small number of eastern larch beetle can successfully reproduce in cut bolts of western larch (Furniss, 1976), but reproduction of eastern larch beetle has never been tested in subalpine larch. The present study was conducted using cut bolts and thus we can only assess host suitability and not host susceptibility, as introducing beetles to live trees outside of the insect’s native range is unwise (Cale et al., 2015; Rosenberger et al., 2017b). We compared the reproductive success of standardized densities of D. simplex in western and subalpine larch to eastern larch, the beetle’s native host. Reproductive success was evaluated in terms of fertility and reproductive gallery length of introduced females, as well as lipid content and size of progeny. By evaluating host suitability of western and subalpine larch, we hope to determine if the eastern larch beetle may pose a threat to these species as global change continues to facilitate range expansions and invasions of forest insects.

Beetles were able to reproduce in cut bolts of all three species of North American Larix, although western larch did not appear to be a highly suitable host (Figure 2). Over a 53-day period, 2019 beetles emerged from bolts of eastern larch, 1,147 beetles from bolts of subalpine larch, and only 153 beetles from bolts of western larch. The differences in numbers of emerged beetles between species were not simply due to differences in the numbers of females that successfully entered the bolts, mated, and laid eggs, as these numbers were similar across species (overall mean ± SE = 4.3 ± 0.1, F_2,43 = 0.12, p = 0.885).

Indeed, the number of offspring that emerged per female, once standardized for bolt area, varied between species (F_2,43 = 18.52, p < 0.0001; Figure 3A). Almost twice as many beetles emerged from bolts of eastern larch (1.9 ± 0.3 beetles/female/dm²) than bolts of subalpine larch (1.0 ± 0.2 beetles/female/dm²); however, this difference was insignificant. Emergence from bolts of western larch (0.2 ± 0.05 beetles/female/dm²) was almost 10 times lower than emergence from bolts of eastern larch. However, there was no significant difference in sex ratio of emerged beetles by species (x² = 1.094, df = 2, p = 0.579). The average percentage of females emerging from eastern larch was 40.0 ± 0.33%, 51.1 ± 0.44% from subalpine larch, and 47.7 ± 0.45% from western larch. On average, 119 ± 20.2 beetles emerged from eastern larch bolts, 72 ± 11.8 beetles from subalpine larch bolts, and 9 ± 2.4 beetles emerged from western larch bolts.

Mean maternal gallery length varied by species (Figure 4; F_{2, 43} = 23.68, p < 0.0001). On average, females tunneled the farthest in subalpine larch (60.4 ± 4.2 cm). This distance was approximately double the length of galleries noted in both eastern larch (36.6 ± 2.2 cm) and western larch (32.7 ± 2.6 cm). In eastern larch, the average number of galleries per bolt was 4.2 ± 0.2, comparable to subalpine larch (4.4 ± 0.2 galleries) and western larch (4.2 ± 0.2 galleries). Multiplying the overall species sex ratios by the mean number of emerged beetles from each bolt within a species, we calculated that, on average, 47.5 females emerged from each eastern larch bolt, 36.6 females came from each bolt of subalpine larch, and 4.6 beetles from each western larch bolt. In all three species, the calculated average of emerged females was greater than the average number of females that successfully colonized each bolt.

When evaluating fertility in terms of total number of offspring (i.e., including all individuals found at debarking), results were similar to above (F_{2, 43} = 21.11, p < 0.0001; Figure 3B). Mating pairs in eastern larch produced 4,798 progeny, those in subalpine larch produced 4,684 progeny, and those in western larch produced 1,638 progeny. A higher proportion of emerged adults versus manually-extracted adult beetles occurred in eastern larch (2019:2604 beetles) than subalpine larch (1,147:3235 beetles), which in turn was higher than the proportion that emerged from western larch (153:1357 beetles). Standardized by bolt size, we found females in bolts of eastern larch produced an average of 4.5 ± 0.3 progeny per dm². These numbers were comparable to females in subalpine larch, which on average produced approximately 4.2 ± 0.3 progeny per dm². In western larch, however, reproductive output dropped by just over half, as each female only produced an average of 1.9 ± 0.3 offspring per dm².

The lipid content in the beetles that were manually extracted from underneath the bark after emergence concluded was higher than the lipid content of beetles that emerged naturally to collection jars (F_1,194 = 176.79, p < 0.0001), so we display these results separately (Figure 5). The mean lipid weight in emerged beetles was 0.495 ± 0.026 mg, while lipid weight for manually extracted beetles averaged 1.377 ± 0.053 mg. Despite these differences in lipid content among emergence status, the effect of species on lipid weight was consistent (F_2,124 = 34.31, p < 0.0001 for emerged beetles, Figure 5A; F_{2, 115} = 16.99, p < 0.0001 for manually extracted beetles, Figure 5B). Beetles that emerged from bolts of eastern and subalpine larch had approximately double the lipid content of insects developing in western larch. This species effect was consistent whether lipid content was measured in dry weight or as a percentage of total body weight (F_{2, 124} = 37.11, p < 0.0001 for emerged beetles, Figure 5C; F_{2, 115=}15.60, p < 0.0001 for extracted beetles, Figure 5D). Mean percent lipid content varied between 9.23 and 20.70% by species for naturally emerging beetles, but was lowest in western larch (Figure 5C). Likewise, the mean percent lipid content varied between 26.71 and 37.47% by species for beetles harvested upon debarking, but was lowest for beetles developing in western larch (Figure 5D).

Emerged beetles were approximately 2% larger than manually extracted beetles (Figure 6; F_{1, 247} = 14.82, p < 0.0001). Among manually extracted beetles, those reared in eastern larch were slightly larger than those reared in subalpine larch, with no difference between subalpine larch- and western larch-reared beetles (F_{2, 115} = 3.39, p = 0.037; Figure 6B). Sex was not associated with differences in lipid weight (F_{1, 194} = 0.72, p = 0.39) nor lipid percentage (F_{1, 194} = 0.34, p = 0.56) when beetle emergence types were pooled. When beetle types were analyzed separately, however, males averaged 0.15 mg (± 0.04 SE) less lipid than females for beetles that had emerged naturally (F_{1, 91} = 11.46, p = 0.0010).

Our findings demonstrate that the eastern larch beetle, Dendroctonus simplex LeConte, can successfully reproduce in cut bolts of all three species of North American Larix. These results indicate that if the range of eastern larch beetle were to expand, western and subalpine larch may be at risk from a novel Dendroctonus pest. In all three species, the average number of females emerging from each bolt were greater than the number of females that colonized each bolt, indicating all three species could prove suitable hosts. Two key unknowns are how attractive western and subalpine larch would be to eastern larch beetles in the field, and if attractive, what levels of defenses (constitutive and inducible) these novel hosts might possess. Due to the limitations of using cut bolts vs. live trees, only constitutive effects are manifested in the current study (Rosenberger et al., 2017a). Lower host defenses have been noted in naïve hosts of mountain pine beetle D. ponderosae (Cudmore et al., 2010; Raffa et al., 2013; Rosenberger et al., 2017b).

Host range expansions benefit from accessible geography, insect sources (e.g., outbreaking populations), and active and passive dispersal. Geographically, the range of D. simplex is sympatric to that of its host, eastern larch, and thus relatively close to but not overlapping the ranges of western and subalpine larch. Range maps produced by Little (Figure 1) indicate the ranges of eastern and subalpine larch are closest in southwestern Alberta, just outside of Banff National Park in Canada (Little, 1971). The shortest distance between the two species at their closest point is only 53 km, not accounting for elevation. From these maps, the distance between eastern and western larch at their closest point is 147 km, also in southwestern Alberta. However, recent models based on available soil water holding capacity predict northward range expansion of western larch that would result in eastern and western larch range overlap near Calgary, Alberta (Mathys et al., 2014). From an outbreak standpoint, the present, ongoing outbreak in Minnesota has been attributed, in part, to ameliorating winters (Venette and Walter, 2008) and longer, warmer summers (McKee, 2015). With the predicted increases in global temperatures from 1.5–4.4°C by 2,100 (IPCC, 2023), the potential for outbreak activity in hosts of suitable species, age, and size classes shows no signs of abating in the future.

The dispersal ability of the eastern larch beetle is currently unknown; however, other Dendroctonus species can disperse on mesoscale atmospheric currents up to 30–100 km/day (Jackson et al., 2008), and have been found at elevations up to 3,048 m above sea level (Furniss and Furniss, 1972). Thus, it is possible that eastern larch beetle could disperse to western or subalpine larch stands, particularly if an outbreak were to occur in the southern edge of the eastern larch range and spread to subalpine larch stands (Howe et al., 2021). The ability of western and subalpine larch to hybridize along the elevation gradient dividing the two species could further facilitate spread across elevations (Carlson and Blake, 1969; Arno, 1990). Given the short growing season and harsh winter conditions experienced at high elevations, further research into effects of temperature on eastern larch beetle development is needed to evaluate if the beetle can successfully complete its life cycle in subalpine larch stands. However, since eastern larch beetle adults and larvae are extremely cold-hardy (down to −42°C and − 49°C, respectively; Venette and Walter, 2008), it is likely the beetle would be able to survive winter temperatures in subalpine larch stands, especially with additional climate warming.

Lower reproductive success of D. simplex in western larch versus subalpine larch and eastern larch (Figures 2, 3) could be caused by higher constitutive host defenses within western larch bolts. Further evidence suggesting high constitutive defenses in western larch is the inability for Douglas-fir beetle Dendroctonus pseudotsugae to successfully reproduce in live trees, despite being able to do so in cut bolts of western larch (Furniss, 1976). Constitutive defenses can be physical as well as chemical, with variation between chemical compounds occurring within different tissues (Krokene, 2015). Differences between monoterpene composition and concentration vary between eastern, western and subalpine larch as well as between tissues. For example, myrcene, a compound toxic to D. simplex, is present in higher concentrations in the twig and needle oil of western larch than eastern and subalpine larch while the concentrations of limonene, another toxic compound, is present in resin of the latter two species in comparable concentrations (Von Rudloff, 1987; Werner, 1995). An earlier study also found that resin of western larch had demonstrably higher levels of α-pinene compared to resin of eastern larch, with the latter having higher levels of 𝛽-pinene and both species having comparable Δ-3-carene levels (Stairs, 1968). Recent work has found high levels of both α-pinene and Δ-3-carene and relatively low levels of 𝛽-pinene within the phloem of eastern larch (Althoff et al., 2023). Alternatively, differences in reproductive success could be due to differences in moisture content or phloem thickness, although we observed these metrics to be qualitatively similar across all species.

Variation in constitutive chemical defenses across species may also explain why maternal galleries constructed in subalpine larch were approximately 50% longer than maternal galleries constructed in the native host, eastern larch (Figure 4). Factors influencing gallery length in eastern larch beetle include female vigor, availability of unused phloem, and host resin production (Werner, 1986; Langor and Raske, 1987a, 1987b). We used beetles of similar vigor across all species and female densities were similarly standardized. Although there were minor differences in bolt size across species, each bolt had unused areas of phloem, so we do not view crowding as an impediment to gallery construction. The relationship between host conifer defenses and elevation is complex (Moreira et al., 2018), but the elevational gradient in plant defense hypothesis (EGPD) proposes that higher-elevation plants are less-defended than those at lower elevations (Ferrenberg et al., 2017). That the longest galleries were constructed in subalpine larch seem to suggest the trees used in this study had lower constitutive defenses against D. simplex than the other species. Lack of host defenses in high-elevation tree species against Dendroctonus bark beetles has been observed in outbreaks of mountain pine beetle in whitebark pine as well (Raffa et al., 2013). Alternatively, the extended gallery lengths in subalpine larch could be due to lower quality phloem or fewer fungal symbionts (Bentz and Six, 2006), requiring adult beetles to tunnel longer to obtain sufficient nutrition to achieve fertility rates similar to those in eastern larch.

Fertility was similar between beetles reared in eastern versus subalpine larch, but lower than western larch (Figure 3). Similar to maternal gallery length, the pattern seen in fertility may suggest higher constitutive defenses in western larch relative to subalpine and eastern larch, although more study is needed. Genetic relatedness between eastern larch, western and subalpine larch and host range expansion and contraction over time may also explain why beetles performed better in subalpine versus western larch. It is worth noting that eastern and western larch are more genetically similar to one another than to subalpine larch (Gros-Louis et al., 2005), and this relatedness may relate to constitutive host tree defenses. Gene flow within subalpine larch populations is more restricted (Khasa et al., 2006), indicating that bottlenecks occurred during or after establishment of subalpine larch populations. This pattern supports the theory that the discontinuous distribution of subalpine larch is attributed to a shift from warmer to cooler climates, resulting in non-contiguous populations of subalpine larch at high elevations as observed in the current range of the species. Similar to how white pine blister rust resistance factors into whitebark pine restoration efforts, the limited genetic diversity of subalpine larch could pose a conservation challenge, particularly if the species in situ is vulnerable to novel insect threats such as eastern larch beetle.

Size and lipid content are often used to evaluate fitness of offspring (Hansen and Bentz, 2003; McKee and Aukema, 2015a; Esch et al., 2016). Eastern larch-reared beetles were slightly larger than those raised in subalpine larch for beetles extracted upon debarking (Figure 6), but the difference is not likely ecologically meaningful. Beetle size may be positively correlated with phloem thickness (Amman et al., 1986), and smaller beetles may be better suited to reproducing in species with thinner phloem (Safranyik et al., 2010). Size differences between emerging versus extracted beetles overall (Figure 6B) could reflect beetle populations in two different physiological states, but would not indicate that a second generation of beetles was laid during the experiment. Some brood adults emerge and colonize new hosts when the growing season permits while other adults remain within the phloem to overwinter in a putative facultative overwintering diapause (Langor and Raske, 1987a, 1987b; McKee and Aukema, 2015b). Given that it takes approximately 35 days to develop from egg to adult at our rearing temperature of 24.7°C (McKee and Aukema, 2015a), and given that this experiment concluded on day 53, there would not have been sufficient time for a second generation of insects that were smaller in size due to deteriorating phloem conditions.

We note that our results of reproduction across species may be conservative and potentially underestimate reproduction in these different hosts. While emergence was occurring, we found several dozen beetles in the laboratory room outside of the rearing tubes and emergence jars and could not trace their origin. No signs of tunneling through the cardboard sides or the plastic lids of emergence tubes were observed during inspections following the experiment.

The ability of eastern larch beetle to successfully reproduce in cut bolts of all three native North American species of larch indicates that western and subalpine larch could function as suitable hosts, albeit a poor host in the case of western larch. Host suitability is different from susceptibility, and it remains unknown if these insects would be attracted to live western and subalpine larch and how they might fare in live hosts with induced defenses as opposed to cut bolts. Nonetheless, subalpine larch as a high-elevation species appears to be especially at risk, given that beetles exhibit similar fertility and lipid content to those reproducing within the beetle’s native eastern larch host, and construct longer ovipositional galleries. Additional work evaluating the dispersal potential of the beetle, its development at different temperatures (McKee and Aukema, 2015b), its performance in live trees, and susceptibility ratings of stands of subalpine and western larch is necessary to determine ultimate host susceptibility to eastern larch beetle in the future.