ABSTRACTDispersal is a key process in the spatial and temporal dynamics of insect populations. Dispersal depends on the flight performance of individual insects, which is influenced by their environment, morphology, and physiological state. Temperature affects flight performance and costs through its effect on the metabolism of ectotherms. It can also induce developmental changes in flight‐related traits that influence flight biomechanics and insect size, affecting the resources available for this activity. We thus need to understand how temperature during flight modulates flight performance and costs, but also how morphology affects them. Using flight mills, models were developed to describe how flight performance and costs of individual spruce budworm females varied over a range of temperatures (10°C–35°C). Variations of barometric pressure and morphological traits were also incorporated in these models. Flight propensity dropped below 20°C, and when female mass increased relative to wing area, suggesting that there is a wing load beyond which flight probability decreases. Speed, duration, and distance of flight decreased as temperature deviated from 23°C, while wingbeat frequency increased consistently with temperature. Females with long and broad wings had greater flight endurance. Mass loss and use of water and fuel (lipids and carbohydrates) increased with temperature, while the remaining lipids were not affected. As a result, female budworms allocated a daily energy budget to flight, which was proportional to their mass. Large females also benefited from an energy saving due to their mass during flight. Water loss was proportional to female mass but decreased with distance traveled, reaching hydric quasi‐homoeostasis at distances greater than 10 km. Our approach reveals the underlying mechanisms of flight and highlights the factors that influence the landing process after dispersal in the spruce budworm. The relationships presented in this study can help develop more realistic models of dispersal for this boreal forest pest.

Active space, the area over which a stimulus elicits a behavioural reaction in a receiving individual, is an important yet seldom investigated aspect of insect behaviour. It defines the spatial scale over which an individual interacts with a stimulus and can influence mating success and population dynamics. From an applied perspective, active space is an important component of the effective range of semiochemical-baited traps used for surveillance of forest and agricultural insects. This study used wing fanning assays, competitive trapping, and portable electroantennography to investigate the active space of spruce budworm, Choristoneura fumiferana Clemens (Lepidoptera: Tortricidae) and spongy moth, Lymantria dispar dispar L. (Lepidoptera: Erebidae) pheromone-baited traps. Wing fanning assays and competitive trapping had similar outcomes for active space distance for both species. For spruce budworm, wing fanning assays estimate an active space of 18.4m and competitive trapping experiments showed interference at distances less than 20m. For spongy moth, wing fanning assays suggest an active space of 81.5m, corroborating results from a previous study that estimated trap interference to occur at distances less than 80m. Portable electroantennography displayed an increase in neural depolarization amplitude at distances less than 30m for spruce budworm and 130m for spongy moth. Overall, the three methods used showed similar results for the active space distance of each species.

The bioinsecticide Bacillus thuringiensis subsp. kurstaki (Btk) is applied over large areas to reduce defoliation caused by the spruce budworm, an insect which affects millions of hectares of coniferous forests every 30 to 40 years in eastern North America. The aim of our study was to determine whether, in addition to its direct lethal effects, aerial spraying of Btk had sublethal effects on spruce budworm populations and their parasitism. Four sites were sprayed with Btk and compared to four control sites in two regions, one where the outbreak had started three years earlier and the other where it had been going on for 10 years. Insects were collected to compare budworm pupal mass and parasitism at different stages (L5, L6 and pupae). Budworm pupae were significantly lighter in Btk-treated sites than in controls, and in the older population than in the younger one. However, pupae collected from Btk-treated sites had the same mass in both regions, suggesting a minimum pupal mass threshold, which may affect population dynamics. Larval parasitism was low, but pupal parasitism was high and strongly influenced by an interaction between Btk and region, with a higher parasitism observed in Btk-treated sites of the younger population than in those of the older population. A significant interaction was observed between Btk treatment and region on the proportion of larvae that failed to complete development, which was particularly high in Btk-treated sites of the older population. Our study confirms the effectiveness of Btk in controlling spruce budworm populations directly but also indirectly through sublethal effects on budworm development, capacity to complete development, pupal size and parasitism. To maximize control efficacy, the timing of Btk applications could vary according to the age of populations during the budworm outbreak cycle.

In northeastern North America, episodic eastern spruce budworm (Choristoneura fumiferana) outbreaks extensively defoliate balsam fir (Abies balsamea) and spruces (Picea glauca, P. rubens, P. mariana). We investigated eastern spruce budworm defoliation in eastern hemlock (Tsuga canadensis), a reported alternate host species, during previously documented outbreaks in Maine, USA. We compiled red spruce and hemlock tree-ring series from nine sites, seven of which showed synchronous radial growth reductions in spruce and hemlock during documented budworm outbreaks. Outbreak evidence varied among sites, but growth reductions were observed in six study sites during a 1914 outbreak. Synchronous growth reductions between spruce and hemlock suggest that when budworm defoliation occurred in spruce at a given site, hemlock typically experienced at least some level of defoliation. Relative budbreak synchrony with preferred host species may contribute to hemlock’s suitability as a host. This research is timely given the recent spruce budworm population increase in the region.

Climate change is having a disproportionate impact on the winter period, although little is known about the implications of shifts in extreme warming events. Changes in the intensity or duration of warm-ups, for instance, may significantly influence insects given their sensitivity to temperature fluctuations. Both ecological and economic implications may be particularly pertinent for pest species such as the spruce budworm (Choristoneura fumiferana [Clem.]), the most destructive defoliator of spruce-fir forests in North America. We subjected the spruce budworm to warming events (factorial combination of 4 warming intensities and 4 durations) during the early winter dormancy phase and measured impacts on survival, development time, body condition, and biochemistry. Results suggested that survival was minimally impacted by either treatment, and there were no effects on development. Body condition varied by sex, but was similarly unaffected by warming. However, both warming treatments influenced energy reserves measured at the end of the winter period; more intense warming reduced lipid concentrations, whereas glycogen concentrations were highest at intermediate treatment levels. Overall, our findings suggest that the impacts of early winter warming events had minimal impact on insect performance. Moreover, the ultimate consequences of shifts in metabolite concentrations likely depend on their contribution to insect energetics following the resumption of development post-dormancy.

Drought may initiate western spruce budworm outbreaks, but multi-year periods of increased moisture availability promote widespread defoliation

Background Stand-replacing wildfires and eastern spruce budworm outbreaks (Choristoneura fumiferana; SBW) are important disturbances in the boreal forest. SBW defoliation can affect fire behaviour by altering fuel loads and connectivity, thereby promoting the transition of low-activity surface fires into crown fires. However, little is known about how these altered fuels impact the effectiveness of fire suppression. Aims To assess key drivers of initial attack (IA) success in Ontario’s boreal forest and determine if incorporating SBW defoliation data improves predictive models. Methods We developed random forest models of fire containment using established predictors including fire weather, fire size at IA and region. We then evaluated if the inclusion of time since SBW defoliation improved model performance. Key results Fire size at IA was the most influential variable for determining whether a fire escaped containment. Contrary to our hypothesis, we did not find evidence that SBW defoliation greatly improved model performance. Conclusions and implications The size of the fire at IA was the most important variable in determining successful containment. Although budworm defoliation has been shown to affect other aspects of fire hazard, we were unable to identify an influence on IA success. Future work could benefit from focused investigation into how historical SBW defoliation affects fire behaviour.

Applying a centennial or millennial perspective to disturbance regimes permits an understanding of how these events have varied in the past in relation to climate change. Correctly interpreting this variability is crucial when preparing sustainable forest management practices for future warming. The eastern spruce budworm (Lepidoptera) is the most important biotic disturbance in the eastern Canadian boreal forest. Adult moths are covered by chitinous scales, and lepidopteran scale records in lake sediments have been analyzed to reconstruct Holocene spruce budworm populations. However, the magnitude of these scale accumulations has yet to be calibrated using an independent proxy. Here, we determine whether the impacts of spruce budworm defoliation are recorded by both sedimentary lepidopteran scale accumulations and tree-ring widths. Agreement between proxies was found at five of nine sites and strongest between the proportion of affected trees and scale accumulations while agreement in signal synchronicity was found at six of nine sites and strongest when comparing scale accumulations to a growth suppression index. A species-based composite chronology relying on white spruce produced the clearest outbreak record for both proxy records. Peak scale accumulations correlated well with smaller tree-ring widths, demonstrating that larger scale accumulations correspond to more severe defoliation events. Therefore, lepidopteran scales provide reliable records of spruce budworm abundance serving as a proxy record ameliorating our understanding of how budworm impacts have fluctuated at centennial and millennial time scales in the context of past climate change.

Abstract By causing widespread tree mortality, spruce budworm outbreaks significantly affect the structural and functional dynamics of northeastern boreal forests. This study examined the understory plant community’s response during the 2005–2020 outbreak in the northeastern boreal forest of Canada. Using a trait-based approach, we analyzed functional diversity, dominance, and resilience across fir-dominated, mixed, and spruce-dominated stands. Results revealed an increase in functional diversity and a decrease in Simpson dominance, particularly in fir-dominated and mixed stands. Changes in soil fertility, light availability, and tree basal area, driven by defoliation and tree mortality, were identified as key factors influencing understory vegetation. Trait shifts, especially the increase in shade-tolerant, mesic, and medium nutrient species and the decline of hydric water preferences and non-vascular growth form, reflected niche-based selection processes under altered conditions. However, the variability in trait distributions and species turnover across stand types also pointed to the role of neutral processes, such as stochastic colonization and dispersal limitation. Functional redundancy varied across stand types, with fir-dominated stands showing greater resilience, likely due to higher trait plasticity and pre-disturbance composition. These findings highlight the complex interplay between niche-driven and stochastic mechanisms in shaping understory dynamics and provide insights for sustainable forest management during and after spruce budworm outbreaks.

Norway spruce (Picea abies (L.) Karst.) has been widely planted beyond its natural range due to its fast growth rate and valuable wood. In Québec, over 200 million seedlings have been planted since 1964. Several of these plantations are now facing a new potential threat, i.e., spruce budworm (Choristoneura fumiferana (Clem.)) infestations. Despite contrasting results, Norway and white spruce (P. glauca [Moench] Voss) apparently sustain a similar degree of budworm defoliation. The main study objective is to quantify defoliation in Norway spruce caused by spruce budworm. We also evaluate the efficacy of Bacillus thuringiensis Berliner spp. kurstaki (Btk) in protecting this exotic host tree. Annual defoliation was assessed in plantations of Norway, white, and black spruce (P. mariana [Mill.] BSP) between 2018 and 2022 in the Bas-Saint-Laurent region. Additional surveys were conducted in Norway and white spruce plantations in the Gaspésie and Côte-Nord to evaluate Btk efficacy. We show that both species exhibit similar defoliation levels, though Norway spruce sometimes sustains greater damage (e.g., 35% vs. 10% in 2019). Btk formulations showed low efficacy in protecting Norway spruce foliage (≥49.32% defoliation in treated plantations). Further studies are needed to understand factors influencing Btk efficacy on this host.

ABSTRACTThe spruce budworm (Choristoneura fumiferana; SBW) is a periodically outbreaking forest insect pest that affects the boreal forests of North America through extensive defoliation and tree mortality. Causes of widespread spatial synchrony of SBW outbreaks remain a key question in the ecology and management of this species. While the Moran effect (correlated favourable environmental conditions) and density‐dependent dispersal (from epicentres of demographic explosions) have been proposed and supported as drivers of synchronised outbreaks, the relative contribution of long‐distance dispersal is still poorly understood. In this study, we use a novel approach to distinguish resident from migrant moths and to assign migrants to likely source clusters with the goal of better characterising regional dispersal. First, we characterise the genetic diversity and structure of resident SBW larvae and three phenologically separated groups of moths over one flight season using Genotyping‐by‐Sequencing. Then, using a novel machine learning approach, we assign putative migrants to their likely source populations. We hypothesised that migrant moths and resident larvae would be genetically distinct and could be assigned to source populations. Our findings revealed complex patterns of moth dispersal and population differentiation within a single season, including two spatially overlapping genetic clusters. We observed subtle but significant genetic differences between resident larvae and migrant moths, supporting the hypothesis that long‐distance dispersal contributes to outbreak dynamics and synchrony. These insights enhance our understanding of SBW population dynamics and suggest that effective management strategies, such as the Early Intervention Strategy (EIS), must account for the role of dispersal in mitigating the detrimental effects of major outbreaks.

Abstract We examine four historical population data sets, each spanning 28–46 years in length, from three locations in eastern Canada, showing that spruce budworm through the period 1930–1997 has exhibited eruptive (i.e., nonlinear) growth dynamics. According to our analysis of density‐dependent recruitment curves, any of several factors, both deterministic and stochastic, could be sufficient to trigger outbreaks. For this reason, eruption timing is inherently unpredictable. There is little evidence that spruce budworm population growth rates are near‐linear second‐order density‐dependent, as required by Moran's Theorem for cycle synchronization. Rather, growth rates are nonlinear first‐order density‐dependent, which makes population eruptions difficult to synchronize by any mechanism, including spatially autocorrelated weather effects and dispersal.

Abstract The concepts of compensation and additive mortality form the ecological basis for understanding animal population responses to exploitation by humans. In the context of pest management, compensation is a density‐dependent response that allows populations to offset control‐related mortality, often via increased survival or reinvasion. Additive mortality, in contrast, accrues when a population's compensatory capacity is insufficient to offset losses, resulting in a net reduction in population size or growth rate. These concepts are rarely considered in forest insect pest management, which tends to emphasise short‐term plant protection over long‐term population control. We used published life table data for a major native forest insect defoliator, the spruce budworm (Choristoneura fumiferana [Lepidoptera: Tortricidae]) to simulate the amount of additive mortality required to suppress an outbreak. Simulations also assessed how the failure to account for different compensatory responses could hinder successful control. Our results suggest that only relatively modest amounts of additive mortality (perhaps as low as approximately 8%–18%) may be needed to stop spruce budworm from outbreaking, with immigration being the strongest potential compensatory hindrance to outbreak suppression. Many of the compensatory responses that thwarted outbreak suppression in the past (e.g., low detection efficiency, immigration, indiscriminate killing of predators and parasitoids) have contemporary solutions that could increase additive mortality and thereby enhance the feasibility of population control strategies for native forest insect pests. Our results suggest that some native forest insect pests may require relatively little additive mortality to suppress outbreaks if compensation‐limiting strategies are used. Incorporating theoretical and strategic frameworks used in vertebrate population management could advance the development of native insect population control programmes.

Purpose of the study: The purpose of this study is to determine the numerical solution of the spruce caterpillar model using the Heun method and the Third Order Runge-Kutta method, as well as to analyze the errors associated with both methods. Methodology: The type of research used in this study is library research. In this study, the data will be analyzed numerically from the data entry stage, data processing and results. The results obtained are from the Heun programming method and the Runge iteration method that have been determined previously. Kutta-Order Three will produce data with the smallest error in the number of. Main Findings:The results of the study showed the solution of the Pinus Lice model for the initial values ​​of B(t₀) = 2, S(t₀) = 10 cm, E(t₀) = 2 cm, at t = 5 years, with h = 0.05. Using the Heun method, it was obtained that B ≈ 3, S = 14.9058 cm, and E = 1.0047 cm, while the Third Order Runge-Kutta method produced B ≈ 3, S = 14.9057 cm, and E = 1.0046 cm. The error calculation showed that the B error was smaller with the Heun method, while the S and E errors were smaller with the Third Order Runge-Kutta method. Novelty/Originality of this study: The novelty of this study lies in the comparative analysis of the errors of the Heun Method and the Third Order Runge-Kutta Method in modeling the dynamics of spruce budworm populations with specific biological parameters.

ABSTRACTAimPredicting climate driven species range shifts requires knowledge of mechanisms limiting species fitness under various climatic conditions. The traditional approach of modelling ranges of herbivorous insects by fitting environmental niche models to occurrence records is generally incapable of differentiating direct effects of climate on insect populations versus indirect effects acting on the range of their host plants. Here, we delimit the southern extent of a major forest defoliator, the spruce budworm, Choristoneura fumiferana, in North America, and investigate whether its range is limited directly by climate or indirectly by the range of its host trees.LocationEastern North America.MethodsWe use pheromone traps to survey populations; these are sensitive tools for detecting low‐density populations. We applied mechanistic models of the effect of temperature on C. fumiferana fitness to evaluate whether these patterns of occurrence can be explained by the direct effect of climate.ResultsWe found that this insect species is widely present through most of the southern distribution of host spruce and fir species, both in the upper Mississippi River valley and the central and southern Appalachian Mountains. The species was present in ca. 50% of spruce‐fir sites surveyed in the Appalachian Mountain region but was absent in the five most southerly sites where spruce and fir occur in eastern North America. Simulation using historical meteorological data indicated that all survey sites are climatically suitable for C. fumiferana.ConclusionsSimulation of climate effects, along with the species' absence from the most southern distribution of hosts, indicate that while the southern range limit of C. fumiferana is limited by host availability it is not directly constrained by climate. Furthermore, populations at the species' southern extent may be in a dynamic state of extinction and recolonisation, thus explaining at least the temporary absence of C. fumiferana from certain locations.

ABSTRACTFungal endophytes can alter plant resistance against herbivores by indirectly influencing plant secondary metabolism or through direct effects of their own metabolism. However, the role of fungal endophytes in conifer defences to insect herbivores remains largely unknown. We characterised the endophytic fungal communities and terpene concentrations of 30 white spruce families across two sites. We determined the effects of fungal endophytes on a defoliating insect, eastern spruce budworm, by testing the budworm responses to media amended with fungal endophytes or exposing them to their volatile organic compounds. We further examined whether the changes in the endophytic fungal communities and abundance alter the terpene concentrations of white spruce by inoculating seedlings with endophytic fungi. Terpene and fungal community compositions in mature trees varied among families and sites. The bioassays showed fungal endophytes can kill budworms or reduce their fitness due to the toxicity of fungal mycelium or volatile compounds. The inoculation experiments demonstrated that the changes in fungal communities and abundance can alter the terpene concentrations in seedlings. We developed a “Plant Partnership Hypothesis” to reflect the role of fungal endophytes in plant resistance to insect herbivores, demonstrating a co‐evolutionary relationship among fungal endophytes, tree defences, and insect herbivores.

Characterizing millennial and multi-millennial variability in disturbance regimes will be crucial in improving knowledge within the context of a changing climate and the development of sustainable forest management practices in the eastern Canadian mixed boreal forest. The major biotic and abiotic disturbances in the mixed boreal forest are the spruce budworm, and fire, respectively. The ability to reconstruct the variability of these disturbance agents under different climate conditions over long time periods will help elucidate the interaction between the agents and their dynamics in the mixed boreal forest. The objective of this observational study was to reconstruct the frequency of large spruce budworm population (LSBP) and fire disturbance events, and describe their interaction in the mixed boreal forest over the course of the Holocene within the context of changing vegetation and climatic conditions. Lepidopteran scales and sedimentary charcoal were used to reconstruct the local/extra-local disturbance history from lake sediment along with pollen to reconstruct changes in tree species composition. Spruce budworm and fire disturbance events were determined using the CharAnalysis software. Regime shifts in disturbance event frequencies along with changes in tree composition were detected using Sequential T-test Analysis of Regime Shifts. Spearman’s correlation was used to determine the relationship between spruce budworm and fire event frequencies. Over the course of the Holocene, 57 LSBP events and 76 fire events were detected with event frequencies ranging between 0.75-6.30 events*kyr-1 and 1.71-10.5 events*kyr-1 respectively. Nine and 7 regime shifts in LSBP and fire event frequencies were detected respectively, along with 2 shifts in vegetation. A significant negative correlation was observed between LSBP and fire event frequencies from 6000-1000 BP suggestive of a linked disturbance interaction. The first local lake sediment multi-millennial disturbance regime reconstruction comprising both spruce budworm and fire in the mixed forest revealed a very peculiar oscillation in disturbance event frequencies. Each disturbance seemingly establishes a positive disturbance-vegetation feedback that favors itself and inhibits the occurrence of the other. Further, rapid climate change events may act as a key trigger in establishing the respective feedback loops resulting in the observed disturbance event frequency oscillation.

Climate change is redefining the dynamics of forest ecosystems globally, particularly through its impact on forest pest populations such as the spruce budworm (SBW, Choristoneura fumiferana [Clem.]), a major defoliator in North American boreal forests. This study investigates the shifts in the population dynamics of spruce budworm across its range in response to recent climate change. We used a process-based, temperature-dependent ecophysiological model combined with the ERA5 reanalysis to assess changes in SBW phenology, reproduction rate, winter survival and population growth rates from 1950 to 2022 across North America. Our findings demonstrate a pronounced northward expansion of suitable climate conditions for SBW, accompanied by earlier phenological events and increased reproduction rates in northern regions. Conversely, the southern parts of its range are experiencing increased winter mortality due to warmer temperatures. This study highlights the significant impact of elevated temperatures, particularly during critical developmental windows such as spring and summer, which are pivotal for spruce budworm survival and reproduction. Additionally, our results reveal that the observed shifts in pest dynamics are more strongly driven by climate change than by changes in landscape composition and structure. We estimated that suitable growth rates have shifted northward by over 68 km on average, but this shift reached more than 200 km in the easternmost portions of its range. Climate-induced shift in suitable conditions for SBW underscores the need for adaptive forest management strategies that consider the rapid ecological changes and the potential for increased forest vulnerability due to climatic and biotic stressors. This study provides vital insights that can inform adaptive management ensuring the sustainability of forest ecosystems in the face of ongoing climate change.

The spruce budworm, Choristoneura fumiferana Clem. (Lepidoptera: Tortricidae), is the most severe defoliator of balsam fir (Abies balsamea) in eastern Canada and northeast U.S.A. A large budworm outbreak is currently underway in the province of Québec, with 10.5 million hectares defoliated in 2023, up 14.1% from the year before. Populations of budworms are controlled using management guidelines of Forest Protection Strategy (FPS); the approach aims at killing defoliating larvae with aerial application of microbial insecticide, with the objective to limit defoliation and prevent tree mortality. The decision to treat/not treat a given forest block is based, in part, on local density of overwintering second instars (L2) collected at ca. 600 sampling points each year across the entire province at a cost of CAD 350 for three branches per site; the threshold for FPS (TFPS) corresponds to 20 L2/branch. Aerial defoliation maps also guide management decisions because FPS generally target areas within or in the vicinity of defoliated forest stands. Budworm abundance rapidly declines with distance to aerial defoliation (in km), to the extent that larval density rarely attains TFPS outside the core range of FPS (>15 km from defoliation). A cost-effective monitoring approach is proposed whereby forest blocks outside the core range of FPS are sampled every second year (as opposed to every year), representing a potential economy of CAD 40,000 annually.

Spruce budworm (SBW; Choristoneura fumiferana Clem.) outbreaks are an important natural disturbance in North America, killing trees over millions of hectares. We related 11 years of SBW defoliation in 87 plots in Gaspé Peninsula, Québec, to 23 stand, site, and climate variables. Defoliation was consistently ordered among host species: balsam fir > white spruce > black spruce. Within the relatively small 200 km2 study area, cluster analyses resulted in four and 10 clusters for balsam fir cumulative and current defoliation, respectively; variation in cumulative defoliation converged over 11 years. Current defoliation was significantly spatially autocorrelated among plots within stands, but autocorrelation weakened at distances >2500 m. Cumulative defoliation was significantly related to insecticide spraying, minimum and maximum summer temperature, and interactions between SBW larvae per branch versus hardwood and white spruce basal area. Tree species, insecticide spraying, and number of defoliating SBW larvae were the main determinants of defoliation. Results showed much higher local spatial variability in current defoliation patterns than previous studies, but over the course of an outbreak, cumulative defoliation patterns converged. Cumulative defoliation patterns similar to these, assigned based on local defoliation severity, can be input into defoliation-based growth models to predict impacts on growth and survival.