Publications by B.J. Bentz

Prepupal diapause and instar iv developmental rates of the spruce beetle, Dendroctonus rufipennis (Coleoptera: Curculionidae, Scolytinae).

Fri, 03 May 2013

The spruce beetle, Dendroctonus rufipennis (Kirby), is an important mortality agent of native spruces throughout North America. The life-cycle duration of this species varies from 1 to 3 years depending temperature. The univoltine cycle (one generation per year) is thought to maximize outbreak risk and accelerate host mortality in established outbreaks. Prepupal diapause is associated with the semivoltine cycle (one generation per 2 years) and we investigated thermal conditions that result in diapause induction. Preliminary experiments used respirometry in an attempt to distinguish the diapause state of experimental insects but the technique was apparently confounded by low respiration before and during pupation, regardless of diapause status. Therefore, diapause induction was deduced using developmental delays. The observed developmental response was not a "switch", with developmental delay either present or absent, but instead varied continuously. We found that temperatures <15°C from instar III through mid-instar IV were associated with developmental delays beyond that expected from cool temperatures. Moreover, the duration of exposure to cool temperatures was important in determining the degree of developmental delay. Small, if any, delays were observed if the cumulative exposure to <15°C was <20 d whereas >40 d cumulative exposure was associated with distinct developmental suppression. Intermediate exposure to cool temperatures resulted in minor developmental delays. We used our results to parameterize a maximum likelihood estimation model of temperature-dependent instar IV developmental rates, including the effect of diapause. This model can be included as part of a spruce beetle phenology model for predicting population dynamics.

Effects of temperature on development, survival and reproduction of insects: Experimental design, data analysis and modeling.

Tue, 24 Jul 2012

The developmental response of insects to temperature is important in understanding the ecology of insect life histories. Temperature-dependent phenology models permit examination of the impacts of temperature on the geographical distributions, population dynamics and management of insects. The measurement of insect developmental, survival and reproductive responses to temperature poses practical challenges because of their modality, variability among individuals and high mortality near the lower and upper threshold temperatures. We address this challenge with an integrated approach to the design of experiments and analysis of data based on maximum likelihood. This approach expands, simplifies and unifies the analysis of laboratory data parameterizing the thermal responses of insects in particular and poikilotherms in general. This approach allows the use of censored observations (records of surviving individuals that have not completed development after a certain time) and accommodates observations from temperature transfer treatments in which individuals pass only a portion of their development at an extreme (near-threshold) temperature and are then placed in optimal conditions to complete their development with a higher rate of survival. Results obtained from this approach are directly applicable to individual-based modeling of insect development, survival and reproduction with respect to temperature. This approach makes possible the development of process-based phenology models that are based on optimal use of available information, and will aid in the development of powerful tools for analyzing eruptive insect population behavior and response to changing climatic conditions.

Climate change and bark beetles of the Western United States and Canada: direct and indirect effects

Mon, 13 Sep 2010

Climatic changes are predicted to significantly affect the frequency and severity of disturbances that shape forest ecosystems. We provide a synthesis of climate change effects on native bark beetles, important mortality agents of conifers in western North America. Because of differences in temperature-dependent life-history strategies, including cold-induced mortality and developmental timing, responses to warming will differ among and within bark beetle species. The success of bark beetle populations will also be influenced indirectly by the effects of climate on community associates and host-tree vigor, although little information is available to quantify these relationships. We used available population models and climate forecasts to explore the responses of two eruptive bark beetle species. Based on projected warming, increases in thermal regimes conducive to population success are predicted for Dendroctonus rufipennis (Kirby) and Dendroctonus ponderosae Hopkins, although there is considerable spatial and temporal variability. These predictions from population models suggest a movement of temperature suitability to higher latitudes and elevations and identify regions with a high potential for bark beetle outbreaks and associated tree mortality in the coming century.

The Status of Our Scientific Understanding of Lodgepole Pine and Mountain Pine Beetles – A Focus on Forest Ecology and Fire Behavior

Mon, 16 Nov 2009

A synthesis of our current knowledge about the effects of the mountain pine beetle epidemic on lodgepole pine forests and fire behavior, with a geographic focus on Colorado and southern Wyoming.

Cross-scale drivers of natural disturbances prone to anthropogenic amplification: The dynamics of bark beetle eruptions

Wed, 04 Jun 2008

Biome-scale disturbances by eruptive herbivores provide valuable insights into species interactions, ecosystem function, and impacts of global change. We present a conceptual framework using one system as a model, emphasizing interactions across levels of biological hierarchy and spatiotemporal scales. Bark beetles are major natural disturbance agents in western North American forests. However, recent bark beetle population eruptions have exceeded the frequencies, impacts, and ranges documented during the previous 125 years. Extensive host abundance and susceptibility, concentrated beetle density, favorable weather, optimal symbiotic associations, and escape from natural enemies must occur jointly for beetles to surpass a series of thresholds and exert widespread disturbance. Opposing feedbacks determine qualitatively distinct outcomes at junctures at the biochemical through landscape levels. Eruptions occur when key thresholds are surpassed, prior constraints cease to exert influence, and positive feedbacks amplify across scales. These dynamics are bidirectional, as landscape features influence how lower-scale processes are amplified or buffered. Climate change and reduced habitat heterogeneity increase the likelihood that key thresholds will be exceeded, and may cause fundamental regime shifts. Systems in which endogenous feedbacks can dominate after external forces foster the initial breach of thresholds appear particularly sensitive to anthropogenic perturbations.

Modeling cold tolerance in the mountain pine beetle, Dendroctonus ponderosae

Thu, 31 May 2007

Cold-induced mortality is a key factor driving mountain pine beetle, Dendroctonus ponderosae, population dynamics. In this species, the supercooling point (SCP) is representative of mortality induced by acute cold exposure. Mountain pine beetle SCP and associated cold-induced mortality fluctuate throughout a generation, with the highest SCPs prior to and following winter. Using observed SCPs of field-collected D. ponderosae larvae throughout the developmental season and associated phloem temperatures, we developed a mechanistic model that describes the SCP distribution of a population as a function of daily changes in the temperature-dependent processes leading to gain and loss of cold tolerance. It is based on the changing proportion of individuals in three states: (1) a non coldhardened, feeding state, (2) an intermediate state in which insects have ceased feeding, voided their gut content and eliminated as many ice-nucleating agents as possible from the body, and (3) a fully cold-hardened state where insects have accumulated a maximum concentration of cryoprotectants (e.g. glycerol). Shifts in the proportion of individuals in each state occur in response to the driving variables influencing the opposite rates of gain and loss of cold hardening. The level of cold-induced mortality predicted by the model and its relation to extreme winter temperature is in good agreement with a range of field and laboratory observations. Our model predicts that cold tolerance of D. ponderosae varies within a season, among seasons, and among geographic locations depending on local climate. This variability is an emergent property of the model, and has important implications for understanding the insect’s response to seasonal fluctuations in temperature, as well as population response to climate change. Because cold-induced mortality is but one of several major influences of climate on D. ponderosae population dynamics, we suggest that this model be integrated with others simulating the insect’s biology.

Impacts of Climate Change on Range Expansion by the Mountain Pine Beetle

Fri, 01 Dec 2006

The current latitudinal and elevational range of mountain pine beetle (MPB) is not limited by available hosts. Instead, its potential to expand north and east has been restricted by climatic conditions unfavorable for brood development. We combined a model of the impact of climatic conditions on the establishment and persistence of MPB populations with a spatially explicit, climate-driven simulation tool. Historic weather records were used to produce maps of the distribution of past climatically suitable habitats for MPB in British Columbia. Overlays of annual MPB occurrence on these maps were used to determine if the beetle has expanded its range in recent years due to changing climate. An examination of the distribution of climatically suitable habitats in 10-year increments derived from climate normals (1921-1950 to 1971-2000) clearly shows an increase in the range of benign habitats. Furthermore, an increase (at an increasing rate) in the number of infestations since 1970 in formerly climatically unsuitable habitats indicates that MPB populations have expanded into these new areas. The potential for additional range expansion by MPB under continued global warming was assessed from projections derived from the CGCM1 global circulation model and a conservative forcing scenario equivalent to a doubling of CO2 (relative to the 1980s) by approximately 2050. Predicted weather conditions were combined with the climatic suitability model to examine the distribution of benign habitats from 1981-2010 to 1941-2070 for all of Canada. The area of climatically suitable habitats is anticipated to continue to increase within the historic range of MPB. Moreover, much of the boreal forest will become climatically available to the beetle in the near future. Since jack pine is a viable host for MPB and a major component of the boreal forest, continued eastward expansion by MPB is probable.

Augmenting the existing survey hierarchy for mountain pine beetle red-attack damage with satellite remotely sensed data

Wed, 19 Jul 2006

Estimates of the location and extent of the red-attack stage of mountain pine beetle (Dendroctonus ponderosae Hopkins) infestations are critical for forest management. The degree of spatial and temporal precision required for these estimates varies according to management objectives and the nature of the infestation. This paper outlines the range of information requirements associated with mountain pine beetle infestations, from the perspectives of forest inventory, planning, and modeling. Current methods used to detect and map red-attack damage form a hierarchy of increasingly detailed data sources. The capability of satellite-based remotely sensed data to integrate into this hierarchy and provide data that is complementary to existing survey methods is presented, with specific examples using medium (Landsat) and high (IKONOS) spatial resolution imagery. The importance of matching the information requirement to the appropriate data source is emphasized as a means to reduce the overhead associated with data collection and processing.

Estimating the probability of mountain pine beetle red-attack damage

Wed, 12 Apr 2006

Accurate spatial information on the location and extent of mountain pine beetle infestation is critical for the planning of mitigation and treatment activities. Areas of mixed forest and variable terrain present unique challenges for the detection and mapping of mountain pine beetle red-attack damage, as red-attack has a more heterogeneous distribution under these conditions. In this study, mountain pine beetle red-attack damage was detected and mapped using a logistic regression approach with a forward stepwise selection process and a set of calibration data representing samples of red-attack and non-attack from the study area. Variables that were considered for inclusion in the model were the enhanced wetness difference index (EWDI) derived from a time series of Landsat remotely sensed imagery, elevation, slope, and solar radiation (direct, diffuse, and global). The output from the logistic regression was a continuous probability surface, which indicated the likelihood of red-attack damage. Independent validation data were used to assess the accuracy of the resulting models. The final model predicted red-attack damage with an accuracy of 86%. These results indicate that for this particular site, with mixed forest stands and variable terrain, remotely sensed and ancillary spatial data can be combined, through logistic regression, to create a mountain pine beetle red-attack likelihood surface that accurately identifies damaged forest stands. The use of a probabilistic approach reduces dependence upon the definition of change by the application of thresholds (upper and lower bounds of change) at the image processing stage. Rather, a change layer is generated that may be interpreted liberally or conservatively, depending on the information needs of the end user.

Augmenting the existing survey hierarchy for mountain pine beetle red-attack damage with satellite remotely sensed data

Wed, 29 Mar 2006

Estimates of the location and extent of the red-attack stage of mountain pine beetle (Dendroctonus ponderosae Hopkins) infestations are critical for forest management. The degree of spatial and temporal precision required for these estimates varies according to the management objectives and the nature of the infestation. This paper outlines the range of information requirements associated with mountain pine beetle infestations, from the perspectives of forest inventory, planning, and modeling.Current methods used to detect and map red-attack damage form a hierarchy of increasingly detailed data sources. The capability of satellite-based remotely sensed data to integrate into this hierarchy and provide data that is complementary to existing survey methods is presented, with specific examples using medium (Landsat) and high (IKONOS) spatial resolution imagery. The importance of matching the information requirement to the appropriate data source is emphasized as a means to reduce the overhead associated with data collection and processing.

Detection and mapping of mountain pine beetle red attack: matching information needs with appropriate remotely sensed data

Thu, 10 Feb 2005

Estimates of the location and extent of the red attack stage of mountain pine beetle (Dentroctonus ponderosae Hopkins) infestations are critical for forest management. The degree of spatial and temporal precision required for these estimates varies according to the management objectives and the nature of the infestation. This paper outlines a hierarchy of information requirements, from the perspectives of forest inventory, planning, and modeling. The capability of remotely sensed data to support these information requirements is presented, with specific focus on medium (Landsat) and high (IKONOS, Quickbird) spatial resolution imagery. The importance of matching the information requirement to the appropriate data source is emphasized as a means to reduce the overhead associated with data collection and processing.

Evaluating satellite imagery for estimating mountain pine beetle-caused lodgepole pine mortality: current status

Thu, 25 Nov 2004

Spatial accuracy in the detection and monitoring of mountain pine beetle populations is an important aspect of both forest research and management. Using ground-collected data, classification models to predict mountain pine beetle-caused lodgepole pine mortality were developed for Landsat TM, ETM+, and IKONOS imagery. Our results suggest that low-resolution imagery such as Landsat TM (30 m) is not suitable for detection of endemic level populations of mountain pine beetle. However, good results were obtained for pixels with groups of red beetle-killed lodgepole pine (> 25 trees killed per 30-m pixel), implying that Landsat imagery is most suited to detection of populations at the building or epidemic phase. Preliminary results using high resolution IKONOS imagery (4 m) suggest that detection of individual or small groups of red beetle-killed lodgepole pine can be accomplished with a relatively high accuracy.