Dendroctonus Brevicomis Research Articles

Western pine beetle voltinism in a changing California climate

Abstract Recent hot droughts in California resulted in ponderosa pine (Pinus ponderosa) mortality attributed to drought and western pine beetle (WPB, Dendroctonus brevicomis). While drought alone can cause tree death, direct warming effects on WPB are a contributing factor. Research on WPB generation timing (voltinism), however, is lacking. We monitored WPB tree attacks and adult emergence timing at two California sites and developed a degree‐day model from field‐observed data. Historical, contemporary, and future temperatures for several California sites were used with the model to examine trends in WPB voltinism. Field data showed a single summer and an overwinter generation at a northern California site. As summer temperatures increased beyond 1900–1980 averages, the predicted number of full and partial WPB generations by 2021 had increased from ~2 annual (one summer and one overwinter) generations historically to ~2.3 at two northern California sites and from ~2.3 to ~3.2 at two warmer California sites. Historical and contemporary data suggest winter warming was not sufficient for an additional generation overwinter. Instead, increases in generations were driven by summer and fall temperatures. Unconstrained increases in the number of future annual generations will be limited by complex, but not well understood, WPB thermal adaptations. Increased knowledge of temperature‐driven WPB population growth will improve forest vegetation models aimed at predicting ponderosa pine mortality in a changing climate.

Applied Chemical Ecology of the Western Pine Beetle, an Important Pest of Ponderosa Pine in Western North America

Western pine beetle (Dendroctonus brevicomis LeConte) is a major cause of ponderosa pine (Pinus ponderosa Dougl. ex. Laws.) mortality in western North America. Twenty-first century epidemics are among the largest in history and have affected hundreds of thousands of hectares. We synthesize literature on the chemical ecology of western pine beetle and on efforts to exploit our understanding of the western pine beetle-ponderosa pine system to reduce host tree losses. This literature dates back to the early 20th century and focuses on populations in California and Oregon, U.S., where western pine beetle exerts its largest impacts. Research in the 1960s–1970s yielded an effective semiochemical attractant (exo-brevicomin, frontalin, and myrcene) that helped inform understanding of the biology, ecology, and management of this species. Later, research focused on isolation and identification of semiochemical repellents. To date, Verbenone Plus (acetophenone, (E)-2-hexen-1-ol + (Z)-2-hexen-1-ol, and verbenone) is the only semiochemical repellent demonstrated effective for protecting ponderosa pines from mortality attributed to western pine beetle in multiple studies in Canada and the U.S.

Carbon stored in live ponderosa pines in the Sierra Nevada will not return to pre-drought (2012) levels during the 21st century due to bark beetle outbreaks

Outbreaks of several bark beetle species can develop rapidly in response to drought and may result in large transfers of carbon (C) stored in live trees to C stored in dead trees (10s of Tg C yr-1 in the western U.S. alone), which over time will be released back to the atmosphere. The western pine beetle (WPB) outbreak incited by the 2012–2015 mega-drought in the Sierra Nevada, California, U.S., could portend more frequent and/or severe bark beetle outbreaks as the temperature warms and drought frequency and intensity increase in the future. However, changes in the frequency and/or severity (resultant levels of host tree mortality) of beetle outbreaks are difficult to predict as outbreaks are complex with non-linear and eruptive processes primarily driven by interactions among beetle populations, the demography of hosts and other tree species, and climate and weather. Using an insect phenology and tree defense model, we projected the future likelihood of WPB outbreaks in the Sierra Nevada with climate drivers from different Earth System Models. Our goal was to understand how host (ponderosa pine, PIPO) recovery and future warming and drought affect the frequency and severity of WPB outbreaks and their C consequences. Our projections suggested that by 2100 the C stored in live PIPO (mean: 1.98 kg C m-2, 95% CI: 1.74–2.21 kg C m-2) will not return to levels that occurred before the 2012–2015 drought (2012: ∼2.30 kg C m-2) due to future WPB outbreaks. However, differences in climate models indicate a wide range of possible WPB outbreak frequencies and severities. Our results suggest that total plot basal area is the most significant factor in the mortality rate of PIPO by WPB in any given year, followed by drought severity and temperature. High levels of host basal area, higher temperature, and extreme drought all contribute to the frequency and severity of future WPB outbreaks. While PIPO basal area may decline under increased drought and warming, limiting high-stand basal area (>60 m2 ha-1) may reduce the severity of future WPB outbreaks in the Sierra Nevada.

Tree Mortality following Thinning and Prescribed Burning in Central Oregon, U.S.

We examined causes and levels of tree mortality one year after thinning and prescribed burning was completed in ponderosa pine (Pinus ponderosa Dougl. ex Laws.) forests at Pringle Falls Experimental Forest, Oregon, U.S. Four blocks of five experimental units (N = 20) were established. One of each of five treatments was assigned to each experimental unit in each block. Treatments included thinning from below to the upper management zone (UMZ) for the dominant plant association based on stand density index values for ponderosa pine followed by mastication and prescribed burning: (1) 50% UMZ (low density stand), (2) 75% UMZ (medium density stand), (3) 75% UMZ Gap, which involved a regeneration cut, (4) 100% UMZ (high density stand), and (5) an untreated control (high density stand). Experimental units were thinned in 2011 (block 4), 2012 (block 2), and 2013 (blocks 1 and 3); masticated within one year; and prescribed burned two years after thinning (2013–2015). A total of 395,053 trees was inventoried, of which 1.1% (4436) died. Significantly higher levels of tree mortality occurred on 100 UMZ (3.1%) than the untreated control (0.05%). Mortality was attributed to prescribed fire (2706), several species of bark beetles (Coleoptera: Curculionidae) (1592), unknown factors (136), windfall (1 tree), and western gall rust (1 tree). Among bark beetles, tree mortality was attributed to western pine beetle (Dendroctonus brevicomis LeConte) (881 trees), pine engraver (Ips pini (Say)) (385 trees), fir engraver (Scolytus ventralis LeConte) (304 trees), mountain pine beetle (D. ponderosae Hopkins) (20 trees), Ips emarginatus (LeConte) (1 tree), and Pityogenes spp. (1 tree).

Changes in tree drought sensitivity provided early warning signals to the California drought and forest mortality event.

Climate warming in recent decades has negatively impacted forest health in the western United States. Here, we report on potential early warning signals (EWS) for drought-related mortality derived from measurements of tree-ring growth (ring width index; RWI) and carbon isotope discrimination (∆13 C), primarily focused on ponderosa pine (Pinus ponderosa). Sampling was conducted in the southern Sierra Nevada Mountains, near the epicenter of drought severity and mortality associated with the 2012-2015 California drought and concurrent outbreak of western pine beetle (Dendroctonus brevicomis). At this site, we found that widespread mortality was presaged by five decades of increasing sensitivity (i.e., increased explained variation) of both tree growth and ∆13 C to Palmer Drought Severity Index (PDSI). We hypothesized that increasing sensitivity of tree growth and ∆13 C to hydroclimate constitute EWS that indicate an increased likelihood of widespread forest mortality caused by direct and indirect effects of drought. We then tested these EWS in additional ponderosa pine-dominated forests that experienced varying mortality rates associated with the same California drought event. In general, drier sites showed increasing sensitivity of RWI to PDSI over the last century, as well as higher mortality following the California drought event compared to wetter sites. Two sites displayed evidence that thinning or fire events that reduced stand basal area effectively reversed the trend of increasing hydroclimate sensitivity. These comparisons indicate that reducing competition for soil water and/or decreasing bark beetle host tree density via forest management-particularly in drier regions-may buffer these forests against drought stress and associated mortality risk. EWS such as these could provide land managers more time to mitigate the extent or severity of forest mortality in advance of droughts. Substantial efforts at deploying additional dendrochronological research in concert with remote sensing and forest modeling will aid in forecasting of forest responses to continued climate warming.

Warming increased bark beetle-induced tree mortality by 30% during an extreme drought in California.

Quantifying the responses of forest disturbances to climate warming is critical to our understanding of carbon cycles and energy balances of the Earth system. The impact of warming on bark beetle outbreaks is complex as multiple drivers of these events may respond differently to warming. Using a novel model of bark beetle biology and host tree interactions, we assessed how contemporary warming affected western pine beetle (Dendroctonus brevicomis) populations and mortality of its host, ponderosa pine (Pinus ponderosa), during an extreme drought in the Sierra Nevada, California, United States. When compared with the field data, our model captured the western pine beetle flight timing and rates of ponderosa pine mortality observed during the drought. In assessing the influence of temperature on western pine beetles, we found that contemporary warming increased the development rate of the western pine beetle and decreased the overwinter mortality rate of western pine beetle larvae leading to increased population growth during periods of lowered tree defense. We attribute a 29.9% (95% CI: 29.4%-30.2%) increase in ponderosa pine mortality during drought directly to increases in western pine beetle voltinism (i.e., associated with increased development rates of western pine beetle) and, to a much lesser extent, reductions in overwintering mortality. These findings, along with other studies, suggest each degree (°C) increase in temperature may have increased the number of ponderosa pine killed by upwards of 35%-40% °C-1 if the effects of compromised tree defenses (15%-20%) and increased western pine beetle populations (20%) are additive. Due to the warming ability to considerably increase mortality through the mechanism of bark beetle populations, models need to consider climate's influence on both host tree stress and the bark beetle population dynamics when determining future levels of tree mortality.

Corticioid basidiomycetes associated with bark beetles, including seven new Entomocorticium species from North America and Cylindrobasidium ipidophilum, comb. nov.

Seven new Entomocorticium species (Peniophoraceae) are described based on morphology and phylogenetic analyses. Along with the type species (E. dendroctoni), Entomocorticium comprises eight species of nutritional symbionts of pine bark beetles in North America. Entomocorticium cobbii is the mycangial associate of the southern pine beetle, Dendroctonus frontalis, and E. parmeteri is the mycangial associate of the western pine beetle, D. brevicomis. Entomocorticium whitneyi, E. portiae, E. kirisitsii, E. oberwinkleri and the previously described E. dendroctoni have been isolated from galleries of D. ponderosae and D. jeffreyi in western North America. Entomocorticium sullivanii forms an ambrosia-like layer of basidia and basidiospores in the pupal chambers of Ips avulsus in the southeastern USA. Entomocorticium is phylogenetically placed within Peniophora, a corticioid genus of wood decay fungi with wind-dispersed basidiospores. At least four species of Entomocorticium produce basidiospores on basidia with reduced sterigmata that apparently do not forcibly discharge basidiospores. Another basidiomycete, Gloeocystidium ipidophilum, was described from Ips typographus galleries in Europe, but it is phylogenetically and taxonomically placed in another genus of wood decay fungi as Cylindrobasidium ipidophilum (Physalacriaceae). The free-living wood-decay fungus Phlebiopsis gigantea (Phanerochaetaceae) has been occasionally associated with bark beetles but is unrelated to C. ipidophilum or Entomocorticium.

Cross-scale interaction of host tree size and climatic water deficit governs bark beetle-induced tree mortality

The recent Californian hot drought (2012–2016) precipitated unprecedented ponderosa pine (Pinus ponderosa) mortality, largely attributable to the western pine beetle (Dendroctonus brevicomis; WPB). Broad-scale climate conditions can directly shape tree mortality patterns, but mortality rates respond non-linearly to climate when local-scale forest characteristics influence the behavior of tree-killing bark beetles (e.g., WPB). To test for these cross-scale interactions, we conduct aerial drone surveys at 32 sites along a gradient of climatic water deficit (CWD) spanning 350 km of latitude and 1000 m of elevation in WPB-impacted Sierra Nevada forests. We map, measure, and classify over 450,000 trees within 9 km2, validating measurements with coincident field plots. We find greater size, proportion, and density of ponderosa pine (the WPB host) increase host mortality rates, as does greater CWD. Critically, we find a CWD/host size interaction such that larger trees amplify host mortality rates in hot/dry sites. Management strategies for climate change adaptation should consider how bark beetle disturbances can depend on cross-scale interactions, which challenge our ability to predict and understand patterns of tree mortality.

Repeated fall prescribed fire in previously thinned Pinus ponderosa increases growth and resistance to other disturbances

In western North America beginning in the late 19th century, fire suppression and other factors resulted in dense ponderosa pine (Pinus ponderosa) forests that are now prone to high severity wildfire, insect attack, and root diseases. Thinning and prescribed fire are commonly used to remove small trees, fire-intolerant tree species, and shrubs, and to reduce surface and aerial fuels. These treatments can be effective at lowering future fire severity, but prescribed burns must be periodically repeated to maintain favorable conditions and are feasible only outside the historical summer wildfire season. This study examines tree growth and mortality associated with spring and fall burning repeated at five (5 yr) and fifteen-year (15 yr) intervals in six previously thinned ponderosa pine stands in the southern Blue Mountain Ecoregion near Burns, Oregon, USA. Each stand consisted of an unburned control, and four season-by-burn interval treatments: spring 5 yr, spring 15 yr, fall 5 yr, and fall 15 yr. Burning was initiated in fall 1997 and spring 1998. Pine height and diameter growth was evaluated in 2013, 15 years following initial treatment. Mortality was assessed annually from 2002 to 2017, when these stands experienced severe defoliation from pine butterfly (PB, Neophasia menapia), followed by a moderate outbreak of western pine beetle (WPB, Dendroctonus brevicomis), allowing us to examine resistance to these disturbances. Pine in the 5 yr fall treatments added more diameter than spring 15 yr and marginally more than spring 5 yr, while fall 15 yr added marginally more diameter than spring 15 yr. In addition, the fall 5 yr treatments had lower mortality associated with prescribed fire, PB, WPB, Ips spp., red turpentine beetle (RTB, D. valens), and mountain pine beetle (MPB, D. ponderosae), but the effect was not always significant. Annosus root disease (ARD, caused by Heterobasidion irregulare) and black stain root disease (BSRD, caused by Leptographium wagneri var. ponderosum) appear to be unaffected by burning. However, BSRD occurrence dramatically declined in all treatments, probably a result of thinning prior to study initiation. Results from this study demonstrate that repeated fall burning, especially at 5-year intervals, improves ponderosa pine diameter growth and may provide resistance to future biotic and abiotic disturbances while spring burning, regardless of frequency, does not.

Stronger influence of growth rate than severity of drought stress on mortality of large ponderosa pines during the 2012-2015 California drought.

Forests in the western United States are being subject to more frequent and severe drought events as the climate warms. The 2012-2015 California drought is a recent example, whereby drought stress was exacerbated by a landscape-scale outbreak of western pine beetle (Dendroctonus brevicomis) and resulted in widespread mortality of dominant canopy species including ponderosa pine (Pinus ponderosa). In this study, we compared pairs of large surviving and beetle-killed ponderosa pines following the California drought in the southern Sierra Nevadas to evaluate physiological characteristics related to survival. Inter-annual growth rates and tree-ring stable isotopes (∆13C and δ18O) were utilized to compare severity of drought stress and climate sensitivity in ponderosa pines that survived and those that were killed by western pine beetle. Compared to beetle-killed trees, surviving trees had higher growth rates and grew in plots with lower ponderosa pine basal area. However, there were no detectable differences in tree-ring ∆13C, δ18O, or stable isotope sensitivity to drought-related meteorological variables. These results indicate that differences in severity of drought stress had little influence on local, inter-tree differences in growth rate and survival of large ponderosa pines during this drought event. Many previous studies have shown that large trees are more likely to be attacked and killed by bark beetles compared to small trees. Our results further suggest that among large ponderosa pines, those that were more resistant to drought stress and bark beetle attacks were in the upper echelon of growth rates among trees within a stand and across the landscape.

Efficacy of verbenone and a blend of verbenone and nonhost volatiles for protecting lodgepole pine from mountain pine beetle (Coleoptera: Curculionidae)

Abstract Significant research has been devoted to the development of semiochemical repellents for mountain pine beetle Dendroctonus ponderosae Hopkins. Several formulations of the beetle's antiaggregation pheromone verbenone (4,6,6‐trimethylbicyclo[3.1.1]hept‐3‐en‐2‐one) are registered to protect pine trees and stands. We compared the efficacy of verbenone and Verbenone Plus for protecting individual lodgepole pine Pinus contorta Dougl. ex Loud. and small stands of P. contorta from mortality attributed to D. ponderosae. Verbenone Plus is a blend of semiochemicals [(−)‐verbenone, acetophenone, and (E)‐2‐hexen‐1‐ol + (Z)‐2‐hexen‐1‐ol] originally developed to inhibit aggregation of western pine beetle D. brevicomis LeConte. It is the only effective semiochemical repellent that protects ponderosa pine P. ponderosa Dougl. ex Laws. from mortality attributed to D. brevicomis, but has yet to be registered or commercialized. Both verbenone and Verbenone Plus were effective for protecting individual P. contorta and small stands of P. contorta from mortality attributed to D. ponderosae. Verbenone alone was as effective as Verbenone Plus. An obstacle to the broader use of semiochemical repellents for the management of bark beetles is the relatively high cost and specificity of the semiochemicals involved. A semiochemical repellent with efficacy across multiple bark beetle species and hosts, such as Verbenone Plus, would be desirable, assuming the additional components are not cost‐prohibitive or limit registration.

Cryptic Species Discrimination in Western Pine Beetle, Dendroctonus brevicomis LeConte (Curculionidae: Scolytinae), Based on Morphological Characters and Geometric Morphometrics.

The western pine beetle (WPB), Dendroctonus brevicomis LeConte, is a major mortality agent of pines in North America. A total of 706 adults of WPB from 81 geographical sites were analyzed with traditional and geometric morphometric methods to evaluate the variation of discrete and quantitative morphological characters with particular attention to the antenna, spermatheca, and seminal rod. Principal coordinates and canonical variate analyses supported three geographical groups in WPB: (1) West, from British Columbia to southern California along the Pacific coast, Idaho, and Montana; (2) East-SMOC, including Nevada, Utah, Colorado, Arizona, New Mexico, Texas, Chihuahua, and Durango; and (3) SMOR, including Coahuila, Nuevo Leon, and Tamaulipas. The pubescence length on the elytral declivity was a robust character for separating West specimens from the other groups. Additionally, the genitalia shape both female and male in dorsal view was a reliable character for discriminating among groups. Based on these results, which agree with genetic and chemical ecology evidence, we herein reinstate Dendroctonus barberi Hopkins (East-SMOC group) and remove it from synonymy with D. brevicomis (West group). Differences in the spermatheca and seminal rod shape of SMOR specimens suggest that these populations might be a different species from D. barberi and D. brevicomis.

Drought-Mediated Changes in Tree Physiological Processes Weaken Tree Defenses to Bark Beetle Attack.

Interactions between water stress and induced defenses and their role in tree mortality due to bark beetles are poorly understood. We performed a factorial experiment on 48 mature ponderosa pines (Pinus ponderosa) in northern Arizona over three years that manipulated a) tree water stress by cutting roots and removing snow; b) bark beetle attacks by using pheromone lures; and c) phloem exposure to biota vectored by bark beetles by inoculating with dead beetles. Tree responses included resin flow from stem wounds, phloem composition of mono- and sesqui-terpenes, xylem water potential, leaf gas exchange, and survival. Phloem contained 21 mono- and sesqui-terpenes, which were dominated by (+)-α-pinene, (-)-limonene, and δ-3-carene. Bark beetle attacks (mostly Dendroctonus brevicomis) and biota carried by beetles induced a general increase in concentration of phloem mono- and sesqui-terpenes, whereas water stress did not. Bark beetle attacks induced an increase in resin flow for unstressed trees but not water-stressed trees. Mortality was highest for beetle-attacked water-stressed trees. Death of beetle-attacked trees was preceded by low resin flow, symptoms of water stress (low xylem water potential, leaf gas exchange), and an ephemeral increase in concentrations of mono- and sesqui-terpenes compared to surviving trees. These results show a) that ponderosa pine can undergo induction of both resin flow and phloem terpenes in response to bark beetle attack, and that the former is more constrained by water stress; b) experimental evidence that water stress predisposes ponderosa pines to mortality from bark beetles.

A major symbiont shift supports a major niche shift in a clade of tree‐killing bark beetles

1. One small clade of bark beetles, out of thousands of species worldwide, has shifted from using phloem to using a combination of phloem and outer bark, or to completely using outer bark.2. The shift to outer bark has been accompanied by a shift to non‐typical bark beetle mutualist fungi in Entomocorticium (Basidiomycota) and Ceratocystiopsis (Ascomycota).3. This study compared the growth and metabolic capabilities of fungi associated with a nearly phloem‐independent species, Dendroctonus brevicomis, with those of mutualist fungi of Dendroctonus ponderosae, a completely phloem‐colonising beetle in a sister clade associated with more typical Ascomycota in Grosmannia and Ophiostoma.4. Only the basidiomycete associated with D. brevicomis could degrade cellulose and lignin, whereas both the ascomycete and basidiomycete could grow in outer bark. Ascomycetes associated with D. ponderosae could not degrade cellulose or lignin or grow in outer bark.5. Beetles and fungi in these mutualisms may best be considered as co‐niche constructors. For niche construction, one organism must modify a resource in a way that enhances its fitness while also influencing fitness of other organisms using the resource. Here, the beetles kill the tree, transport the fungi into the tree, and modify the woody substrate for use by the fungi. They have also evolved mycangia to ensure vertical dissemination of the fungi. In turn, the fungi modify tree tissues and provision nutrients to the host and have evolved traits that support their acquisition and transport by the beetle host in mycangia.

Forest structure and climate mediate drought-induced tree mortality in forests of the Sierra Nevada, USA.

Extreme drought stress and associated bark beetle population growth contributed to an extensive tree mortality event in California, USA, resulting in more than 129million trees dying between 2012 and 2016. Although drought is an important driver of this mortality event, past and ongoing fire suppression and the consequent densification of forests may have contributed. In some areas, land management agencies have worked to reduce stand density through mechanical treatments and prescribed fire to restore forests to less dense, more open conditions that are presumably more resilient to disturbance and changing climate. Here, we evaluate if stand structural conditions associated with treated (e.g., thinned and prescribed burned) forests in the Sierra Nevada of California conferred more resistance to the bark beetle epidemic and drought event of 2012-2016. We found that, compared to untreated units, treated units had lower stand densities, larger average tree diameters, and greater dominance of pines (Pinus), the historically dominant trees. For all tree species studied, mortality was substantially greater in climatically drier areas (i.e., lower elevations and latitudes). Both pine species studied (ponderosa pine [Pinus ponderosa] and sugar pine [Pinus lambertiana]) had greater mortality in areas where their diameters were larger, suggesting a size preference for their insect mortality agents. For ponderosa pine, the tree species experiencing greatest mortality, individual-tree mortality probability (for a given tree diameter) was significantly lower in treated stands. Ponderosa pine mortality was also positively related to density of medium- to large-sized conspecific trees, especially in areas with lower precipitation, suggesting that abundance of nearby host trees for insect mortality agents was an important determinant of pine mortality. Mortality of incense cedar (Calocedrus decurrens) and white fir (Abies concolor) was positively associated with basal area, suggesting sensitivity to competition during drought, but overall mortality was lower, likely because the most prevalent and effective mortality agents (the bark beetles Dendroctonus brevicomis and D.ponderosae) are associated specifically with pine species within our study region. Our findings suggest that forest thinning treatments are effective in reducing drought-related tree mortality in forests, and they underscore the important interaction between water and forest density in mediating bark beetle-caused mortality.

Some Lessons Learned on Early Survival and Growth of Containerized, Locally-Sourced Ponderosa Pine Seedlings in the Davis Mountains of Western Texas, US

The ponderosa pine forests in the Davis Mountains of western Texas recently experienced a major mortality event caused, in part, by an extended regional drought that predisposed trees and stands to mortality from both western pine beetle and wildfires. The loss of many overstory pines and the scarcity of natural ponderosa pine regeneration pose a considerable challenge to restoration. A commissioned study investigated artificial regeneration using containerized ponderosa pine seedlings with multiple planting seasons and vegetation management alternatives. Early survival was statistically greater for dormant season plantings than monsoon season plantings. Vegetation management treatments influenced early growth, survival, and herbivory rates. Physical weed control, which consisted of fibrous weed mats around the base of planted seedlings, showed early advantages over some vegetation management treatments in growth, survival and herbivory deterrence, but all vegetation management treatments had similar survival and herbivory results after 2.5 years. Early survival was poor in all treatments, mainly due to herbivory, which was identified as the principal short-term obstacle to artificial regeneration of ponderosa pine in the Davis Mountains. The larger question regarding feasibility of recovery in this isolated population, particularly if local climatic conditions become increasingly unfavorable, remains.

Extreme ecological stoichiometry of a bark beetle–fungus mutualism

1. Ecological stoichiometry theory was applied to investigate how a consumer contends with an extreme elemental mismatch between its food and its body via symbiotic facilitation.2. The beetle Dendroctonus brevicomis LeConte develops in bark, a substrate extremely low in nitrogen (N) and phosphorus (P). Its survival there depends on interactions with mutualist and antagonist fungi.3. This study found that mutualists transfer N and P from sapwood and phloem into bark, where beetles feed, whereas the antagonist moves these elements only to phloem, resulting in starvation of the insect. However, even with mutualists, N and P concentrations remained low in bark, resulting in low N and extremely low P concentrations in the beetle.4. The N:P ratios found in D. brevicomis larvae were the highest thus far reported for beetles and among the highest for insects and invertebrates. This suggests that the beetle has evolved additional, nutrient‐sparing adaptations.

Predicting post-fire attack of red turpentine or western pine beetle on ponderosa pine and its impact on mortality probability in Pacific Northwest forests

In ponderosa pine forests of western North America, wildfires are becoming more frequent and affecting larger areas, while prescribed fire is increasingly used to reduce fuels and mitigate potential wildfire severity. Both fire types leave trees that initially survive their burn injuries, but will eventually die. Predicting delayed tree mortality has received considerable research attention to aid in post-fire planning and management. The amount of crown scorched is recognized as the most useful variable for discriminating between trees that live or die, but models gain discrimination with additional variables such as bole scorch, bud or cambium necrosis, and post-fire bark beetle attack (Coleoptera: Curculionidae: Scolytinae). Here, logistic regression was used to determine what fire-injury variables are most associated with red turpentine beetle (RTB; Dendroctonus valens LeConte), or western pine beetle (WPB; D. brevicomis LeConte) attack within three years post-fire. This was tested on 7343 ponderosa pine representing a wide diameter range from 18 wild and prescribed fires in Oregon and Washington, and repeated on a subset of 884 large trees >53.3 cm diameter. Bole scorch height was most associated with RTB or WPB attack on trees across all diameters, but model predictive ability was poor, whereas for pines >53.3 cm, the models provided moderate discrimination for predicting attack by each beetle. In addition, mortality models using crown scorch proportion and bole scorch proportion were compared to models with an additional variable for RTB, or WPB attack in year one, or attack by either beetle in year one, or year three. Models including any one of these beetle variables outperformed the models using just bole scorch and crown scorch proportions. For both tree diameter groups, the models including RTB year one performed similar to, or better than models with any other beetle variable, and are preferred for predicting delayed mortality because the RTB attack functions as an additional tree injury indicator similar to cambium kill, not captured by the bole scorch proportion, or crown scorch proportion variables. Furthermore, RTB attack can be assessed within the first year post-fire, and is much faster and easier to evaluate than direct sampling of cambium necrosis.

Tree mortality following drought in the central and southern Sierra Nevada, California, U.S.

Much of California, U.S. experienced a severe drought in 2012–2015 inciting a large tree mortality event in the central and southern Sierra Nevada. We assessed causal agents and rates of tree mortality, and short-term impacts to forest structure and composition based on a network of 11.3-m fixed-radius plots installed within three elevation bands on the Eldorado, Stanislaus, Sierra and Sequoia National Forests (914–1219, 1219–1524 and 1524–1829 m on the Eldorado, Stanislaus, Sierra; 1219–1524, 1524–1829, and 1829–2134 m on the Sequoia), where tree mortality was most severe. About 48.9% of trees died between 2014 and 2017. Tree mortality ranged from 46.1 ± 3.3% on the Eldorado National Forest to 58.7 ± 3.7% on the Sierra National Forest. Significantly higher levels of tree mortality occurred in the low elevation band (60.4 ± 3.0%) compared to the high elevation band (46.1 ± 2.9%). Ponderosa pine, Pinus ponderosa Dougl. ex Laws., exhibited the highest levels of tree mortality (89.6%), with 39.4% of plots losing all P. ponderosa. Mortality of P. ponderosa was highest at the lowest elevations, concentrated in larger-diameter trees, and attributed primarily to colonization by western pine beetle, Dendroctonus brevicomis LeConte. About 89% of P. ponderosa in the three largest diameter classes were killed, representing loss of an important structural component of these forests with implications to wildlife species of conservation concern. Sugar pine, P. lambertiana Dougl., exhibited the second highest levels of tree mortality (48.1%). Mortality of P. lambertiana was concentrated in the mid-diameter classes and attributed primarily to colonization by mountain pine beetle, D. ponderosae Hopkins. White fir, Abies concolor (Gord. & Glend.) Lindl. ex Hildebr., and incense cedar, Calocedrus decurrens (Torr.) Florin, exhibited 26.3% and 23.2% mortality, respectively. Only one Quercus died. Tree mortality (numbers of trees killed) was positively correlated with tree density and slope. A time lag was observed between the occurrence of drought and the majority of tree mortality. Tree regeneration (seedlings and saplings) was dominated by C. decurrens and Quercus spp., representing a potential long-term shift in composition from forests that were dominated by P. ponderosa. About 22.2% of plots contained plant species considered invasive, including cheatgrass, Bromus tectorum L., ripgut brome, Bromus diandrus Roth, bull thistle, Cirsium vulgare (Savi) Ten., and yellow star-thistle, Centaura solstitalis L. The implications of these and other results to recovery and management of drought-impacted forests in the central and southern Sierra Nevada are discussed.

Genomoviruses associated with mountain and western pine beetles

Genomoviruses are circular single-stranded DNA viruses (∼2 kb in size) classified into nine genera, they are highly diverse and have been identified in a variety of samples ranging from fungi to animal sera. Here we identify five genomoviruses belonging to the Gemycircularvirus genus and one to the Gemykibivirus genus from mountain pine beetle and western pine beetle sampled in Arizona. Collectively these six viral genomes share <77% genome-wide pairwise identity and hence represent six new species of genomoviruses. Four of the gemycircularviruses from the mountain pine beetles are recombinant, with one having a recombinant region that spans the entire capsid protein. Pine beetles have a symbiotic relationship with certain tree pathogenic fungi. Therefore given that Sclerotinia sclerotiorum hypovirulence-associated DNA virus 1, a gemycircularvirus, induces hypovirulence in the plant pathogenic fungus Sclerotinia sclerotiorum and infects the mycophagous insect Lycoriella ingenua, it is possible that the six genomoviruses identified here may be directly associated with the pine beetle fungal symbionts and/or with the insects themselves.