Southwestern pine beetle, Dendroctonus barberi Hopkins (Coleoptera: Curculionidae), is a pest of ponderosa pine, Pinus ponderosa Dougl. ex Laws. (Pinales: Pinaceae), in the southwestern United States and northern Mexico. D. barberi is closely related to the western pine beetle, D. brevicomis LeConte, but was reinstated as a distinct species in 2019. Verbenone Plus, a semiochemical blend of (-)-verbenone, acetophenone, (E )-2-hexen-1-ol, and (Z )-2-hexen-1-ol, was found to be inhibitory to D. brevicomis in 2009 and later reported to reduce mortality of P. ponderosa attributed to D. brevicomis. We evaluated the effects of Verbenone Plus on inhibition of D. barberi at the Davis Mountains Preserve, Texas, United States. A commercial formulation of (-)-verbenone (SPLAT Verb, EPA Reg. No. 80286-20, ISCA Inc., Riverside, California, United States) was used in all experiments. SPLAT Verb is a flowable matrix containing 10.0% (-)-verbenone by weight. Acetophenone, (E)-2-hexen-1-ol, and (Z)-2-hexen-1-ol were loaded into pouches with (E)-2-hexen-1-ol + (Z)-2-hexen-1-ol combined in a single pouch. Trapping assays were conducted from 16 to 23 June 2024 and from 2 to 9 October 2024 to evaluate responses of D. barberi to a low dose (3.5 g of (-)-verbenone/trap, VPL) and high dose (7.0 g of (-)-verbenone/trap, VPH) of Verbenone Plus in multiple-funnel traps baited with a D. barberi lure (Synergy Semiochemical Corp., Delta, BC, Canada). VPL and VPH inhibited attraction of total D. barberi, female D. barberi, and male D. barberi in both trapping assays. In the October assay, VPH captured fewer total D. barberi and female D. barberi than VPL, whereas no differences were observed between doses in the April assay. A zone-of-inhibition assay was conducted from 7 to 17 October 2025 to evaluate responses of D. barberi to multiple-funnel traps baited with a D. barberi lure at 5 distances (1.5, 3, 6, 9, and 12 m) from P. ponderosa treated with VPL and VPH. No significant differences in captures occurred among distances for VPL. For VPH, significant reductions in captures occurred at 1.5, 3, and 6 m compared to 12 m, with maximum inhibition (defined as the farthest distance at which the highest levels of inhibition occurred) observed up to 3 m. In a tree protection study, 5 plots containing 25 P. ponderosa were treated with VPH and 5 were left untreated in 2024 and 2025. Significantly less P. ponderosa mortality occurred on plots treated with VPH (27.4 ± 3.5%) than in the control (52.4 ± 3.9%) during the 2-year period. The implications of these and other results to management of D. barberi are discussed.

Mountain pine beetle, Dendroctonus ponderosae Hopkins, is among the most significant forest insect pests, particularly in lodgepole pine, Pinus contorta Dougl. ex Loud., forests in western North America. We established a network of 125, 0.081-ha circular plots in P. contorta forests in Colorado, Idaho, Montana, Utah, and Wyoming to document tree mortality and stand-level responses to D. ponderosae outbreaks. We found notable reductions in several key metrics but also observed significant variability in per-year (0–71%) and cumulative mortality (2–87%). We used this long-term monitoring dataset and employed generalized linear mixed effects modeling to determine the biophysical drivers of the variation in P. contorta mortality. Models were fitted to the entire (2005–2024) dataset, and to two truncated datasets: outbreak years only (2005–2012) and maximum per year P. contorta mortality per plot. Explanatory variables included tree, plot, and environmental factors (temperature and water availability). Our models highlighted several significant drivers of P. contorta mortality. The most significant and consistent were individual tree dbh and quadratic mean diameter (QMD). Larger individual tree dbh and larger QMD positively influenced the probability of P. contorta mortality. Tree dbh interacted significantly with water availability variables, particularly November-March snowpack. Increased snowpack and greater densities of nonhost tree species decreased mortality. These results suggest that management efforts should account for the frequency of large-diameter P. contorta and that density reduction and/or increasing tree species (nonhost) and structural diversity may provide mitigation strategies for reducing P. contorta mortality attributed to D. ponderosae in the Intermountain West.

Spruce beetle, Dendroctonus rufipennis (Kirby) (Coleoptera: Curculionidae), is the most significant pest of spruce, Picea spp. (Pinales: Pinaceae), in western North America. Several doses of 3-methylcyclohex-2-en-1-one (MCH), the primary antiaggregation pheromone of D. rufipennis, alone and combined with non-host volatiles have been demonstrated effective for Picea protection. Herein, we evaluate the effects of MCH dose on D. rufipennis captures in baited trapping assays in Alaska and Colorado, United States. Twenty-five, 12-unit, multiple-funnel traps were baited with a D. rufipennis lure (frontalin + MCOL + spruce terpenes; Synergy Semiochemical Corp., Delta, British Columbia, Canada) and randomly assigned to one of 5 treatments in each assay: SBL (baited control); SBL + 1 g MCH; SBL + 3.5 g MCH; SBL + 7 g MCH; and SBL + 10 g MCH. SPLAT MCH (experimental formulation ISR: MCH-001R1, ISCA Inc., Riverside, California, United States), a flowable matrix containing 10.0% MCH by weight, was used in both assays with dose manipulated by the number and size of SPLAT MCH dollops (release points) attached to traps. In both Alaska and Colorado, all MCH doses (1, 3.5, 7, and 10 g) significantly reduced D. rufipennis captures compared to SBL. No significant differences were observed among MCH doses. Males and females responded similarly to MCH doses. The implications of these and other results to management of D. rufipennis are discussed.

In the western U.S., the research, development and application (RD&A) of semiochemical inhibitors for protecting conifers from bark beetles has focused on antiaggregation pheromones and, to a lesser extent, nonhost volatiles. The sequence begins with identifying potential inhibitors by searching relevant literature; sampling the volatile profiles of bark beetle extracts, frass, infested host materials, hosts, nonhosts, and the forest environment; and/or conducting gas chromatographic-electroantennographic detection analyses of bark beetle antennae. Promising inhibitors are then evaluated for levels of inhibition in trapping and tree protection assays under field conditions. Additional assays are required to refine inhibitors and to inform the spacing and distribution of release devices. Factors affecting levels of tree protection include (a) the population density of the target bark beetle species, (b) forest structure and composition, (c) variability in bark beetle responses to inhibitors, (d) the stereochemistry and chemical stability of inhibitors in the forest environment, (e) release rates of inhibitors, (f) strength of inhibition, (g) zone of inhibition, and (h) the presence of other stressors and disturbances. Once an inhibitor has been successfully developed, there are substantial costs and data requirements associated with registration. The commercial availability of inhibitors for protecting conifers from bark beetles is limited but evidence suggests there are favorable benefit-to-cost returns to be realized with additional investments in RD&A. Opportunities include (a) developing new (or better) inhibitors, inhibitory blends, and formulations, (b) automating (e.g., via unmanned aerial vehicles) and optimizing applications of inhibitors in the field, (c) developing cheaper and more efficient means of synthesizing inhibitors, (d) streamlining regulatory processes and incentivizing RD&A to expedite the commercial availability of products, and (e) integrating inhibitors with other management tactics. We review recent advances in the use of inhibitors for western pine beetle, Dendroctonus brevicomis LeConte, southwestern pine beetle, D. barberi Hopkins, mountain pine beetle, D. ponderosae Hopkins, Douglas-fir beetle, D. pseudotsugae Hopkins, spruce beetle, D. rufipennis (Kirby), northern spruce engraver, Ips perturbatus (Eichhoff), and pine engraver, I. pini (Say). Research on these and other bark beetles has informed the RD&A of inhibitors for other forest and agricultural pests.

Spruce beetle (Dendroctonus rufipennis (Kirby)) is a major cause of spruce (Picea spp.) mortality in western North America. We synthesized the literature on the chemical ecology of spruce beetle, focusing on efforts to reduce host tree losses. This literature dates back to the mid-20th century and focuses on spruce beetle populations in Alaska, U.S., western Canada, and the central and southern Rocky Mountains, U.S. Spruce beetle aggregation pheromone components include frontalin (1,5-dimethyl-6,8-dioxabicyclo[3.2.1]octane), seudenol (3-methyl-2-cyclohexen-1-ol), MCOL (1-methyl-2-cyclohexen-1-ol), and verbenene (4-methylene-6,6-dimethylbicyclo[3.1.1]hept-2-ene). The attraction of spruce beetle to one aggregation pheromone component is enhanced by the co-release of other aggregation pheromones and host compounds (e.g., α-pinene). Several baits that attract spruce beetles are commercially available and are used for survey and detection, population suppression, snag creation, and experimental purposes. The antiaggregation pheromone is MCH (3-methyl-2-cyclohexen-1-one), which has been evaluated for reducing colonization of felled spruce since the 1970s. Beginning in the early 2000s, MCH has been evaluated for protecting live, standing spruce from colonization by and mortality attributed to spruce beetle. With a few exceptions, significant reductions in levels of spruce beetle colonization and/or spruce mortality were reported. More recent efforts have combined MCH with other repellents (e.g., nonhost compounds) in hope of increasing levels of tree protection. Today, several formulations of MCH are registered for tree protection purposes in the U.S. and Canada.

Disturbance plays a critical role in the ecology of forests including influencing the abundance and diversity of fauna. Although numerous studies have focused on forest responses to various disturbance events, less attention has been given to arthropod community responses. California experienced an extreme, multi-year drought from 2012 to 2015 which severely stressed trees and incited epidemics of several bark beetle species (Coleoptera: Curculionidae: Scolytinae). Water stress and bark beetles contributed to a significant mortality event of hundreds of millions of trees in the central and southern Sierra Nevada, causing significant structural and compositional changes in forests. Our study sought to characterize woodborer and wood-decay-related beetle responses to various levels of tree mortality and snag (dead standing tree) retention resulting from this tree mortality event. Of particular interest were responses to differences in the orientation of dead wood, standing snags versus fallen snags. Ethanol-baited panel flight intercept traps were deployed for multiple weeks in 2022 and 2023 on plots representative of 3 disturbance classes: (i) low tree mortality (<30%), (ii) high tree mortality (>50%) with low snag fall (≤50%), and (iii) high tree mortality (>50%) with high snag fall (>60%). Woodborers and wood-decay-related beetle assemblages were compared at the family and species level. Our analyses revealed several significant differences in community assemblages among disturbance classes. Despite these differences, our results failed to reveal clear, qualitatively distinctive assemblages among disturbance classes. Rather, we could only conclude general patterns from the observed dissimilarities in richness and abundance. In general, we observed a greater diversity of woodborers on high-mortality plots than on low-mortality plots. Similarly, the diversity of wood-decay-related beetles generally increased with greater amounts (basal area) of snag fall. The amount of tree mortality and snag fall were positively related to several woodborer and wood-decay-related beetles. Observed beetle assemblages, their corresponding life histories, and the influences of altered habitat availability are discussed.

Spruce beetle, Dendroctonus rufipennis (Kirby) (Coleoptera: Curculionidae), poses a significant threat to mature spruce (Picea spp.) in western North America. We evaluated 3-methylcyclohex-2-en-1-one (MCH), the primary anti-aggregation pheromone of D. rufipennis, for tree protection at 3.5 and 7 g MCH/tree (SPLAT MCH, ISCA Inc., Riverside, CA) alone and in combination with acetophenone + (E)-2-hexen-1-ol + (Z)-2-hexen-1ol (PLUS) or with linalool + β-caryophyllene + (Z)-3-hexanol (AKB). Dendroctonus rufipennis colonization and subsequent mortality of individually treated spruce and of untreated spruce within 0.041-ha circular plots (11.3-m radius) surrounding individually treated spruce were assessed in two studies, one on the Kenai Peninsula, Alaska, and one on the San Isabel National Forest, Colorado. In a third study, we evaluated two release devices containing MCH, SPLAT MCH, and Synergy Shield MCH Double Bubble caps (Synergy Semiochemical Corp. Delta, BC), for tree protection at three doses (1, 3, and 7 g MCH/tree) on the Kenai Peninsula. In each study, treated trees and untreated control trees were baited with frontalin, the primary aggregation pheromone of D. rufipennis, to induce D. rufipennis attacks. Separately, we conducted three zones of inhibition studies to determine maximum inhibition from semiochemical point sources (treated spruce) based on D. rufipennis captures in baited traps. All semiochemical treatments, except SPLAT MCH alone, significantly reduced D. rufipennis colonization of treated spruce and neighboring spruce in all studies compared to the controls. All semiochemical treatments significantly reduced the mortality of treated spruce and neighboring spruce in all studies compared to the controls. Only two (of 300) semiochemical-treated spruce died during these studies while 47% to 80% mortality occurred in the controls (32 of 50 spruce across all studies). Maximum inhibition was statistically constant to 4 m from semiochemical point sources in Alaska and to 12 m in two separate studies in Colorado. The implications of these results are discussed.

DOI: 10.18474/JES24-31 Abstract Bark beetles (Coleoptera: Curculionidae, Scolytinae) cause extensive tree mortality in conifer forests in the western United States. One method to protect conifers from bark beetles involves applications of liquid formulations of insecticides to the tree bole using high-pressure (e.g., >= 2,241 kPa) ground-based sprayers. Several active ingredients and products are effective when properly applied in accordance with the label. Researchers recently have developed more portable methods that inject small quantities of systemic insecticides directly into trees. The purpose of this review is to synthesize information on the efficacy, residual activity, and environmental safety of insecticides commonly used to protect conifers from bark beetles in the western United States so that informed, judicious decisions can be made about the use of these insecticides. This review serves as an update to "Advances in insecticide tools and tactics for protecting conifers from bark beetle attack in the western United States" (Fettig et al. 2013a) and focuses, where applicable, on relevant literature published since 2012.

Wildfire activity in the western U.S. has highlighted the importance of effective management to address this growing threat. The Lookout Mountain Thinning and Fuels Reduction Study (LMS) is an operational-scale, long-term study of the effects of forest restoration and fuel reduction treatments in ponderosa pine (Pinus ponderosa Dougl. ex Laws.) and mixed-conifer forests in central Oregon, U.S. The broad objectives of the LMS are to examine the effectiveness and longevity of treatments on wildfire risk and to assess the collateral effects. Treatments include four levels of overstory thinning followed by mastication of the understory vegetation and prescribed burning. Stands were thinned to residual densities of 50, 75, or 100% of the upper management zone (UMZ), which accounts for site differences as reflected by stand density relationships for specific plant communities. A fourth treatment combines the 75 UMZ with small gaps (~0.1 ha) to facilitate regeneration (75 UMZ + Gaps). A fifth treatment comprises an untreated control (UC). We examined the causes and levels of tree mortality that occurred 2–9 years after treatments. A total of 391,292 trees was inventoried, of which 2.3% (9084) died. Higher levels of tree mortality (all causes) occurred on the UC (7.1 ± 1.9%, mean ± SEM) than on the 50 UMZ (0.7 ± 0.1%). Mortality was attributed to several bark beetle species (Coleoptera: Curculionidae) (4002 trees), unknown factors (2682 trees), wind (1958 trees), suppression (327 trees), snow breakage (61 trees), prescribed fire (19 trees), western gall rust (15 trees), cankers (8 trees), mechanical damage (5 trees), dwarf mistletoe (4 trees), and woodborers (3 trees). Among bark beetles, tree mortality was attributed to western pine beetle (Dendroctonus brevicomis LeConte) (1631 trees), fir engraver (Scolytus ventralis LeConte) (1580 trees), mountain pine beetle (Dendroctonus ponderosae Hopkins) (526 trees), engraver beetles (Ips spp.) (169 trees), hemlock engraver (Scolytus tsugae (Swaine)) (77 trees), and Pityogenes spp. (19 trees). Higher levels of bark beetle-caused tree mortality occurred on the UC (2.9 ± 0.7%) than on the 50 UMZ (0.3 ± 0.1%) which, in general, was the relationship observed for individual bark beetle species. Higher levels of tree mortality were attributed to wind on the 100 UMZ (1.0 ± 0.2%) and UC (1.2 ± 1.5%) than on the 50 UMZ (0.2 ± 0.02%) and 75 UMZ (0.4 ± 0.1%). Higher levels of tree mortality were attributed to suppression on the UC (0.5 ± 0.3%) than on the 50 UMZ (0.003 ± 0.002%) and 75 UMZ + Gaps (0.0 ± 0.0%). Significant positive correlations were observed between measures of stand density and levels of tree mortality for most causal agents. Tree size (diameter at 1.37 m) frequently had a significant effect on tree mortality, but relationships varied by causal agent. The forest restoration and fuels reduction treatments implemented on the LMS increased resistance to multiple disturbances. The implications of these and other results to the management of fire-adapted forests are discussed.

Ecological forestry experiments typicaly use large treatment units and silvicultural prescriptions that commonly increase within-unit heterogeneity in structural complexity and species composition in large treatment units. Increased heterogeneity influences processes affecting tree responses (e.g., competition) that operate at the neighborhood-level, posing challenges to analysis and interpretation. To investigate whether examining within-unit heterogeneity offers a more meaningful evaluation of project success than comparing categorical treatment effects, we used 20-year data from the Goosenest Adaptive Management Area (AMA) ecological forestry experiment in northern California, U.S. Designed to evaluate management alternatives for reducing fuels and accelerating development of late-seral forest characteristics in ponderosa pine (Pinus ponderosa)/white fir (Abies concolor) mixed-conifer forests, the Goosenest AMA study consists of (1) an untreated Control, (2) a Big Tree treatment using thinning from below to favor retention of large trees of any species, (3) a Pine Emphasis treatment combining thinning from below with radial thinning to favor percent ponderosa pine while increasing structural complexity, and (4) a Pine Emphasis with Fire treatment with added post-thinning prescribed burns. Our objectives were to evaluate 1) how treatments affect within-unit variation in neighborhood competition, structural complexity, and tree species composition, and 2) whether categorical treatment effects versus within-unit variation in competition, complexity, and composition influence individual-tree basal area increment (BAI), vigor as indicated by live crown ratio (ΔLCR), and tree mortality. To accomplish this, we developed and compared a series of generalized linear mixed models. Our analysis included the first investigation into whether fuel-reduction treatments alter tree species mixture-effects in ponderosa pine/white fir mixed-conifer forests. We found that all treatments similarly reduced neighborhood competition relative to the Control. The two Pine Emphasis treatments promoted greater variation in neighborhood competition and higher Percent pine relative to the Big Tree treatment, consistent with restoration objectives. Reduced neighborhood competition improved both ponderosa pine and white fir BAI. Reduced neighborhood competition helped to offset or reverse crown vigor decline in ponderosa pine and white fir, respectively. Species-mixture effects were negative for both small and large ponderosa pine BAI. For white-fir, trees grew faster in neighborhoods with a higher percentage of pine but the probability of mortality increased. Categorical treatment differences consistently reduced ponderosa pine and white fir mortality, except for the Pine Emphasis with Fire treatment, which increased white fir mortality. Our findings suggest that restoring historical ponderosa-pine forest reference conditions could accelerate the development of fire-resistant ponderosa pine tree sizes and sustain large pine growth.

Forests play an essential role in conserving pollinating insects and supporting pollination services in mixed-use landscapes and are particularly important to species that require resources restricted to forests. However, some forests provide higher quality habitat for these organisms than others. The primary objectives of this article are to 1) review how pollinator communities are influenced by changes in forest structure, composition, and age, 2) explore how these patterns differ between conifer and broadleaf forests, and 3) provide recommendations for managers interested in optimizing forest conditions for pollinating insects. Although biodiversity generally increases as forests mature and become more structurally and compositionally complex, patterns exhibited by pollinating insects vary depending on forest type and prevailing disturbance regimes. For example, conifer forests can either sustain pollinator diversity comparable to open habitats or experience reduced pollinator diversity depending on the openness of the canopy. In broadleaf forests, pollinator diversity often increases with age (following the stem exclusion stage) and increasing tree diversity, and diversity in these areas can exceed what is observed in open habitats even under closed-canopy conditions. Such patterns likely reflect the importance of flowering broadleaf trees to pollinators, including many forest-dependent species, and suggest that optimal management practices for conserving pollinators differ between conifer and broadleaf forests. We conclude that: 1) the quality of forests to pollinating insects is a function of forest structure and composition as mediated by forest age and disturbance history and 2) best management practices need to be developed separately for conserving pollinators in conifer and broadleaf forests. Research aimed at better understanding the value of different broadleaf tree taxa to pollinators, especially forest-dependent species, is needed.

The European spruce bark beetle, Ips typographus (L.), is the most important forest pest in Europe due to the profound impacts of periodic outbreaks on ecosystem goods and services. Herein, we evaluated the responses of I. typographus to different doses of verbenone (SPLAT (R) Verb, 10% (-)-verbenone by weight; ISCA Inc., Riverside, CA, USA) in traps baited with its aggregation pheromones. Results are based on 1,492,289 I. typographus collected in five experiments over 3 years. SPLAT (R) Verb inhibited the response of I. typographus to baited traps out to 14 m from the point of release (dollop) and for >80 days at a dose of 75 g per dollop. Reductions in trap catch ranged from 34% to 93% depending on the dose of verbenone, age of SPLAT (R) Verb dollops, distance from dollops and the environment. In forest stands, significant reductions in trap catch were observed at distances up to 14 m from the point of release, with the largest reductions observed at 0 m (93%) and 2 m (64%). In an open area, significant reductions in trap catch were observed at distances up to only 2 m from the point of release, with the largest reduction observed at 0 m (66%). The much lower active inhibitory range of verbenone in the open area appears to be explained by less stable accumulations of verbenone in the surrounding air. There was a significant negative correlation between trap catch and the amount of verbenone measured in air in the vicinity of traps. We also observed inhibition of the sixtoothed spruce bark beetle, Pityogenes chalcographus (L.), another important forest pest in Europe, at all doses (20, 40, 75 and 100 g) of SPLAT (R) Verb that were evaluated. The implications of these and other results to the management of I. typographus are discussed.

Outbreaks of European spruce bark beetle, Ips typographus, often follow storms, and can result in losses of Norway spruce, Picea abies, that largely exceed those caused by storms alone. Management actions to reduce P. abies losses attributed to I. typographus mainly consist of salvage and sanitation logging of windthrown P. abies, which are sometimes limited by logistic capacities and constraints. Recent outbreaks across Europe have destroyed large areas of P. abies forests, and are expected to further intensify as a consequence of climate change, so that additional tools are needed for integrated bark beetle management. In a 4-yr study, we applied (-)-verbenone (SPLAT (R) Verb, ISCA Inc., Riverside, CA, U.S.), a common bark-beetle inhibitor, to natural and artificially generated windthrows of P. abies (5-6 trees) to examine its efficacy for reducing infestation by I. typographus in spring. In two of four years, SPLAT (R) Verb caused delay of infestation of similar to 3 wks, which may provide a relevant gain of time for logging operations. Infestation probability and density were reduced by up to 78% and 76%, respectively. Treatment effectivity increased with dose per volume of windthrown P. abies and seemed to decrease with greater infestation pressure. Elevated concentrations of verbenone in the active airspace of forests could only be detected in close proximity (<50 cm) of SPLAT (R) Verb application points (dollops). We propose a decision pathway for management of windthrown P. abies that includes verbenone as an environmentally friendly tool within the integrated management of I. typographus in Central Europe.

Spruce beetle, Dendroctonus rufipennis (Kirby) (Coleoptera: Curculionidae), is the most destructive pest of mature spruce (Picea) in western North America. Recent outbreaks in Alaska and other western US states highlight the need for tools to protect Picea from D. rufipennis. The primary antiaggregation pheromone of D. rufipennis (3-methylcyclohex-2-en-1-one, MCH) and various combinations of potential repellents (1-octen-3-ol, exo-brevicomin, endo-brevicomin, ipsdienol, ipsenol, limonene, and verbenone) were tested for their ability to disrupt the response of D. rufipennis to attractant-baited multiple-funnel traps. Two assays were conducted on the Kenai Peninsula, Alaska, in June and July 2021. All treatments significantly reduced the mean number of D. rufipennis caught compared to the baited control. No other significant differences were observed among treatments. Informed by these and other data, tree protection studies were established in Lutz spruce, Picea x lutzii, on the Kenai Peninsula in 2022 and in Engelmann spruce, Pi. engelmannii, in the Uinta Mountains, Utah, in 2021. All experimental trees were baited with frontalin. Repellent treatments included MCH (SPLAT MCH, ISCA Inc., Riverside, CA, USA) and at least 1 additional repellent combination. In Alaska, all treatments significantly reduced colonization (strip attacks + mass attacks) and mortality of individually treated Pi. x lutzii and all Picea within 11.3-m radius of each treated Pi. x lutzii compared to the control. In Utah, all treatments except for SPLAT MCH + octenol significantly reduced colonization compared to the control. Only SPLAT MCH + Acer kairomone blend (AKB) and SPLAT MCH + octenol reduced Pi. engelmannii mortality compared to the control. SPLAT MCH + AKB and SPLAT MCH + acetophenone and green leaf volatiles (PLUS) were the most effective across both studies. The implications of these and other results to the development of an effective semiochemical repellent for D. rufipennis are discussed.

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.

Mixed tree-species forest management can increase forest resilience by reducing the impacts of disturbances that disproportionately affect a single tree species or closely related groups of tree species. Beyond disturbance-risk reduction, tree-species diversification may foster functional-diversity effects (e.g., complementarity or facilitation) that alter the performance of a given tree species in mixed versus pure stands, potentially benefitting carbon sequestration and wildlife habitat. Tree species-mixture effects have been explored to only a limited degree in western US forests and, particularly, in California. Establishing whether vigor, growth, and mortality of common tree species vary with stand composition would help inform restoration and modeling of these forests under climate change. Using data from USDA Forest Service Forest Inventory and Analysis (FIA) plots from California, we examined how individual-tree vigor, as indicated by live crown ratio (LCR), periodic basal area increment (BAI), and mortality odds varied with functional dissimilarity (FDis). We quantified FDis using an index based on 11 traits related to resource acquisition, competition, environmental tolerances, and fire ecology. We classified major tree species into ponderosa pine (Pinus ponderosa), Jeffrey pine (Pinus jeffreyi), incense-cedar (Calocedrus decurrens), true firs (Abies spp.), Douglas-fir (Pseudotsuga menziesii), live oaks (Quercus spp.), and deciduous oaks (Quercus spp.) response groups. We tested for the main effects of FDis on tree responses, as well as for interactions with tree, site, stand, and climate factors. We found that initial tree height modulated the effects of FDis on ponderosa pine, Jeffrey pine, incense-cedar, and true fir LCR, whereas FDis interacted with climate to alter live oak and deciduous oak LCR. FDis decreased BAI in ponderosa pine and increased BAI in live oaks. FDis interacted with tree size to influence BAI for Jeffrey pine, Douglas-fir, and true firs. We found no evidence that climate or site quality modulated FDis effects on BAI for any species group. Tree mortality was not responsive to FDis, except for the true firs, where both initial tree height and competition interacted with FDis to increase and decrease mortality odds, respectively. FDis effects commonly shifted from positive to negative along gradients of stand structure and site quality, indicating that these effects vary with site and stand conditions. Our results have implications for balancing the ecosystem benefits of mixed stands, such as disturbance risk, carbon sequestration, and habitat during forest restoration projects in the region, as well as for more accurate modeling of complex stands.

We assessed attraction of pine engraver, Ips pini (Say) (Coleoptera: Curculionidae; Scolytinae), to pheromone-baited funnel traps treated with repellent semiochemicals in ponderosa pine, Pinus ponderosa var. scopulorum Engelm., forests in northern Arizona. Treatments included: 1) baited control (B, ipsdienol + lanierone), 2) 70 g of SPLAT Verb (a flowable, biodegradable formulation containing 10% verbenone, ISCA Technologies Inc., Riverside, CA, USA) + B, 3) 70 g of SPLAT Verb + (E)-2-hexen-1-ol+(Z)-2-hexen-1-ol + acetophenone + B, 4) 7.84-g verbenone pouch (Product #3413, Synergy Semiochemicals Corp., Delta, British Columbia, Canada) + B, and 5) 7.84-g verbenone pouch + (E)-2-hexen-1-ol+(Z)-2-hexen-1-ol + acetophenone + B. In total, 472 I. pini were collected. Trap catches were highest in baited traps and declined significantly with the addition of both formulations of verbenone. Traps treated with SPLAT Verb caught significantly fewer I. pini and male I. pini than those treated with verbenone pouches. The addition of (E)-2-hexen-1-ol+(Z)-2-hexen-1-ol + acetophenone to SPLAT Verb and the verbenone pouch had no effect on trap catch. Verbenone has potential as an effective tool for protecting P. ponderosa trees and slash from I. pini in northern Arizona, but the addition of (E)-2-hexen-1-ol+(Z)-2-hexen-1-ol + acetophenone to verbenone is unwarranted.

AbstractIntegrated pest management (IPM) is perhaps best described as “…the maintenance of destructive agents, including insects, at tolerable levels by the planned use of a variety of preventative, suppressive or regulatory tactics that are ecologically and economically efficient and socially acceptable.

These files include the main processed data and R script used in the analysis for the publication Characterizing ground and surface fuels across Sierra Nevada forests shortly after the 2012–2016 drought accepted in the journal Forest Ecology and Management. Abtract The 2012–2016 hotter drought in the Sierra Nevada, California, USA led to the mortality of millions of trees. This disruption to the disturbance regime is an example of how climate extremes can exacerbate current and future wildfire risks. In this study, we used data from an extensive network of forest plots across 13 different sites in the Sierra Nevada to characterize ground (i.e., duff) and surface fuels and their potential drivers shortly after the drought (2016–2018), but before snag (standing dead tree) fall associated with this massive tree mortality event occurred. Overall, we found high biomass of fuels for most sites (138.2 ± 21.7 Mg ha-1, Mean ± 95% CI), especially in areas lacking recent fire or active management, with values up to five times higher than estimates for pre-settlement conditions for mixed-conifer forests in the region. Four major groups of forest overstory structure were identified with some distinct fuel characteristics. These ranged from a cluster of ponderosa pine (Pinus ponderosa Douglas ex Lawson)-dominated plots with the highest density of snags, lowest live tree basal area, and lowest total ground and surface fuel biomass (mean = 90.1 Mg ha-1) to a group of giant sequoia (Sequoiadendron giganteum (Lindl.) J. Buchholz)-dominated plots with the highest live tree basal area and highest total fuel biomass (mean = 547.6 Mg ha-1). Although we found relatively weak relationships between forest characteristics and fuel loads, their inclusion in a model selection framework that also considered biophysical variables and disturbance history explained more than 80% of the observed variation in litter + fine woody debris loads. The baseline fuel conditions described here will not only inform the management of drought-impacted forests in the Sierra Nevada, but also help identify key drivers of fuel succession in a changing environment.

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.