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Population dynamics of the spruce budworm Choristoneura fumiferana
... Despite these insights, the extent to which outbreaks are initiated, as opposed to accelerated or supplemented, by immigration of egg-bearing moths, remains debatable [15,16]. Further, we are uncertain whether the periodic nature and large-scale spatial synchrony of spruce budworm outbreaks are dominated by common, intrinsic responses to widespread ecological conditions through the Moran effect [4,16] or if dispersal plays a distinct role in these large-scale features of their population ecology [17]. This has significant implications for management of outbreaks. ...
... His re-analysis of the Green River data concluded migration of gravid moths was a source of external noise that perturbs the details of the intrinsic temporal pattern during an outbreak cycle, but neither triggers nor causes spatial synchrony of outbreaks. In contrast, a theoretical argument was proposed that migration could be a strong source of population synchrony in spruce budworm outbreaks [17]. ...
... As extensive, annual records of defoliation of outbreaks became available, the research approach on dispersal shifted to empirical, geo-statistical techniques to infer patterns of direction and rate of 'spread' in defoliation that might reveal the ecological structure of moth dispersal [4,10,48]. Skeptical of these empirical estimates, theoretical models simplified the difficulty by assigning a random effect with respect to population rates of change [17,49,50]. ...
Egg recruitment quantifies the relative importance of realized fecundity and migration rates in the population dynamics of highly mobile insects. We develop here a formal context upon which to base the measurement and interpretation of egg recruitment in population dynamics of eastern and western spruce budworms, two geographically separated species that share a very similar ecology. Under most circumstances, per capita egg recruitment rates in these budworms are higher in low-density populations and lower in high-density populations, relative to the regional mean: Low-density populations are nearly always migration sinks for gravid moths, and dense populations nearly always sources. The slope of this relationship, measured on a log scale, is negatively correlated with migration rate, and ranges between 0 and −1. The steeper the slope, the more marked net migration. Using our western spruce budworm observations, we found strong evidence of density-dependent emigration in budworms, so migration is not simply a random perturbation in the lagged, density-dependent stochastic process leading to budworm outbreaks. It is itself statistically and biologically density-dependent. Therefore, moth migration is a synchronizing factor and a spread mechanism that is essential to understanding the development and expansion of spruce budworm outbreaks at regional scales in the boreal forests of North America.
... Its signifi cance is refl ected in an extensive history of research to support modeling and management activities (e.g., Morris 1963a ;Greenbank et al. 1980 ;Royama 1984 ;Sanders et al. 1985 ). The most recent review of spruce budworm modeling was by Régnière and Lysyk ( 1995 , but see also Cooke et al. 2007 ). Since 1995 (when spruce budworm became primarily endemic), 1103 papers were published with the keyword C. fumiferana (Web of Science, accessed December 2014), indicating a strong need for new synthesis. ...
... For the latter, dispersal acts as successive triggering of eruptions through a spatial chain reaction, similar to a "domino-effect" (Clark et al. 1979 ) also known as a traveling wave . This so-called "epicenter hypothesis" fell out of favor with the advent of the harmonic oscillation paradigm that emphasized the Moran effect ( Royama 1984 ). Régnière and Lysyk ( 1995 ) later proposed a high-resolution spatial model that illustrated how dispersal may act as a significantly more robust synchronization process that forces independently oscillating systems to converge to a common cycling frequency even if their intrinsic cycling frequencies differ (as illustrated in Fig. 5.4 b). Régnière and Lysyk ( 1995 ) fundamentally changed the nature of the discussion of cycle synchronization to place equal emphasis on weather and dispersal as potential synchronizing forces (e.g., Peltonen et al. 2002 ). ...
... Régnière and Lysyk ( 1995 ) later proposed a high-resolution spatial model that illustrated how dispersal may act as a significantly more robust synchronization process that forces independently oscillating systems to converge to a common cycling frequency even if their intrinsic cycling frequencies differ (as illustrated in Fig. 5.4 b). Régnière and Lysyk ( 1995 ) fundamentally changed the nature of the discussion of cycle synchronization to place equal emphasis on weather and dispersal as potential synchronizing forces (e.g., Peltonen et al. 2002 ). More recently, Régnière et al. ( 2013 ) empirically documented an " Allee effect" ( Allee 1931 ) within low-density budworm populations-not in the form of a predator pit as proposed by Holling and colleagues (Ludwig et al. 1978 ), but due to low mate-fi nding success at very low densities. ...
Forest landscape disturbances are a global phenomenon. Simulation models are an important tool in understanding these broad scale processes and exploring their effects on forest ecosystems. This book contains a collection of insights from a group of ecologists who address a variety of processes: physical disturbances such as drought, wind, and fire; biological disturbances such as defoliating insects and bark beetles; anthropogenic influences; interactions among disturbances; effects of climate change on disturbances; and the recovery of forest landscapes from disturbances—all from a simulation modeling perspective. These discussions and examples offer a broad synopsis of the state of this rapidly evolving subject.
... The interpretation of the budworm-forest relationship was, however, neither crystal clear nor without debate. Academic differences of opinion that became prominent during the 1980s (Regniere and Lysyk 1995) over budworm population dynamics show that the spray program was not based on as solid a scientific foundation as may have been assumed initially (for example by Flieger, 1953;or Balch, 1952). Ironically, the different interpretations were actually derived from the same data, which were collected from studies in an area of New Brunswick called the Kedgewick Check area. ...
... Pressure has continually been arising to more clearly determine the underlying processes in budworm-forest dynamics (Regniere and Lysyk 1995). This is not surprising considering the proposed future of forest management planning in New Brunswick-a desire for increased control over timber yield and budworm outbreaks through a computerized decision support system. ...
... As well, the Blais model places more importance on the role of forest maturation: when the forest matures, if it is of the spruce-fir variety, a budworm outbreak is extremely likely. Under this line of thinking, budworm outbreaks are avoidable because forest composition can be altered to reduce the likelihood of an outbreak(Regniere and Lysyk 1995). Under the Roy ama model, the predator-prey scenario is more important, where there are gradual build-ups and declines of budworm populations in relation to predator numbers, parasite levels, and budworm food supply. ...
... At the population level, this should result in the greatest emigration of gravid moths at intermediate densities where moths are abundant and well-nourished. Consequently, a net exchange of moths between locations would be more difficult to detect during highly synchronized outbreaks when populations are at similar densities within a region (Régnière and Lysyk 1995), or where the number of locations sampled is few (Royama 1984). Recent evidence with more spatial elements and representing a greater range of the outbreak phase, including this study, show these exchanges are common and play an important role in the dynamics of outbreaks (Régnière et al. 2012, Régnière andNealis 2019b). ...
... In Royama's view, however, the food web was restricted to the spruce budworm and its natural enemies. We see no theoretical inconsistency with including the resource as a food web element, conditioning the direction and amplitude of population fluctuations (Régnière and Lysyk 1995). Indeed, the inclusion of bottom-up processes reduces uncertainties impeding reconciliation of competing paradigms (Pureswaran et al. 2016). ...
We sampled outbreak populations of western spruce budworm, Choristoneura occidentalis (Lepidoptera: Tortricidae), between 1997 and 2016 in Douglas‐fir forests in the interior of British Columbia, Canada. Annual rates of change in population densities were correlated with generation survival, modulated by egg recruitment via dispersal of moths. Most temporal variation in generation survival was the result of variation in survival of small, non‐feeding larval stages. Survival rates of feeding larval and pupal stages determined the magnitude of generation survival but contributed little to temporal trend. Survival of small larvae was the product of density‐related losses of larvae dispersing to and from hibernation sites and weather‐related stress caused by warm temperatures in late summer and early spring. Overwinter survival improved in cooler years and at higher elevations. Survival of feeding larvae and pupae was associated with mortality by natural enemies, in particular a few species of dominant, ubiquitous endoparasitoids, and an episodic, virulent baculovirus. Recruitment of eggs indicated emigration of gravid moths independent of defoliation but proportional to density of local moths. During periods of increasing and decreasing population densities over a large outbreak area, a marked differential in adult densities among locations resulted in greater than expected per‐capita egg recruitment to areas of relatively low density and vice versa, indicative of net exchange of gravid moths from high‐ to low‐density populations. The result was homogenization of egg densities among locations, apparent synchrony of the outbreak at the large scale, and extended duration of the outbreak at some locations, despite declining generation survival. These results are compared with field studies of the closely related spruce budworm, Choristoneura fumiferana. We suggest our interpretation applies more generally and outbreaks of conifer‐feeding budworms are maintained for several years by compensatory survival of sequential life‐history stages and spatio‐temporal smoothing of densities via dispersal of gravid moths. Parasitoids and pathogens can cause sudden declines in densities, but populations inevitably wane as a result of cumulative degradation of their resource. The interaction and temporal sequence of bottom‐up and top‐down factors explain the similarities and differences in outbreak characteristics within‐ and among‐budworm species.
... A core tenet of the oscillatory hypothesis is that the cycles occur over relatively large areas, with disparate pockets rising more or less synchronously across the region [19][20][21]. Moth dispersal, although frequent and often conspicuous during outbreaks [22][23][24], does not drive outbreak spread per se but rather acts in tandem with weather to draw regional outbreak trends into closer synchrony [20,25]. ...
... Hypothetical recruitment curves for spruce budworm showing how population density and forest type might interact to influence forest susceptibility and action priorities (adapted from[19]). ...
The spruce budworm, Choristoneura fumiferana, Clem., is the most significant defoliating pest of boreal balsam fir (Abies balsamea (L.) Mill.) and spruce (Picea sp.) in North America. Historically, spruce budworm outbreaks have been managed via a reactive, foliage protection approach focused on keeping trees alive rather than stopping the outbreak. However, recent theoretical and technical advances have renewed interest in proactive population control to reduce outbreak spread and magnitude, i.e., the Early Intervention Strategy (EIS). In essence, EIS is an area-wide management program premised on detecting and controlling rising spruce budworm populations (hotspots) along the leading edge of an outbreak. In this article, we lay out the conceptual framework for EIS, including all of the core components needed for such a program to be viable. We outline the competing hypotheses of spruce budworm population dynamics and discuss their implications for how we manage outbreaks. We also discuss the practical needs for such a program to be successful (e.g., hotspot monitoring, population control, and cost-benefit analyses), as well as the importance of proactive communications with stakeholders.
... SBW population dynamics are driven by several processes such as competition for food, disease propagation, predation, parasitism, and migration (Régnière 1984;Royama 1984;Régnière and Nealis 2007). Some causal mechanisms have been proposed to explain large-scale synchrony, such as temporal variation in apparent fecundity and natural enemies (Royama 1984) or moth dispersal (Régnière and Lysyk 1995;Royama et al. 2005), but these hypotheses were not examined over large spatial scales, and connections between year-to-year weather fluctuations and specific SBW population processes remain little explored. ...
... Short-frequency oscillations in late epidemic, collapsing populations were also attributed to weather, but through an interaction with the natural enemies that build up when host population density increases, a top-down trophic process (Royama 1984;Régnière and Nealis 2007). Year-to-year variations in the emigration/migration ratio also has been mentioned as a potential explanation (Régnière and Lysyk 1995;Royama et al. 2005). These other factors might explain why annual oscillations were relatively poorly predicted by the selected variables during the early portion of the time series (e.g., 1986-1995) (Fig. 4), which coincides with the collapse of the previous outbreak cycle (Morin et al. 2007). ...
Understanding the mechanisms that cause large-scale synchrony in insect population dynamics might yield key insights for predicting potential outbreak occurrence. Here, we evaluated which environmental factors best explain synchronous population fluctuations in the spruce budworm (Choristoneura fumiferana Clem.) (SBW), a major defoliator of coniferous forests in North America. SBW population levels were assessed with pheromone traps during the 1986–2014 period across a 625 000 km² territory located in the province of Québec (Canada). The populations were characterized by abundance fluctuations that were often synchronized across the whole study area. Interannual population fluctuations were correlated with host tree cone production (a source of food) and high May temperatures, suggesting that synchrony was influenced by food availability and phenological mismatch during shoot development. Cone production was itself correlated with low precipitation during the previous summer. This study indicates that bottom-up trophic factors can drive spatiotemporal synchrony in insect populations and contribute to explain important and sustained population increases during some years. We also suggest that several biological processes, all influenced by weather, are likely to interact to explain population synchrony during the different phases of the approximately 35 year SBW outbreak cycle, complicating the prediction of climate change effects on this insect.
... Insect dispersal can link independently oscillating populations, synchronizing the dynamics of spatially disjunct populations (Kaitala and Ranta 1998, Williams and Liebhold 2000, Peltonen et al. 2002, Tobin and Bjornstad 2005). While evidence supports the idea that regional synchrony and other spatiotemporal outbreak patterns may be caused by dispersal of insects from areas of high to low abundance (Royama 1984, Williams and Liebhold 2000, Johnson et al. 2004, Nealis and Régnière 2004a, Royama et al. 2005), the role of dispersal in explaining these patterns remains unclear due to the difficulty of quantifying dispersal and its impacts on populations (Royama 1984, Royama et al. 2005). A greater understanding of the processes underlying dispersal may improve our ability to predict outbreak spread and focus effective intervention measures (Greenbank et al. 1980, Sharov and Liebhold 1998, Johnson et al. 2006). ...
... Moths can easily disperse 20 km and the maximum recorded dispersal distance is 450 km (Greenbank et al. 1980). In addition, evidence indicates that emigration rates increase with increasing levels of defoliation (Royama 1984, Nealis and Régnière 2004a). ...
Dispersal has been proposed as an important mechanism in the broad-scale synchronisation of insect outbreaks by linking spatially disjunct populations. Evidence suggests that dispersal is influenced by landscape structure, phenology, temperature, and air currents; however, the details remain unclear due to the difficulty of quantifying dispersal. In this study, we used data on the abundance and distribution of spruce budworm Choristoneura fumiferana larvae (potential dispersers) and adult male moths (dispersers) to make inference on the effects of air currents and host-species abundance on dispersal. Hierarchical-Bayesian and inverse modeling was used to explore 4 dispersal models: 1) isotropic dispersal; 2) directional-dispersal; 3) directional-and-host-species dispersal; and 4) host-species dispersal. Despite their strong dependence on balsam fir Abies balsamea and spruce species Picea spp., the mapped basal area of these host species did not influence the pattern of dispersed moths. The model that best fit the data was the directional-dispersal model, which showed that the prevailing dispersal direction was from the northwest (328°). We infer that the strong pattern of directional dispersal was due to a prevailing wind from the same direction. Our interpretation was corroborated by independent wind data during the period of active adult male budworm flight, particularly in the region with high larval abundance. Our results indicate that there was a relatively high probability of individuals flying at least 48 km with the wind where larvae abundance at source locations was also high. Such findings emphasize the importance of long-distance dispersal on spatial distribution of adult male spruce budworms. Insight into the population-level consequences of such dispersal patterns requires additional research.
... In this theoretical context, outbreaks develop first in forests where the rate of increase of budworm populations exceeds predation rates and spread from these 'epicentres' by migration of moths. Outbreaks remain high until food depletion forces a collapse (see discussion in Régnière & Lysyk, 1995 ). Royama (1977Royama ( , 1981aRoyama ( , b, 2001 ) developed a rigorous set of analytical techniques in population dynamics and applied them to these same Green River data ( Morris, 1963 ). ...
... The female flight period was determined from captures in Malaise traps deployed each year in each study site (Black Sturgeon Lake data courtesy of the late Dr C. J. Sanders). In years where no female capture data were available (Black Sturgeon Lake: 1983, 1995-1997Gargantua: 1985;Armagh: 1986), Régnière's (1987Régnière's ( , 1990) Spruce Budworm Phenology model was used to calculate the expected female flight period based on daily minimum and maximum air temperature records. ...
Abstract 1. Stage-specific survival and recruitment of spruce budworm were measured by frequent sampling of foliage in four outbreak populations over a 15-year period in Ontario and Quebec, Canada.
2. Patterns of change in population density during the outbreak collapse phase were closely linked to changes in survival of the late immature stages, and were determined largely by the impact of natural enemies.
3. Host-plant feedback also contributed significantly to survival patterns throughout the outbreak: annual defoliation influenced survival of fourth and fifth instars and fecundity while cumulative defoliation influenced survival of the very early larval stages (first and second) via impacts on stand condition.
4. Inclusion of this host-plant feedback reveals spruce budworm population dynamics as a function of density-related trophic interactions that vary in their order and strength of influence over time. This view re-introduces the importance of forest interactions as a component of dynamics of the spruce budworm.
... SBW emigration, the departure of adults from a given location, is best understood as a density-dependent process resulting from crowding and deteriorating habitat conditions (including ongoing defoliation), both of which can affect the expected availability of food resources for the next generation of SBW during the outbreak (Régnière and Nealis, 2007;2019;Van Hezewijk et al., 2018;Rhainds, 2020). Dispersal of adult SBW is also thought to support the redistribution and mixing of subpopulations and to promote both spatial synchrony in SBW outbreaks over large areas (Régnière and Lysyk, 1995;Williams and Liebhold, 2000;Peltonen et al., 2002;Nenzén et al., 2018;Régnière and Nealis, 2019) and the endemic persistence of SBW populations between outbreak events (Bouchard and Auger, 2014;Bouchard et al., 2018b). Theoretically, several years of flight-supporting weather can help local populations overcome Allee effects (Nealis and Régnière, 2004;Régnière et al., 2019a), including potential mating failure (Contarini et al., 2009;Tobin et al., 2009;Rhainds, 2010;Régnière et al., 2013). ...
Long-term studies of insect populations in the North American boreal forest have shown the vital importance of long-distance dispersal to the maintenance and expansion of insect outbreaks. In this work, we extend several concepts established previously in an empirically-based dispersal flight model with recent work on the physiology and behavior of the adult eastern spruce budworm (SBW) moth, Choristoneura fumiferana (Clemens). An outbreak of defoliating SBW in Quebec, ongoing since the mid-2000s, already covers millions of hectares of forests in eastern Canada and threatens to spread into neighboring areas through annual summertime episodes of long-distance dispersal. Such flight events in favorable conditions frequently include billions of SBW moths dispersing in the warm atmospheric boundary layer, typically starting around sunset and often lasting through several hours of wind-driven transport over hundreds of kilometers. Successful SBW dispersal to possibly distant host forest areas depends acutely on the weather. Here we describe the components and results of SBW–pyATM, an open-source individual-based modeling framework developed in Python for the simulation of these weather-driven SBW dispersal events. Using seasonal SBW phenology results from BioSIM at known outbreak locations and high-resolution Weather Research and Forecasting (WRF) model output, we focus on modeling dispersal flights over two successive nights in July 2013 in southern Quebec. Our flight model closely reproduces the SBW spatial patterns and motions observed by weather surveillance radar over the St. Lawrence estuary. With SBW–pyATM we can estimate landing locations for both male and female SBW and the resulting spatial patterns of egg distribution, allowing us eventually to forecast future larval defoliation activity in new locations where immigration could help overcome local limitations on SBW populations. This information could then support forest management decisions where SBW outbreaks threaten valuable resources.
... Recent observations of the density dependence of SBW survival in the larval and pupal stages in rising outbreak populations [14] support Morris' [7] "predator pit" hypothesis. Prey-switching would lead to less pronounced outbreak cycles, with a somewhat higher intrinsic frequency in more diverse meridional forests, a conclusion that is in accordance with the findings of [49], as discussed by [50]. ...
The impact of avian predation on a declining population of the spruce budworm, Choristoneura fumifereana (Clem.), was measured using single-tree exclosure cages in a mature stand of balsam fir, Abies balsamea (L.), and white spruce, Picea glauca (Moench.) Voss. Bird population censuses and observations of foraging and nest-feeding activity were also made to determine the response of budworm-linked warblers to decreasing food availability. Seasonal patterns of foraging. as well as foraging success in the declining prey population was compared to similar information from birds observed in another stand where the spruce budworm population was rising. Avian predation was an important source of mortality between the 4th instar and moth emergence in the declining outbreak population. Mortality by predation increased from negligible to over 98% as budworm density dropped from 100 to <1 larva/kg of host foliage, over 3 years. Calculations based on nest-feeding activity and basic metabolic demands support these observed rates. Seasonal and yearly differences in predation rates observed between the two host-tree species correspond to equivalent shifts in bird foraging behavior in response to dropping insect density. In particular, a preference for searching on white spruce disappeared, although budworm-linked birds remained more efficient at finding food on this plant. The ability to change foraging behavior as prey density dropped differed between bird species.
... Historically, there are two interpretations of how SBW moth dispersal contributes to outbreaks. One interpretation favoured until recently is that moth dispersal functions mainly to synchronize outbreak cycles over large geographic areas (Régnière and Lysyk, 1995;Anderson and Sturtevant, 2011;Larroque et al., 2019;Régnière and Nealis, 2019). More recent evidence based in part on life table data suggests that dispersal may play a more complex role in outbreaks than previously thought, helping to synchronize outbreaks but also driving outbreak spread to outlying areas that might not otherwise reach outbreak densities . ...
Outbreaking insects often undergo rapid population growth synchronously in multiple locations separated by large distances. Dispersal may play an essential role in synchronizing outbreaks over large geographic areas but its role in outbreak spread remains unclear. In our study, we used population genetics to assess how much dispersal contributes to the spread of a major forest pest, the spruce budworm (Choristoneura fumiferana Clemens). Using spatial analyses of genotypic data, we determined the extent to which dispersing individuals from epidemic populations in Quebec leave their genetic signature in adjacent, rising populations in New Brunswick and Maine. A lack of genetic differentiation between endemic and epidemic populations would indicate that outbreak spread depends on effective dispersal from epidemic to endemic regions. In contrast, en-demic populations that are distinct from epidemic population would suggest that dispersal plays a reduced role in fomenting outbreak rise. An intermediate possibility with differentiation between epidemic and endemic population as a continuous spatial gradient might suggest a temporal lag in genetic differentiation that may take the form of a genetic travelling wave. PCA, cluster analyses, isolation by distance, and sPCA were used to characterize spatial genomic variation using 300 SBW larvae sampled in 2015 and genotyped at thousands of genome-wide SNPs. We found support for a genetic travelling wave pattern, matching the SBW density pattern. These results indicate that dispersal may trigger the transition of populations from the endemic to the epidemic state and thereby drive outbreak spread. By supplementing persisting endemic populations, migrants may drive populations to beyond levels that can be controlled by local biotic constraints. These results support the underlying justification of ongoing outbreak containment efforts in Atlantic Canada (i.e., the Early Intervention Strategy), although continued sampling as the outbreak progresses is needed confirm the temporal stability of the observed patterns.
... Synchrony in insect population fluctuations often occurs at the landscape scale, and such large-scale synchronous increases in insect population density during one year and persistence over subsequent years can lead to rising insect populations and outbreak initiation (Royama et al., 2005;Liebhold et al., 2012;Bouchard et al., 2018). Several underlying mechanisms of spatial synchrony of insect outbreaks and local population fluctuations have been proposed, including regional stochasticity, dispersal, and trophic interactions (Régnière and Lysyk, 1995;Myers, 1998;Williams and Liebhold, 2000;Liebhold et al., 2012). Research on various insects has shown how seasonal drought can accelerate outbreak initiation (e.g., spruce beetle (Dendroctonus rufipennis); Hart et al., 2017), stands with more abundant host species attract insect initial attack (e.g., mountain pine beetle (Dendroctonus ponderosae Hopkins); Klutsch et al., 2009), and how parasitism produces regional insect pest population patterns during the outbreak onset phase (e.g., western tussock moth (Orgyia vetusta); Maron et al., 2001). ...
We determined effects of local spruce budworm (Choristoneura fumiferana Clem.; SBW) population level, proximity to sites with high SBW populations, insecticide spray, and environmental variables on SBW populations from 2014 to 2018, the outbreak initiation period in northern New Brunswick, Canada. SBW second instar larvae (L2) per branch data collected at 1100–2000 sample points per year were used to create annual interpolated population rasters. Fishnet sample points extracted from these rasters were overlaid with georeferenced layers of 46 possible predictor variables including forest composition, climate, topography, site quality, and insecticide treatment. Results showed that local SBW population in the previous year, proximity to sites with high SBW populations, and early spring climate were consistently the most important predictors over the 5 study years. Simultaneous autoregressive models were used to address spatial autocorrelation when forecasting the SBW L2 population, and a linear mixed effects model was fit to aggregate data for 2015–2018. The models reduced spatial dependence in the residuals, and explained 68–79% of variance in annual L2 levels and 53% of variance over the 4 years combined. Sensitivity analysis showed that locations with 5–10 more SBW L2 per branch than observed values, or 20–40 km closer to high population sites in the previous year could have up to 24 more L2 in the current year. Cumulative degree days in April helped to estimate the upper and lower bounds of the population. Expansion and retraction of SBW outbreak initiation were mathematically described. Understanding which variables influence SBW outbreak initiation and population level assists in design of small area target-specific insecticide spray applications and helps focus SBW L2 sampling on predicted outbreak hot spots.
... It reaches outbreak levels in more or less regular intervals of 30-40 years (Blais 1965;Morin 1994). Several factors influencing its population dynamics have been identified, one of which are natural enemies, in particular parasitoids (Royama 1984;Régnière et al. 1995;Régnière and Nealis 2007). Spruce budworm parasitoid communities change in terms of species occurrence and relative importance as mortality factors depending on their host's population densities (Eveleigh et al. 2007). ...
Tranosema rostrale (Brischke) (Hymenoptera: Ichneumonidae) is an important parasitoid of low-density spruce budworm Choristoneura fumiferana (Clemens) (Lepidoptera: Tortricidae) populations. To investigate the effectiveness of this parasitoid in attacking low-density spruce budworm populations, we conducted a detailed laboratory study on its reproductive biology and behavior including mating behavior, potential fecundity, longevity, host searching, and oviposition behavior. We found that the occurrence of mating increases with the number of males present in the cage but that it drastically decreases when females mated previously. Females may mate multiple times with different males in one breeding session and mating duration is significantly longer when a male mates the second time with the same female. Dissections of T. rostrale’s oviducts showed that it is a synovigenic species emerging with about 17 % of its lifetime egg load and develops most of its eggs in the first three days after emergence at 20 °C. Sugar, but not pollen, significantly increased the insect’s longevity compared to water. Spruce budworm larvae, silk from larvae and damaged balsam fir foliage triggered probing in T. rostrale females significantly more often than larval feces. The sequence of the parasitoid’s behaviors leading to successful attack is described as antennation, probing, insertion of the ovipositor, oviposition, and subsequent disinterest or resuming of the sequence. Defense mechanisms of the host larva such as vigorous movements, biting, and/or regurgitation and behavioral countermeasures by T. rostrale are described in detail.
... This univoltine insect undergoes six larval instars per year and feeds on balsam fir Abies balsamea (L.) Miller (Pinaceae) and several spruce species (Greenbank 1963). Parasitism plays an important role as a mortality factor in the population dynamics of spruce budworm (Royama 1984, Régnière and Lysyk 1995, Régnière and Nealis 2007. The community composition of parasitoid species changes with the population density of spruce budworm (Eveleigh et al. 2007). ...
The seasonal pattern of parasitism by a parasitoid can be influenced by many factors, such as interspecific competition and host instar preference. We conducted field and laboratory experiments to describe the seasonal pattern of parasitism of spruce budworm Choristoneura fumiferana (Clemens) larvae by Tranosema rostrale (Brischke), and to investigate whether this pattern can be explained by interaction with other parasitoid species, or by host instar preference. Larval survival, developmental time, sex ratio, and adult size of T. rostrale developing in different host instars were also measured to further assess the potential importance of host instar on parasitoid life history. Parasitism by T. rostrale increased over the season, reaching the highest rate during the fourth-instar larva, and then decreased again until the sixth-instar. At the same time, parasitism by another parasitoid, Elachertus cacoeciae (Howard), increased over the season, and multiparasitism with T. rostrale suggests potential competition between these two parasitoids. Tranosema rostrale showed no host instar preference when third- to sixth-instar larvae were exposed simultaneously in a manipulative field experiment. The proportion of females emerging from spruce budworm larvae increased over the season; however, no difference in sex ratio was observed in the manipulative field experiment. Only male pupal development time and adult size were marginally increased in fifth-instar spruce budworm larvae. We conclude that T. rostrale’s seasonal phenology or competition with E. cacoeciae, but not host instar preference, were possibly responsible for the observed seasonal pattern of parasitism.
... Susceptibility of forest stands to some pathogens can be altered by modifying species composition. Severity and frequency of spruce budworm outbreaks can be reduced by managing forest tree species composition to increase the hardwood component or by reducing the proportion of balsam fir (Blais 1983, Hardy et al. 1983, Régnière and Lysak 1995, MacLean 1996, Su et al. 1996. Similarly, damage by Armillaria root disease can be decreased by favouring more resistant species in planting and early stand density management operations (Hagle andShaw 1991, Morrison andMallett 1996). ...
... In other words, if these so-called epicenters are also the initial points of outbreak collapse, then they may not function as mere sources of outbreaks. If they are also a source of predators, then they may be acting as relay points in what is a loosely coupled network of oscillating predator and prey populations (Régnière and Lysyk, 1995). ...
Recurrent outbreaks of forest insect populations have been discussed extensively in the ecological literature but rarely from the perspective of disturbance ecology. The reason lies, in part, in the traditional focus of quantitative animal ecologists on the species of interest contrasted with the traditional emphasis of plant ecologists on whole communities. This chapter presents the argument that herbivorous insects constitute a class of forest disturbance that is distinct from fire, windthrow, or flooding. Insect outbreaks tend to be spatially synchronized and temporally periodic (periods of high impact followed by periods of low impact over extensive areas), and insect outbreaks are predictably selective, and therefore result in different legacies than do abiotic disturbances. The chapter suggests that recognition of the distinct nature of disturbances caused by insect outbreaks may shed light on the analysis of other disturbances that are, by nature, aperiodic and difficult to forecast. It reviews the dynamics of several outbreak systems. To study these relationships, the chapter uses a comparative, process-oriented approach and illustrates the approach by comparing the ecologies of four species of insect defoliators.
... The Moran effect has been suggested as an important factor contributing to the regional outbreak synchrony for a wide range of Lepidopteran species (Ranta et al. 1997;Bjørnstad et al. 1999;Myers and Cory 2013), including species such as the Douglas-fir tussock moth (Orgyia pseudotsugata [McDunnough]) with wingless females incapable of flight. Nonetheless, adult dispersal is another process by which independently oscillating populations can become synchronized (Régnière and Lysyk 1995;Kaitala and Ranta 1998;Williams and Liebhold 2000). Imperfect synchronization can manifest as "epicentres" spreading to adjacent areas (Berryman 1987), or as "travelling waves" where outbreaks spread across large geographic areas (e.g., Bjørnstad et al. 2002;Tenow et al. 2013). ...
... Royama (1984) a discrédité cette hypothèse de déclenchement climatique chez la tbe. Cependant, l'influence du climat sur les fluctuations annuelles de la fécondité apparente, reliée surtout à la migration des papillons, pourrait servir de mécanisme de synchronisation (Royama, 1984 ;Régnière et Lysyk, 1995). ...
Résumé
La compilation de toutes les chronologies publiées sur les épidémies de la tordeuse des bourgeons de l'épinette établies à partir de la largeur des cernes de croissance des arbres hôtes a été utilisée pour étudier l'évolution spatiale et temporelle des épidémies survenues au cours du XX e siècle dans l'est de l'Amérique du Nord et particulièrement dans les régions nordiques. L'étude inclue 19 chronologies comprenant 1 257 arbres échantillonnés. Les données sont nombreuses pour certaines régions du Québec mais fragmentaires dans d'autres régions comme l'est et le centre de l'Ontario. On observe un remarquable synchronisme des plus faibles croissances radiales associées à la défoliation entre les différentes régions représentées pour les trois épidémies enregistrées au XX e siècle. La plus faible croissance radiale a été atteinte dans la majorité des régions vers 1914 pour la première épidémie du XX e siècle, entre 1948 et 1952 pour la deuxième et entre 1977 et 1979 pour la troisième. L'analyse des chronologies existantes ne montre pas une expansion des épidémies des régions méridionales vers les régions plus nordiques à moins forte proportion de sapin. Les chronologies du sud du Québec sont synchrones avec celles de la réserve des Laurentides et du nord du Saguenay - Lac Saint-Jean pour les trois épidémies, jusqu'au 50 e parallèle où les populations de sapin sont marginales. Il y a un léger décalage vers l'est pour la première épidémie, dans le Bas-Saint-Laurent et en Gaspésie. Les résultats des analyses dendrochronologiques présentées appuient l'hypothèse selon laquelle la dynamique des populations de tbe dans les forêts boréales nordiques du domaine de la pessière noire ne serait pas fondée sur un débordement des populations plus méridionales en période d'épidémies graves mais résulterait d'une augmentation relativement synchrone de populations présentes à l'état endémique.
... There is also evidence of outbreaks of western and two-year cycle spruce budworms for several centuries (Zhang and Alfaro 2002;Alfaro et al. 2014). Interpreting these patterns assembled from a combination of direct and indirect evidence gathered at different scales and variable sampling intensities is hazardous in detail (Régnière and Lysyk 1995) but nonetheless represents our best evidence of historical population patterns. Blais (1954Blais ( , 1965aBlais ( , 1983Blais ( , 1985 used tree rings as indicators of spruce budworm outbreaks across the southern boreal forest of eastern Canada and concluded that outbreaks have been regional in scale but not clearly periodic as the intervals between outbreaks for a given location were highly variable (Blais 1968). ...
The comparative ecology of conifer-feeding budworms in the genus
Choristoneura
Lederer (Lepidoptera: Tortricidae) in Canada is reviewed with emphasis on publications since 1980. Systematics and life history are updated and historical outbreak patterns and their current interpretation summarised. Recent evidence is analysed in the context of ecological interactions among three trophic levels; host plant, budworm herbivore, and natural enemies. The influence of weather and climate are viewed as modulating factors. The population behaviour of budworms is interpreted as the result of tri-trophic interactions that vary at different scales. The result of these multi-scale interactions is that despite shared phylogenetic constraints and common adaptations, different budworm species display different population behaviour because of specific ecological relationships with their respective hosts and natural enemies.
... This insect pest causes recurrent and considerable damages and mortality in balsam fir stands [87], but is also essential for their natural regeneration [88]. Major outbreaks are estimated to occur every 35 years on average [82,89,90], which would roughly represent one balsam fir generation (average sexual maturity reached at around 25 years [41]). During these major outbreaks, balsam fir mortality can reach 91%, while P. glauca is generally less affected (52% [91]). ...
The phylogeographic structure and postglacial history of balsam fir (Abies balsamea), a transcontinental North American boreal conifer, was inferred using mitochondrial DNA (mtDNA) and chloroplast DNA (cpDNA) markers. Genetic structure among 107 populations (mtDNA data) and 75 populations (cpDNA data) was analyzed using Bayesian and genetic distance approaches. Population differentiation was high for mtDNA (dispersed by seeds only), but also for cpDNA (dispersed by seeds and pollen), indicating that pollen gene flow is more restricted in balsam fir than in other boreal conifers. Low cpDNA gene flow in balsam fir may relate to low pollen production due to the inherent biology of the species and populations being decimated by recurrent spruce budworm epidemics, and/or to low dispersal of pollen grains due to their peculiar structural properties. Accordingly, a phylogeographic structure was detected using both mtDNA and cpDNA markers and population structure analyses supported the existence of at least five genetically distinct glacial lineages in central and eastern North America. Four of these would originate from glacial refugia located south of the Laurentide ice sheet, while the last one would have persisted in the northern Labrador region. As expected due to reduced pollen-mediated gene flow, congruence between the geographic distribution of mtDNA and cpDNA lineages was higher than in other North American conifers. However, concordance was not complete, reflecting that restricted but nonetheless detectable cpDNA gene flow among glacial lineages occurred during the Holocene. As a result, new cpDNA and mtDNA genome combinations indicative of cytoplasmic genome capture were observed.
... Various factors are often associated with the onset of a spruce budworm outbreak and typically affect mortality rates of host trees (e.g., Lynch and Witter, 1985;Hix et al., 1987;Régnière and Lysyk, 1995;MacLean and MacKinnon, 1997;Magnussen et al., 2004). Pilon and Blais (1961) observed in Quebec that the presence of mature balsam fir, moth dispersal and certain climatic conditions (early summer drought) were needed for an outbreak to commence. ...
... Increases in predator populations due to increased availability in prey have been documented in several natural systems, such as the spruce budworm (Choristoneura fumiferana). Spruce budworm populations periodically outbreak (Royama 1984), and these transitory increases in density have affected the bird populations that prey upon them (e.g., Venier et al. 2009). Passerine bird populations (Crawford and Jennings 1989) often increase, and several warbler species increased density more than 10-fold (Venier et al. 2009). ...
Genetically engineered (GE) insects have the potential to radically change pest management worldwide. With recent approvals of GE insect releases, there is a need for a synthesized framework to evaluate their potential ecological and evolutionary effects. The effects may occur in two phases: a transitory phase when the focal population changes in density, and a steady state phase when it reaches a new, constant density. We review potential effects of a rapid change in insect density related to population outbreaks, biological control, invasive species, and other GE organisms to identify a comprehensive list of potential ecological and evolutionary effects of GE insect releases. We apply this framework to the Anopheles gambiae mosquito - a malaria vector being engineered to suppress the wild mosquito population - to identify effects that may occur during the transitory and steady state phases after release. Our methodology reveals many potential effects in each phase, perhaps most notably those dealing with immunity in the transitory phase, and with pathogen and vector evolution in the steady state phase. Importantly, this framework identifies knowledge gaps in mosquito ecology. Identifying effects in the transitory and steady state phases allows more rigorous identification of the potential ecological effects of GE insect release.
... In most of the study area, stand ages (obtained from Forest Resource Inventory maps) in upland and riparian areas usually ranged from 60 to 90 years, with occasional stands of 110–140 years. The boreal forest in general has a disturbance-driven ecology (Attiwill 1994), with wildfire (Johnson 1992) and periodic spruce budworm (Choristoneura fumiferana) outbreaks (Regniere and Lysyk 1995) having considerable influence on stand structure in boreal mixedwoods. The frequency of disturbance can vary across boreal landscapes, but Day and Harvey (1981) estimated a fire frequency of 75 ± 50 years for boreal mixedwoods in Ontario. ...
The inputs, characteristics and function of large woody debris (LWD) were measured in 16 mid-order (3-7 m wide), moderate-gradient (1-5% slope) streams in mixedwood forests of the Boreal Shield in Ontario. Wood pieces in these streams were less abundant, smaller, less stable and, consequently, less functional than in streams of most other forest regions. The average frequency (19.9 pieces/100m), size (16.7 cm diameter), and aggregation (2.4 debris dams/100m) of LWD were less than in most previous studies. Fewer than 15% of the pools in these streams were influenced by LWD. The relatively-low LWD loading and influence on pool formation reflected the boulder-dominated substrates, where bed scouring or aggregation around LWD to influence channel morphology was limited. The dominance of coarse bed materials conferred a high degree of instability on many LWD pieces and increased the likelihood of fluvial transport. Given the limited functional role that LWD seems to have on channel morphology in these types of streams, selection-based logging in riparian zones of Boreal Shield streams may be a viable option to enhance riparian stand quality while maintaining stream channel integrity. Companion studies to examine other potential impacts of selection logging in riparian reserves of boreal streams are ongoing.
... It has long been recognized that spruce budworm outbreaks are very large-scale events, typically consisting of many infestations that occur in different localities within a region at approximately the same time (Prebble 1975). A number of authors (e.g., Wellington et al. 1950;Greenbank 1956;Pilon and Blais 1961;Greenbank et al. 1980;Blais 1985;Régnière and Lysyk 1995) have reported that, in general, the large areas of spruce budworm defoliation seem to shift from the west to the east during an outbreak. Blais (1985) succinctly states one widely held view of the relevance of this observation to theories of outbreak spread: "The general progress of outbreaks from west to east indicates that spread is attributable, at least in part, to the dispersal of moths by prevailing westerly winds." ...
Survey records of spruce budworm (Choristneura fumiferana Clem.) defoliation in Ontario, taken annually since 1941, were analysed using geographic information systems (GIS), spatial statistics, and time-series methods. Cumulative frequency maps indicated that the 41 x 106 ha of Ontario that had been defoliated in at least one year since 1941 could be split into three zones of frequent defoliation separated by two approximately 100 km wide, longitudinally oriented corridors of lower frequency. Analysis of annual records of the total area defoliated showed that the fluctuations in this time series are the result of a basic oscillation of approximately 36 years, which is modified by secondary fluctuations and occasionally by sharp drops. The secondary fluctuations are at least partially due to asynchrony in otherwise remarkably similar long-wave oscillations in the eastern (25.5 x 106 ha) and western (9.6 x 106 ha) zones of frequent defoliation. Analysis of this asynchrony showed that outbreaks in the eastern zone occurred 5 or 6 years before outbreaks in the central (6.6 x 106 ha) and western zones, which were synchronous. These observations contradict previous reports of the large-scale spread of outbreaks from west to east.
... The recurrence cycle of outbreaks has averaged about 32-40 years over the last three centuries (Royama 1984;Krause 1997;Morin 1998;Boulanger and Arseneault 2004). These population fluctuations are known to be synchronized over very large areas (Williams and Liebhold 2000;Peltonen et al. 2002;Boulanger and Arseneault 2004) through stochastic and spatially correlated phenomena of density-independent regulation (Moran effect, e.g., climate), change in forest landscape vulnerability (Blais 1983), or population levels of the natural enemy complex (Royama 1984;Régnière and Lysyk 1995;Peltonen et al. 2002). ...
In this study we used dendrochronology to reconstruct the history of eastern spruce budworm (Choristoneura fumiferana (Clem.)) outbreaks over the last 450 years in the Bas-Saint-Laurent region of southeastern Quebec. In total, 260 tree cores were sampled from 204 beams in seven historic buildings and 12 trees in a virgin forest stand. Eight previously documented outbreaks (1975-1992, 1947-1958, 1914-1923, 1868-1882, 1832-1845, 1805-1812, 1752-1776, 1710-1724) and three presumed previous outbreaks (1678-1690, 1642-1648, 1577-1600) were identified based on periods of growth reduction. Of these 11 confirmed or presumed outbreaks, six were documented for the first time in eastern Quebec. Such data suggest that outbreak frequency has remained quite stable, with a mean interval of about 40 years between the midpoint of successive outbreaks since the mid-16th century. In addition, together with previous studies, our results indicate a strong spatial synchrony of spruce budworm outbreaks across central and eastern Quebec during the last 300 years. Consequently, our study does not support the hypothesis that spruce budworm outbreak frequency and synchrony increased during the 20th century.
... In most of the study area, stand ages (obtained from Forest Resource Inventory maps) in upland and riparian areas usually ranged from 60 to 90 years, with occasional stands of 110–140 years. The boreal forest in general has a disturbance-driven ecology (Attiwill 1994), with wildfire (Johnson 1992) and periodic spruce budworm (Choristoneura fumiferana) outbreaks (Regniere and Lysyk 1995) having considerable influence on stand structure in boreal mixedwoods. The frequency of disturbance can vary across boreal landscapes, but Day and Harvey (1981) estimated a fire frequency of 75 ± 50 years for boreal mixedwoods in Ontario. ...
The inputs, characteristics, and function of large woody debris (LWD) were assessed in 16 mid-order (average channel widths of 37 m), moderate-gradient (approx. 1%5% channel slopes) streams in mixedwood forests of the Boreal Shield in Ontario. Three of the streams were adjacent to clearcuts, with the remainder in areas that have not been logged or recently (>70 years) burned. The average frequency (19.9 pieces·100 m1) and size (mean diameter 16.7 cm) of LWD in these streams were less than reported in most other regions and forest types. Averaged across sites, input sources were undetermined for about 50% of the LWD owing to fluvial displacement from the points of origin. Natural mortality (24%) and windthrow (15%) were primary input sources of the remaining LWD. Windthrow was highly variable and mostly associated with nearby clear-cut logging. At the three sites near clearcuts, windthrow contributed 34%62% of LWD in streams. In study reaches where active beaver colonies were observed, beaver-felled trees accounted for up to 47% of LWD inputs. The average frequency of debris dams (2.4 dams·100 m1) was less than those reported from studies in other areas and was positively correlated with an index of bottom substrate size (r = 0.72). Less than 15% of the pools in these streams were formed or influenced by LWD. Most wood pieces appeared to be ineffective as pool-forming agents because of their relatively small size and instability. In these Boreal Shield forests, it appears that most riparian trees do not live long enough or grow to sufficient size to contribute functional LWD and influence stream morphology or pool formation.
... Natural disturbances by wild - fire and insect irruptions play a dominant role in most Cana - dian forest landscapes , and the area affected varies considerably , both inter - annually ( Stocks et al . 2003 ) and over longer - term cycles ( Royama 1984 ) . ...
Increasing the amount of organic-carbon stored in the biomass of terrestrial ecosystems is an effective way to reduce the net anthropogenic emissions of greenhouse gases to the atmosphere. This can be done by conserving existing ecological reservoirs of fixed organic-carbon, maintaining or enhancing the rate of sequestration, and restoring stocks that have been depleted by past land-use practices. Most trading systems for greenhouse-gas offsets recognize the validity of projects that gain ecological offsets, and permit them to sell carbon credits in an emerging marketplace for these novel commodities. Although ecological carbon-offset projects have been criticized from a variety of perspectives, most of the supposed problems can be satisfactorily mitigated. In addition to offsetting emissions of greenhouse gases, ecological projects that accumulate carbon credits may have a strong cross-linkage to the conservation of natural values, which in itself is an important action for society to undertake. This is, however, less of a consideration for projects that are based on anthropogenic ecosystems, such as no-till agricultural systems and plantation forests, which provide relatively few benefits to native biodiversity and might even detract from that objective if developed on newly converted natural habitat. Moreover, the existing rules for carbon-offset systems exclude some kinds of ecological projects from the trading markets, even though they would result in avoided emissions or enhanced sequestration of organic-carbon. As the emerging marketplace for carbon offsets grows, it will be important to understand the co-benefits and side effects of offset projects on non-carbon values, including native biodiversity.
... The recurrence cycle of outbreaks has averaged about 32-40 years over the last three centuries (Royama 1984;Krause 1997;Morin 1998;Boulanger and Arseneault 2004). These population fluctuations are known to be synchronized over very large areas (Williams and Liebhold 2000;Peltonen et al. 2002;Boulanger and Arseneault 2004) through stochastic and spatially correlated phenomena of density-independent regulation (Moran effect, e.g., climate), change in forest landscape vulnerability (Blais 1983), or population levels of the natural enemy complex (Royama 1984;Régnière and Lysyk 1995;Peltonen et al. 2002). ...
It is argued that spruce budworm (Choristoneura fumiferana (Clemens)) (SBW) outbreaks have tended to be more frequent, severe, and spatially synchronized since the beginning of the 20th century. However, few studies have as- sessed the long-term (>200 years) variations in SBWoutbreak dynamics. We reconstructed the SBWoutbreak history at the northern limit of the temperate forest in southern Quebec using dendrochronological material from old buildings and five old-growth stands. Our regional tree-ring chronology (1551–1995) represents one of the longest and most replicated insect outbreak reconstructions in North America. Nine potential outbreaks were identified (1976–1991, 1946–1959, 1915–1929, 1872–1903, 1807–1817, 1754–1765, 1706–1717, 1664–1670, and 1630–1638) with three additional uncertain outbreaks (1647–1661, 1606–1619, and 1564–1578). Results suggested that southern Quebec has experienced frequent and synchron- ized outbreaks throughout the last 400 years. Although outbreak frequency was higher during the 20th century (approxi- mately 30 years) as compared with the 1660–1850 period (approximately 50 years), similar or even higher outbreak frequency might have occurred prior to 1660 (approximately 28 years). We did not find any evidence that the recent out- break dynamics in southern Quebec is outside its historical range of the last 400 years. Previous studies based on living trees may have underestimated outbreak frequency and synchrony prior to 1900.
... When abundant, larvae can defoliate trees, and outbreaks may extend over millions of hectares. Several theories about the dynamics of budworm populations have been advanced based alternatively on the effects of weather, hosts, and natural enemies (Wellington 1950; Greenbank 1956, 1957; Morris 1963; Hardy et al. 1983; Royama 1984, 1992; Blais 1985; Régnière and Lysyk 1995). Most of the theories were developed from intensive life table data collected in specific forest stands (Morris 1963). ...
Using two tests for direct density dependence and standard techniques of time series analysis, we identified density dependence
in defoliation time series of the spruce budworm across its outbreak range in eastern North America over the years 1945–1988.
We carried out analyses for the entire region and for grid cells of defoliation maps at five spatial scales created by aggregating
the smallest grid cells. The rate of detection of direct density dependence, as assessed by two previously published methods,
decreased with increasing spatial scale. Using both methods, density dependence was detected more frequently at the periphery
of the outbreak range, where defoliation rate was lower. This result suggested that density-dependent regulation may be stronger
in those areas. The first order autoregressive process was the basic model for defoliation dynamics overall and the most common
model across spatial scales. Second-order processes were encountered much less frequently, and those commonly identified as
resulting from delayed density dependence generally occurred across spatial scales at a rate expected by chance alone. Our
results were similar to those of other published studies, which have found the detection of density dependence to decrease
at larger spatial scales. The results also reinforced the importance of considering spatial scale when diagnosing population
processes using time series of abundance for single species.
... Even among the best-studied systems there is substantial debate about the nature of population regulation and the cause of outbreaks. For example, despite decades of research on the spruce budworm, Choristoneura fumiferana Clemens (Lepidoptera: Tortricidae), it is still unclear whether outbreaks are caused by an eruptive mechanism whereby regulation about a low-density equilibrium state occasionally fails (Morris 1963, Ludwig et al. 1978, Clark and Holling 1979, Blais 1983, Campbell 1993 ), or by a noneruptive mechanism whereby populations cycle loosely about a single equilibrium state because of lagged effects of parasitism (Royama 1984Royama , 1992, Ré gniè re and Lysyk 1995, Berryman 1996). The case of the forest tent caterpillar, Malacosoma disstria (Hü bner) (Lepidoptera: Lasiocampidae), is remarkably similar in this regard. ...
Overwintering mortality of forest tent caterpillar (Malacosoma disstria (Hubner)) eggs was estimated over a 360 km2 grid of 83 plots in north-central Alberta over the period 1992Ð1996 during a local outbreak. Egg mortality in the trembling aspen (Populus tremuloides Michaux) canopy was generally low; however, 20% of the eggs laid in the summer of 1995 failed to hatch in the spring of 1996. In the shrub layer, 70% failed to hatch. In both the canopy and shrub layers, the spatial pattern of mortality was density-independent, with high mortality occurring in low-lying areas. Daily tem- perature records suggested that the proximal cause of death was freezing during mid-winter. Cater- pillar populations peaked in 1995, before perturbation, and collapsed during the summer of 1996, largely as a result of larval parasitism. The timing of this perturbation-assisted population collapse coincided loosely with the penetration of larvae down into the shrub layer. We illustrate how winter temperature, albeit a density-independent factor, probably acts in a partially density-dependent manner through interactions with density-dependent behavioral and physiological processes that inßuence spatial variation in vulnerability and susceptibility to winter cold. We argue that cold winter temperatures are an important factor inßuencing the long-term dynamics of forest tent caterpillar populations in northern climates.
This is a professional account of my personal experience with spruce budworm in Canada, from 2001-2023. I explain what I predicted in 2004, before the outbreak began, and on what basis, and discuss how the outbreak has unfolded since. The account is biased, and personal.
One of the most significant categories of insect that cause damage to trees are the defoliators. While many orders of insects feed on tree foliage, in this chapter we will focus on Lepidoptera, as there are so many Lepidopteran larvae (caterpillars) that are known for their extensive tree damage. In this chapter we review the impact of foliage feeders on forest trees and stand composition, and the ways in which densities of these species or the defoliation they cause are monitored. We do not cover insects attacking ornamental trees in the landscape, nor do we cover insects feeding exclusively on foliage tips or buds.
The spruce budworm is a defoliator of boreal forests in North America and can cause major impacts to spruce and balsam fir, which are important tree species in the logging industry. While most years the spruce budworm density is low and little harm comes to these trees, periodically throughout history large outbreaks of the spruce budworms occurred. These outbreaks caused major losses to the trees and to economic industries tied to the trees. As a result, the spruce budworm is one of the most well-studied—empirically and theoretically—ecological systems. Here, we introduce and explore the spruce budworm system. In our examination of the spruce budworm, we begin with a simple model describing the budworm dynamics alone. Our analysis and simulation of even the simple model demonstrates the complicated dynamics it can produce, as a function of a parameter that depends on the health and size of the forest. We then expand the system to include the forest as a variable itself and include feedbacks with the spruce budworm. We extend our analyses and simulation for this more complete system.
Insect defoliators constitute a distinct class of forest disturbance because they are selective, extensive, periodic, and spatially synchronous, albeit imperfectly. This chapter discusses the nature of the mechanisms that generate cycles and synchronize these cycles across the landscape. A template is presented that could be used to structure a process-oriented insect disturbance model and a tritrophic herbivory model of cycle induction is applied to four systems: forest tent caterpillar, western spruce budworm, spruce budworm, and jack pine budworm. All four systems can be interpreted within the context of the same tritrophic regulatory structure and show that the ecological relationships between the defoliating insect, its resource, and its natural enemies are critical processes governing insect-caused disturbance.
Landscape-level forest management has long been hypothesized to affect forest insect outbreak dynamics, but empirical evidence remains elusive. We hypothesized that the combination of increased hardwood relative to host tree species, prevalence of younger forests, and fragmentation of those forests due to forest harvesting legacies would reduce outbreak intensity, increase outbreak frequency, and decrease spatial synchrony in spruce budworm (Choristoneura fumiferana Clem.) outbreaks. We investigated these hypotheses using tree ring samples collected across 51 sites pooled into 16 subareas distributed across a large ecoregion spanning the international border between Ontario (Canada), and Minnesota (USA). This ecoregion contains contrasting land management zones with clear differences in forest landscape structure (i.e., forest composition and spatial configuration) while minimizing the confounding influence of climate. Cluster analyses of the 76-years time-series generally grouped by subareas found within the same land management zone. Spatial nonparametric covariance analysis indicated that the highest and lowest degree of spatial synchrony of spruce budworm outbreaks were found within unmanaged wilderness and lands managed at fine spatial scales in Minnesota, respectively. Using multivariate analysis, we also found that forest composition, configuration, and climate together accounted for a total of 40% of the variance in outbreak chronologies, with a high level of shared variance between composition and configuration (13%) and between composition and climate (9%). At the scale of our study, climate on its own did not explain any of the spatial variation in outbreaks. Outbreaks were of higher frequency, lower intensity, and less spatially synchronized in more fragmented, younger forests with a lower proportion of host species, with opposing outbreak characteristics observed in regions characterised by older forests with more concentrated host species. Our study is the first quantitative evaluation of the long-standing “silvicultural hypothesis” of spruce budworm management specifically conducted at a spatio-temporal scale for which it was intended.
Three main hypotheses have been postulated over the past century to explain the outbreaking population dynamics of eastern spruce budworm, Choristoneura fumiferana (Clemens). The Silviculture Hypothesis first arose in the 1920s, with the idea that outbreaks were driven by forestry practices favoring susceptible softwood species. In the 1960s, it was proposed that populations were governed by Multiple Equilibria, with warm weather conditions releasing low-density populations from the regulatory control of natural enemies. Dispersal from outbreak foci, or " epicenters, " was seen as causing widespread outbreaks that eventually collapsed following resource depletion. However, in the 1980s, following the re-analysis of data from the 1940s outbreak in New Brunswick, this interpretation was challenged. The alternative Oscillatory Hypothesis proposed that budworm population dynamics were governed by a second-order density-dependent process, with oscillations being driven by natural enemy–victim interactions. Under this hypothesis, weather and resource availability contribute to secondary fluctuations around the main oscillation, and weather and moth dispersal serve to synchronize population cycles regionally. Intensive, independent population studies during the peak and declining phases of the 1980s outbreak supported the principal tenet of the Oscillatory Hypothesis, but concluded that host plant quality played a more important role than this hypothesis proposed. More recent research on the early phase of spruce budworm cycles suggests that mate-finding and natural-enemy-driven Allee effects in low-density populations might be overcome by immigration of moths, which can facilitate the onset of outbreaks. Even more recent research has supported components of all three hypotheses attempting to explain spruce budworm dynamics. In the midst of a new rising outbreak (2006-present), we discuss the evolution of debates surrounding these hypotheses from a historic perspective, examine gaps in current knowledge, and suggest avenues for future research (e.g., intensive studies on low-density populations) to better understand and manage spruce budworm populations. The ecological mechanisms that drive periodic oscillations in animal populations have been a subject of intrigue and debate among population ecologists for at least a century (Elton 1924, Graham 1939, Berryman 1996, Liebhold and Kamata 2000). Outbreak-prone her-bivorous insects have attracted particularly intense study, because many of them have widespread and severe impacts on agricultural and forestry systems. This has included many empirical studies, of course, but herbivorous insects have also provided the foundations for theoretical debates about the mechanisms of animal population dynamics (reviewed in Berryman 2003). Such theoretical debates cry out for long-term data sets to help resolve them, and while these are unfortunately uncommon, they exist for a number of herbivorous insects. Long-term studies tend to generate a daunting quantity of data, and surprisingly often, the concordance between long-term data and theoretical expectations is not obvious. This can result in conflicting literature and in the lengthy persistence of conflicting hypotheses about population dynamics. The eastern spruce budworm (Choristoneura fumiferana (Clemens), Lepidoptera: Tortricidae; henceforth, just " spruce budworm " or " budworm ") exemplifies this conundrum. Despite more than 65 yr of intensive research, multiple long-term studies , and hundreds of published articles, our understanding of budworm population dynamics has seen paradigms shifting through the decades, but no broad consensus has been reached on the ecological factors that drive and modulate population oscillations (Sturtevant et al. 2015).
Spruce budworm (Choristoneura fumiferana) is a native insect that defoliates needleleaf trees, especially balsam fir (Abies balsamea) and spruces (Picea spp.), in northern North America. Spruce budworm can defoliate millions of hectares of forest during an infestation, depressing regional economies that depend on the timber industry. Ecosystems, though, can benefit from spruce budworm because outbreaks rejuvenate the forest, maintaining optimal levels of primary production, and thereby carbon sequestration. Although many ecologists, entomologists, geographers, and resource managers have studied the effects of spruce budworm on spruce–fir forests throughout the region, no single explanation of what causes the number of insects in a forest to rise and fall is universally accepted. Spruce budworm populations can reach ‘outbreak’ levels, or densities high enough to defoliate and kill balsam fir and spruce on a landscape scale, on average every 30–40 years. We review the biology of spruce budworm, the processes that scientists follow to reconstruct spruce budworm outbreaks, the leading hypothesis to explain population dynamics and outbreak events, and the complexity of forecasting possible future trends of populations and distributions of spruce budworm. Reconstructions of past outbreaks help us understand their severity, frequency, and spatial synchrony, which might be linked in complex ways to climate, forest, and stand characteristics. Future spruce budworm dynamics are difficult to predict because the insect is part of a complex food web. In the coming decades, spruce budworm probably will survive climate change because it is adapted to a wide range of temperatures and precipitation amounts.
Food shortage is a common situation in nature but little is known about the strategies animals use to overcome it. This lack of knowledge is especially true for outbreaking insects, which commonly experience nutritional stress for several successive generations when they reach high population densities. The aim of this study is to evaluate the life history consequences of chronic nutritional stress in the outbreaking moth Choristoneura fumiferana. Larvae were reared on two different artificial diets that emulate nutritional conditions larvae face during their natural population density cycle (low and medium quality artificial diets). After four generations, a subset of larvae was fed on the same diet as their parents, and another on the opposite diet. We explored larval life-history strategies to cope with nutritional stress, its associated costs and the influence of nutritional conditions experienced in the parental generation. We found no evidence of nutritional stress in the parental generation increasing offspring ability to feed on low quality diet, but the contrary: compared to offspring from parents that were fed a medium quality diet, larvae from parents fed a low quality diet had increased mortality, reduced growth rate and reduced female reproductive output. Our results support a simple stress hypothesis because the negative effects of malnutrition accumulated over successive generations. Density-dependent deterioration in plant quality is thought to be an important factor governing the population dynamics of outbreaking insects and we hypothesize that chronic nutritional stress can be a driver of outbreak declines of C. fumiferana, and of forest insects in general.
The Canadian registration in 2007 of Disrupt SBW Micro-Flakes®, a pheromone-based product for control of spruce budworm, Choristoneura fumiferana (Clemens), paved the way for large-scale trials to test the practicality of mating disruption as a commercial pest management strategy. We review results from field and laboratory experiments on pheromone-based mating disruption of spruce budworm conducted from 1974 to 2008. Application of pheromone from the ground or the air consistently reduced the orientation of males toward pheromone sources. Mating disruption also reduced the mating success of caged or tethered females in 15 of 16 field studies where this parameter was recorded, but had only a limited effect on the mating success of feral females. No consistent difference in the density of egg masses in control and treated plots was observed, which has often been attributed to immigration of gravid females into pheromone-treated plots. Laboratory studies suggest that false-trail following is the predominant mechanism underlying mating disruption in spruce budworm. The enhanced mating success of females with increasing population density suggests that mating disruption should target low-density emergent populations during the initial phase of an outbreak. Constraints that may limit the potential of mating disruption as a management tool include (1) difficulties associated with obtaining accurate sampling estimates at low population density to forecast the onset of outbreaks, (2) potential behavioral adaptations by which females enhance their mating success when the atmosphere is treated with pheromone, and (3) long-range dispersal of females by flight.
The present study evaluated an oviposition threshold for flight in a moth species with an unusual inter-reproductive migration strategy, the spruce budworm (SBW), Choristoneura fumiferana (Clem.) (Lepidoptera: Tortricidae). Females typically lay some eggs in their natal patch then ascend by flight above the plant canopy to the atmospheric boundary layer where they can get carried several hundreds of kilometres way. Estimates of residual fecundity (= proportion of eggs remaining in the abdomen of females relative to potential fecundity) were calculated based on the relationship between the wing length and dry weight of 985 field-collected females. The females were sampled using three procedures, light traps deployed in the tree canopy or forest clearings, or by fogging trees with insecticides; the experiment was replicated at nine plots. The data were interpreted under the realistic assumption that females captured at light traps had to fly, whereas samples collected by fogging trees included females that were capable or not capable of flight. The lower residual fecundity of females captured at light traps compared with those collected on fogged trees revealed that females have to lay some eggs before flight. Very few females captured at light traps had laid fewer than 50 % of their egg complement, indicative of an oviposition threshold for flight corresponding to 50 % residual fecundity. The threshold appeared robust as it did not vary much over the flight season or the location of the light traps, and it was consistent for different sets of equations related to residual fecundity. The importance of the oviposition threshold for flight on the nature of spruce budworm population dynamics remains to be investigated.
We investigated the spatial synchrony of outbreaks of the spruce budworm, Choristoneura fumiferana, over much of its outbreak range in eastern North America during the period 1945-1988. Spatial synchrony decreased with distance between local populations and approached zero near 2000 km. Investigation of the synchrony of local population time series with cluster analysis revealed a pattern of geographically distinct blocks of clusters oriented along an east-west axis. Spatial synchrony also was identified in monthly temperature and precipitation time series at 18 weather stations over the same time period and geographical range as the spruce budworm outbreaks. Cross correlations decreased linearly with distance between stations and approached zero near 3000 km and 1800 km, respectively. We developed a spatially explicit lattice model for a single species occupying multiple patches. Within patches, the model had first order logistic dynamics, and patches were linked by dispersal that depended upon their separation distances. Both local and regional stochasticity (i.e., a Moran effect) were present The modeled lattice had the Same spatial configuration as the outbreak region to facilitate investigating the relative effects of a Moran effect and dispersal on spatial synchrony. Simulations with and without a simple region-wide Moran effect and three levels of dispersal did not produce the decrease in spatial synchrony with distance observed with spruce budworm time series. However, when run at the highest dispersal rate, those simulations produced cluster maps similar to that observed for spruce budworm defoliation. Simulations with a spatially autocorrelated disturbance that had either zero or high local variability and three levels of dispersal produced decreases in spatial synchrony with distance similar to that observed in the historical data. When run at the highest dispersal rate, simulations yielded cluster maps similar to the cluster map for defoliation. We discuss the potential significance of the spatially autocorrelated disturbance factor in understanding regional insect outbreaks. We also consider the plausibility of dispersal rates used in our simulations. We suggest in conclusion that spruce budworm outbreaks were synchronized by a combination of a spatially autocorrelated Moran effect and a high dispersal rate.
The four-year oscillations of the number of spawning sockeye salmon that return to their native stream within the Fraser River basin in Canada are a striking example of population oscillations 1{3 . The period of this oscillation cor- responds to the dominant generation time of these fish. Different stocks can have their popula- tion maximum in different years. Various - not fully convincing - explanations for these oscil- lations have been proposed, ascribing this phe- nomenon either to transient effects or to stochastic influences 4 , to depensatory predation 5 , to fishery 6 , or to genetic effects 7 . Here, we show that these oscillations can be explained as a stable dynami- cal attractor of the population dynamics, resulting from a strong resonance near a Neimark Sacker bi- furcation. This explains not only the long-term persistence of these oscillations, but also repro- duces correctly the sequence of one strong year followed by one intermediate year and two weak years. Furthermore, it explains the observations that these periodic oscillations occur only in large oligotrophic lakes, but not in ultra-oligotrophic or mesotrophic lakes, and that they are usually not observed in salmon species that have a 5-year gen- eration time. Four-year sockeye salmon oscillations were reported as early as the 19th century and are evident for instance in the extremely high catches by fisheries every fourth year 8 . Figure 1 shows three examples of sockeye salmon oscilla- tions in the Fraser River basin. These salmon return to spawn in their native stream or lake in late summer and die, thus providing an important phosphorus input into the lake. The hatched fry migrate downstream to the re-
This paper investigates the effects of spruce budworm (Choristoneura fumiferana) on balsam fir (Abies balsamea) and white spruce (Picea glauca) at Itasca State Park in northwestern Minnesota. We studied the species composition, age structure, and radial growth patterns in tree rings along five belt transects at sites infested with spruce budworm. Our objectives were to: (1) discover when the latest spruce budworm outbreak started; (2) determine whether tree growth was similarly reduced in earlier decades, suggesting earlier spruce budworm outbreaks; and (3) test whether radial tree growth and the start of the outbreak(s) were correlated with climate. We used the computer program OUTBREAK to determine that the current spruce budworm infestation began to reduce tree growth in the 1990s, before it was detected by park officials in 2001. The tree-ring record indicated that growth of the host-species trees was periodically reduced at all sites prior to the 1990s. We found no consistent relationship between temperature/precipitation and the initiation of spruce budworm outbreaks, as reconstructed by the OUTBREAK program, at the southwestern edge of the distribution of balsam fir. The Palmer Drought Severity Index, however, was positive (i.e., wetter-than-normal conditions) before increased spruce budworm activity at all sites. Outbreaks are related to climatic conditions, but they also depend on other factors such as the availability of sufficient food for the spruce budworm.
The effects of harvesting on seed bank dynamics in a boreal mixedwood forest were studied on replicated 10-ha treatment blocks harvested by different clear-cutting or partial-cutting systems in the fall of 1993. From 1994 to 1995 we monitored seed rain, soil seed banks, and seasonal changes in species composition in understory vegetation and seed banks in all harvest blocks plus three uncut controls. No persistent conifers were found in the soil seed banks of any treatment. The number of seeds of other species generally decreased with soil depth in all treatments, with the lower layer of organic soil yielding the highest numbers of seedlings. Many seeds of sedges and some herbs were found in the upper mineral soil horizon, indicating significant longevity. While disturbance by harvesting operations altered the distribution of seeds in the soil profile, harvesting method had little effect on the total number of species present in post-harvest seed banks or understory vegetation. There were no differences in seasonal compositional changes between treatments. Seed rain monitoring indicated that few conifer seeds were added to the seed bank. Betula papyrifera Marsh. was the dominant tree species in seed rain in the partial cutting treatments. However, in the second post-harvest year on clear cut sites sedges and grasses increased from less than 1 to 14% of seed rain. The results suggest that predominantly hardwood stands with prolific understory vegetation will initially develop on the treated sites, with a variable, but depleted conifer content.Key words: boreal mixedwood forest, natural regeneration, seed rain, seed bank, succession, vegetative propagation.
The ideas of community structure and the expression of dominance, that of bio-logical succession, and finally that of climax, are based largely upon the assumption of long-term stability in the physical habitat. Remove this assump-tion and the entire theoretical structure becomes a shambles. . . . Disturbances have been so frequent and so generally effective that the expected "climaxes," or "equilibria," recede into pure speculation. Natural successions either do not occur at all or are limited to such incomplete frag-ments as can be accomplished between upheavals.
Randomization and simulation are used to detect bias in k-factor analysis. In nine previously published data sets there is strong evidence of bias. This may result from either non-independence of observations or the arithmetic relationship used to estimate k-factors, which can generate spurious correlations. Randomization can be used to test for density dependence without bias. This procedure confirms the existence of densitydependent effects in 8 of the 9 populations and 11 of the 16 k-factors previously thought to have density-dependent effects.
The circum-polar boreal forest has played an important role in the wealth of northern nations since the 15th century. Its natural resources spurred strategic geopolitical developments beginning in the 16th century but intense development of the boreal forest is largely limited to the 20th century. Insects cause considerable loss of wood that has an adverse effect on the balance of carbon sequestered by forests. Current understanding of processes that lead to stand-replacing outbreaks in three insect species is reviewed in this paper. Many of these processes depend on climate either directly, such as reduced survival with extreme weather events, or indirectly, mainly through effects on the host trees. In the boreal zone of Canada, pest-caused timber losses may be as much as 1.3–2.0 times the mean annual depletions due to fires. Pests are thus major, but consistently overlooked forest ecosystem components that have manifold consequences to the structure and functions of future forests. Global change will have demonstrable changes in the frequency and intensity of pest outbreaks, particularly at the margins of host ranges. The consequent shunting of carbon back to the atmosphere rather than to sequestration in forests as biomass is thought to have positive feedback to global warming. Whereas significant progress has been made in developing carbon budget models for the boreal forests of Canada, enormous problems remain in incorporating pest effects in these models. These problems have their origins in the nature of interactions among pests with forest productivity, and problems with scaling. The common problems of verification and validation of model results are particularly troublesome in projecting future forest productivity. The interaction of insects with fires must be accounted for if realistic carbon sequestration forecasts in a warming climate are to be made. These problems make assessments of mitigation and adaptation of pest management alternatives difficult to evaluate at present. Nevertheless, the impacts of stand-replacing insect population outbreaks is important in formulating future resource management policy.
Historical records of defoliation by spruce budworm in Quebec from 1965 to 1992 were analyzed to determine the degree of variability in the spatial and temporal patterns of defoliation. Approximately 80% of the provincial variation in the 28-year record could be described by 25 representative defoliation patterns after first removing the geographic trends in onset year of outbreak. The spatial distribution and probable host impact of these geo-referenced representative defoliation patterns are discussed. These patterns can be used in the context of forest protection and resource management decision-making, especially if they are incorporated into a decision support system with a geo-referenced database of stand characteristics and a stand growth model. Such a system can improve estimates of expected losses due to defoliation by spruce budworm and of the potential benefits of foliage protection. The spatial and temporal patterns of defoliation can be used also to investigate the influence of environmental conditions on the characteristics of budworm outbreaks. They also constitute a basis for the validation of spatial patterns predicted by epidemiological models of the insect's population dynamics.
Pine wilt disease (PWD) is caused by a non-native pest that has spread extensively throughout Japan. Previous research has indicated that most infected trees have died and the litter deposited has resulted in changes to stream-water chemistry, particularly increased nitrate (NO
3−) concentrations. In this study, we divided stream nitrogen (N) export into N loss due to PWD and baseline N leakage without disturbance based on long-term monitoring. The annual N export was 110.0 mol N ha−1 year−1 in 1990 and 749.8 mol N ha−1 year−1 in 1997, and had decreased to 37.0 mol N ha−1 year−1 in 2005. N export under PWD influence was estimated to be 3697 mol N ha−1, and N loss due to PWD was 2810 mol N ha−1. N loss due to PWD was three times larger than baseline N leakage for the disturbed period. These changes in plant–herbivore relationships could affect N status in a forest ecosystem. So-called “semi-natural” disturbances related to non-native species invasion and increases of atmospheric N deposition caused by human activity will increase. Long-term monitoring studies of various aspects are necessary to offer insight into this ecosystem.
The four-year oscillations of the number of spawning sockeye salmon (Oncorhynchus nerka) that return to their native stream within the Fraser River basin in Canada are a striking example of population oscillations. The period of the oscillation corresponds to the dominant generation time of these fish. Various-not fully convincing-explanations for these oscillations have been proposed, including stochastic influences, depensatory fishing, or genetic effects. Here, we show that the oscillations can be explained as an attractor of the population dynamics, resulting from a strong resonance near a Neimark Sacker bifurcation. This explains not only the long-term persistence of these oscillations, but also reproduces correctly the empirical sequence of salmon abundance within one period of the oscillations. Furthermore, it explains the observation that these oscillations occur only in sockeye stocks originating from large oligotrophic lakes, and that they are usually not observed in salmon species that have a longer generation time.
The braconid parasitoid Meteorus trachynotus Vier. was found in overwintered larvae of the obliquebanded leafroller, Choristoneura rosaceana (Harr.), on foliage of a variety of deciduous species, well before the parasitoid’s attack on the spruce budworm, Choristoneura fumiferana (Clem.). In the spring, percentage parasitism of C. rosaceana by M. trachynotus was 8.0 and 18.8% in 1986 and 1987, respectively. Maximum parasitism on C. fumiferana (35 and 4% in 1986 and 1987, respectively) was reached in late June, at a time when over 50% of M. trachynotus cocoons found on deciduous vegetation had emerged. Twenty days later, parasitoids produced on C. fumiferana reached 50% adult emergence. The catches of adult parasitoids on sticky traps placed on deciduous trees and on conifers showed two periods of adult activity. Based on the sex ratio of captured parasitoids, male M. trachynotus were active at emergence sites, whereas females were found mostly in the habitats of available hosts. The phenology of C. rosaceana was observed in 1987. As indicated by light-trap captures, males fly sooner than females. Eggs were laid in July, and so the early larval stages were available to female M. trachynotus as overwintering hosts. Several parasitoids common to C. rosaceana and C. fumiferana were identified.
Examined the hypothesis that predation by forest birds restricts expansion of low-density populations of spruce budworm (Lepidoptera: Tortricidae). Bird populations were censuses in spruce-fir stands of low to moderate budworm density in N New Hampshire and W Maine. In plots where spruce budworm densities ranged from low to transitional, the entire bird community as well as specific groups of birds (overstory warblers and golden-crowned kinglets Regulus satrapa; sparrows, vireos, and juncos; and nuthatches, thrushes, and purple finches Carpodacus purpureus) showed significant functional responses to increasing budworm density. When transitional budworm densities were eliminated from the analysis, only the warbler-kinglet group exhibited a significant functional response. Only Canada warbler Wilsonia canadensis and golden-crowned kinglet showed numerical responses to increasing budworm numbers. The percentage of the budworm population consumed by birds decreased as budworm numbers increased. Birds consumed 84% of the larvae and pupae where budworm populations were low, 22% where budworm populations were intermediate between low and high density. Once 4th instar larvae exceeded 106 individuals/ha, bird predation was ineffectual. The hypothesis that birds can effectively limit budworm increase is compatible with theories to explain budworm outbreaks. -from Authors
The extensive coniferous stands in northwestern Ontario have been subjected to repeated widespread outbreaks of the spruce budworm, Choristoneura fumiferana (Clem.) (5). About 1939 two such outbreaks originated almost simultaneously, one on the west shores of Lake Nipigon and the other south of Lac Seul approximately 180 miles farther west. Each outbreak eventually covered thousands of square miles and was responsible for the destruction of hundreds of square miles of balsam fir forest. Studies of the parasite complex attacking the spruce budworm in northwestern Ontario and the assessment of its general influence on budworm population trends were initiated in the Lake Nipigon outbreak in 1946 by Professor N. R. Brown, now of the University of New Brunswick, and were further developed by the senior author from 1947 to 1950. Similar investigations were initiated by the junior author in the Lac Seul outbreak in 1950 and continued until 1954. Studies in both areas were continued on a limited basis until 1956 through the co-operation of several research officers.
Based on a simple graphic analysis, I conclude that blackheaded budworm population density cycles are caused by delayed negative feedback processes, probably due to the numerical response of insect parasitoids. The parasitoid–host interaction, however, has a stable solution and continuous population density cycles can only be maintained in a variable environment.
A theory for the population behavior of the eastern blackheaded budworm is presented. The qualitative properties and key processes of the system are identified and a description of the theory as a simulation model is given. Two domains of stability exist for the insect; one is caused by bird predation, the second by the effects of food limitation. The model predicts that the budworm’s cyclical population fluctuations are produced by the disappearance and reappearance of these stable domains and consequent movement of densities to new levels. Parasitism is identified as the principal mechanism causing this behavior. A qualitative comparison of model behavior with historical behavior is made and a critical field experiment to test the validity of the theory is proposed. Finally it is suggested that the population behavior of many forest defoliators may be explained using this theory.
Annual population estimates of spruce budworm, Choristoneura fumiferana (Clem.), larvae and the incidence of larval parasitism were monitored in northern New Brunswick following the collapse of the 1949–1959 epidemic in the region. The most common parasitoid among the very sparse hosts was Synetaeris tenuifemur, although it had been rarely found during the epidemic. Larval parasitism sometimes exceeded 50% but it could not be shown as a key factor associated with changing host abundance.
Earlier attempts to correlate spruce budworm, Choristoneura fumiferana (Clem.), outbreaks and weather conditions were usually carried out by comparing seasonal data and spruce budworm outbreak development in broad terms: it was found that warm, dry weather favored outbreak development, while cool, wet weather retarded development. In this paper, laboratory experiments and historical data are examined to determine the effect of temperature and precipitation on second-instar larvae, just prior to and after spring emergence. Results tend to show that prolonged rain and freezing temperatures during and shortly after emergence are detrimental and may, in some cases, have a profound impact on outbreak development.
A spruce budworm outbreak started in Quebec and adjoining parts of New Brunswick in 1949 and spread each year until by 1955 most balsam fir stands within an area of 32,000 square miles were attacked. Approximately 45 per cent of this area was in the Lower St. Lawrence and Gaspe regions of Quebec. In Quebec, important population reductions took place in various sectors of the outbreak area as a result of unfavourable weather conditions in 1956 and 1957 (Blais, 1958a, 1958b), and the final collapse occurred in 1958 when populations returned to the endemic level throughout the area (Blais and Martineau, 1958). In New Brunswick, a relatively small outbreak area remained at the end of the summer in 1958 (Webb et al. , 1959). The large-scale aerial applications of DDT begun in New Brunswick in 1952, and in Quebec in 1954, were continued until 1958. Biological studies in connection with the spraying were carried out in both provinces. In Quebec these studies consisted of assessing spruce budworm population fluctuations in both sprayed and unsprayed areas and in determining, where possible, the causes for these fluctuations. All factors associated with changes in population could not be adequately measured over such a large territory, but a real attempt was made to obtain information on parasitism. The present paper deals with this aspect of the studies.
New Brunswick outbreaks of the spruce budworm which began in 1912 and 1949 are considered in relation to the theory of climatic release. Studies on a natural population show that larval development is more rapid in dry and sunny weather than in humid and cloudy weather. Polar air masses bring the favorable conditions, and tropical air masses and cyclones the unfavorable. June precipitation and temperature records, analyzed in conjunction with weather maps, show that climatic changes took place in regions where the outbreaks developed. The outbreaks were preceded by dry and sunny summers during four or five consecutive years. Although direct mortality of the budworm due to adverse weather conditions has not been observed, favorable climatic conditions may have indirectly promoted population increase from the endemic to the outbreak level in the following ways. Flower production became more frequent in periods of dry years and larvae which fed on staminate flowers developed more rapidly. Larvae that developed early in the season gave rise to more fecund females than larvae that developed late. The age of current foliage consumed is also related to fecundity. The rate of development of balsam fir relative to the budworm varies from year to year. It is postulated that the average fecundity of the budworm increases in the pre-outbreak years as the result of favorable climatic conditions and greater than usual flower production. Larval mortality is greater in years with a prolonged developmental period although the increase is not statistically significant. The theory that outbreaks in New Brunswick resulted from the spread of populations from outbreak areas to the west will be considered in the second part of this two-part paper.
In Part I of this paper consideration was given to the role of climate in the initiation of outbreaks of the spruce budworm in New Brunswick. Analysis of the available weather data showed that the 1912 and 1949 outbreaks developed after several consecutive dry summers. Support was given to the theory of climatic release, which explains the time and place of outbreaks on a climatic basis. However, the recorded history of the spruce budworm also shows that high populations appeared in New Brunswick shortly after "spreading" through Quebec, and this suggests that the New Brunswick outbreaks are also a continuation of this spread. In the present part of the paper consideration is given to the role of dispersal. Moth dispersal is a more effective agent of spread than larval dispersal. Moths may be transported by convectional and turbulent air currents for long distances. Light traps used to detect the incidence of moth movements, showed that large segments of a population may be transferred from one area to another. Unspent females often predominate in these movements. Moth invasion was not detected before the 1949 outbreak although there is evidence from other sources that it occurred in 1948. When deposited in dense, mature, softwood stands, the moths can create outbreaks, but when deposited in young, open, or mixed-wood stands the ensuing high populations soon decline unless bolstered by repeated invasions. Populations in New Brunswick showed gradual and general increases as early as 1947. It is thought probable that these increases resulted from the build-up of local populations through climatic release. The nearest highly-populated centers were over 100 miles to the west in 1947. Later, invasion of moths from centers outside of the Province may have hastened the process.
Apparent parasitism and the population trend during the larval stage of the host are the basic data for assessing budworm larval mortality caused by parasites. Apparent parasitism is estimated from samples of the host population, and three methods of obtaining this estimate within predetermined error limits are outlined. The seasonal history of the parasite is divided into a 'pre-emergence' period, when the parasite is developing within the host, and an 'emergence' period, when the host is actually killed. Percentage mortality is then based on host density at the beginning of the emergence period so the results will not be influenced by the mutual interference of other factors during the pre-emergence period. One field example illustrates the technique and its application to life tables.
Populations of the spruce budworm were studied on flowering and nonflowering balsam fir trees. Generally more eggs were found on the flowering trees. The flowering balsam fir trees were found to harbor higher populations in the early larval stages owing to the presence on these trees of staminate flowers and flower cups. The behavior of the larvae in relation to staminate flowers and flower cups was studied in both the field and the laboratory. Larvae that fed partially on pollen developed more rapidly than larvae that fed exclusively on foliage. Pollen as a food did not appear to have any direct effect on survival or fecundity. Other experiments showed that mortality was higher, development retarded, and fecundity reduced in insects forced to feed on old foliage in contrast with those fed on the current year's growth. Defoliation was more severe on flowering trees in the earlier stages of the infestation. However, as populations increased, wandering increased owing to competition for food. This resulted in an overflow of larvae from flowering to nonflowering trees.
The need for computer-assisted decision-making in entomology has been emphasised by the requirements of integrated pest management, where the information base and the possibilities for increasingly sophisticated control strategies have both expanded greatly in the last decade. Attributes of a computerised system are outlined, and types and characteristics of computer-based decision aids are reviewed. -P.J.Jarvis
The characterization of the vegetation found in seven epicenters where the current spruce budworm outbreak got underway in Quebec has allowed for the elaboration of new concepts on the epidemiology of the spruce budworm. The epicenters were found to be mostly covered with mixwood stands and the presence of meridional species such as sugar maple, yellow birch, and white pine was common to every epicenter. Softwood stands occupied less than 30 percent of the cover and the most common age class of the host species, balsam fir and white spruce, was 50. Moreover, every epicenter had been subjected to some ecological perturbation that resulted in the presence of pioneer species which in turn seem to exert an influence upon the abundance of the host species. Applied to the current outbreak, the concept of zones of abundance stated by Cook indicated that the outbreak developed in a concentric manner with differential periods of occupation, which in turn could affect the control strategies commonly in use against this insect. Forest Sci. 29:715-725.
Pest populations are frequently regulated below their potential levels of abundance by natural enemies, host resistance, or other biological interactions. However, if these regulating processes operate imperfectly, or are intolerant to variations in pest density, then we may observe periodic outbreaks of the pest. In effect, intolerant regulating processes create thresholds separating distinct dynamic behaviors, usually referred to as endemic and epidemic behaviors. If threshold functions can be defined in terms of measurable system variables, they offer a powerful approach for evaluating the risk of epidemics in managed ecosystems. Methods for defining threshold functions and constructing risk decision models are discussed.
Analysis of rawfiber content of balsam fir, Abies balsamea (L.) Miller, needles in conjunction with laboratory-reared larvae of spruce budworm, Choristoneura fumiferana (Clemens), during the 1985 growing season indicated significant variations in the quality of food available for consumption by the insect. Two consecutive years of defoliation as well as poor drainage were directly related to higher rawfiber content of the current year's foliage which, in turn, caused a decrease in pupal weight, larval development rate, and survival.
Field trials with four types of pheromone traps and two types of commercial lures were evaluated to develop a population-monitoring technique for spruce bud worm, Choristoneura fumiferana (Clemens). When data were stratified by state or province, mean catch in several treatments (trap model/lure combinations) was significantly correlated with densities of larval stages L3-L4 (generations n and n + 1) and L2. Relationships varied among regions and hosts. There was no significant difference in moth catch between traps within five-trap clusters when traps were spaced 40 m apart. Within-cluster variation in trap catch was high; coefficients of variation averaged 35% for the covered funnel trap/Conrel lure treatment and 49% for the covered funnel trap/Hercon lure treatment. Application of pheromone-baited traps to monitor spruce budworm populations is discussed and recommendations are made to improve the monitoring system.
This paper explores the prospects for combining elements of the ecological and policy sciences to form a substantive and effective science of ecological policy design. This exploration is made through a case study whose specific focus is the management problem posed by competition between man and an insect (the spruce budworm, Choristoneura fumiferana) for utilization of coniferous forests in the Canadian Province of New Brunswick. We used this case study as a practical testing ground in which we examined the relative strengths, weaknesses, and complementarities of various aspects of the policy design process. Where existing approaches proved wanting, we sought to develop alternatives and to test them in turn. In particular, we used a combination of simulation modeling and topological approaches to analyze the space-time dynamics of this ecosystem under a variety of natural and managed conditions. Explicit consideration was given to the development of invalidation tests for establishing the limits of model credibility. An array of economic, social, and environmental indicators was generated by the model, enabling managers and policy makers to evaluate meaningfully the performance of the system under a variety of management proposals. Simplified versions of the models were constructed to accomodate several optimization procedures, including dynamic programming, which produced trial policies for a range of possible objectives. These trial policies were tested in the more complex model versions and heuristically modified in dialogue with New Brunswick's forest managers. We explored the role of utility functions for simplifying and contrasting policy performance measures, paying special attention to questions of time preferences and discounting. Finally, the study was shaped by a commitment to transfer the various models and policy design capabilities from their original academic setting to the desks and minds of the practicing managers and politicians. An array of workshops, model gaming sessions, and nontraditional communication formats was developed and tested in pursuit of this goal.This paper reports some specific management policies developed, and some general lessons for ecological policy design learned in the course of the study.
To successfully manage spruce budworm (Choristoneura fumiferana (Clemens)) populations, we need more knowledge about the insect's nutritional ecology. Here we address this issue by considering variations in host suitability for the budworm due to tree species, age, nutrient levels, defensive compounds, inducible defenses, and stress. Tree age has minor effects on foliar nutritional quality. Nitrogen levels (as an index of protein) in the diet appear to be the most significant factor affecting performance both in the field and laboratory. Tannins/phenolics, and supraoptimal levels of calcium in host foliage may interfere with the bioavailability of some minerals such as iron. Monoterpenes can be toxic at very high levels but otherwise seem to have minor negative effects on budworm performance. There is little evidence of short-term inducible defenses in fir foliage. Likewise, there is little evidence that drought stress makes fir trees more nutritionally favorable for spruce budworm. However, drought-induced flowering may be very important to larval survival. Neither is there evidence for substantial variation among trees in their inherent susceptibility to budworm. We hypothesize that variations in host-plant quality are less important in regulating budworm dynamics than other factors, such as the physical environment and natural enemies. However, spruce-budworm/host-plant phenological synchrony and host flowering status may be very important variables that deserve more research. Management of budworm will require more detailed knowledge about the entire complex of community factors that contribute to the increased likelihood of outbreak development.
The population dynamics of microparasites and their invertebrate hosts Synchronous fluctuations in spatially separated populations of cyclic forest insects
- R M Anderson
- R M May
- D A Barbour
Anderson, R.M.; May, R.M. 1981. The population dynamics of microparasites and their invertebrate hosts. Philo. Trans. Royal Soc. London 291: 451-524, Barbour, D.A. 1990. Synchronous fluctuations in spatially separated populations of cyclic forest insects. Pages 339- 346 in A.D. Watt, S.R. Leather, M.D. Hunter and N.A.C. Kidd (Eds.), Population dynamics of forest insects. Intercept. Andover, England
Staminate flowers and spruce budworm abundance
- Ha Bess
Bess, HA. 1946. Staminate flowers and spruce budworm abundance. Can. Dep. Agric. Bi-Monthly Prog. Rep. 2: 3-4
Trends in the frequency, extent and severity of spruce budworm outbreaks in eastern Canada The ecology of the eastern spruce budworm: A review and discussion. Pages 49-59 Recent r104 Chapter 8 advances in spruce budworms research
- Chron Blais
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