V.G. Nealis

Potential for range expansion of mountain pine beetle into the boreal forest of North America.

Abstract The potential for mountain pine beetle, Dendroctonus ponderosae Hopkins (Coleoptera: Curculionidae: Scolytinae), to expand its historical range in North America from west of the continental divide into the eastern boreal forest was assessed on the basis of analyses of the effects of climate and weather on brood development and survival, and key aspects of the interaction of mountain pine beetle with its hosts and associated organisms. Variation in climate suitability and high host susceptibility in the boreal forest create a finite risk of establishment and local persistence of low-level mountain pine beetle populations outside their historical range. Eventually, these populations could become widespread and cause epidemic infestations, creating an ecological pathway eastward through the boreal forest. Such infestations would reduce the commercial value of forests and impose an additional disturbance on native ecological systems.

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

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.

Ecological mechanisms of population change during outbreaks of the spruce budworm

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.

15 – Insect Defoliators as Periodic Disturbances in Northern Forest Ecosystems

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.

A lagged, density-dependent relationship between jack pine budworm Choristoneura pinus pinus and its host tree Pinus banksiana.

Abstract. 1. Survival of newly emerged jack pine budworm Choristoneura pinus pinus is related to the density of available pollen cones (microsporangiate strobili) produced by its host tree, jack pine Pinus banksiana.

The phenological window for western spruce budworm: seasonal decline in resource quality

1 Western spruce budworm Choristoneura occidentalis Free. (Lepidoptera: Tortricidae) emerge in the spring before budburst and then face a rapidly deteriorating host quality each season. 2 Measures of fitness, survival and fecundity, were made on cohorts of final‐instar spruce budworms deployed on host trees at several times during the season in four field locations in coastal and interior British Columbia, Canada. 3 Survival and fecundity were strongly correlated throughout the season and varied as much as four‐fold from maxima at mid‐season to minima at the end of the season. 4 Fitness values overall were greatest in the coastal compared with interior locations. Among interior locations, fitness was greatest at the highest elevation and least at the lowest elevation. Both cohort and sample‐based estimates of survival of wild, final‐instar budworms were relatively high in these outbreak populations. 5 The influence of the phenological window and degree of synchrony with the host plant on herbivore abundance often depends on other processes affecting population rates of change.

Risk of dispersal in western spruce budworm

1 Western spruce budworm Choristoneura occidentalis Free. larvae emerge in the spring before buds have expanded and spend a variable period of time foraging on branches and mining needles.

Two sides of a coin: host plant synchrony fitness trade‐offs in the population dynamics of the western spruce budworm

Conifer‐feeding budworms emerge from overwintering sites as small larvae in early spring, several days before budburst, and mine old needles. These early‐emerging larvae suffer considerable mortality during this foraging period as they disperse in search of available, current‐year buds. Once buds flush, surviving budworms construct feeding shelters and must complete maturation before fresh host foliage senesces and lignifies later in the summer. Late‐developing larvae suffer greater mortality and survivors have lower fecundity when feeding on older foliage. Thus, there is a seasonal trade‐off in fitness associated with host synchrony: early‐emerging budworms have a greater risk of mortality during spring dispersal but gain better access to the most nutritious foliage, while, on the other hand, late‐emerging larvae incur a lower risk during the initial foraging period but must contend with rapidly diminishing resource quality at the end of the feeding period. We investigate the balance that results from these early‐season and late‐season synchrony fitness trade‐offs using the concept of the phenological window. Parameters associated with the variation in the phenological window are used to estimate generational fitness as a function of host‐plant synchrony. Because defoliation modifies these relationships, it is also included in the analysis. We show that fitness trade‐offs characterizing the phenological window result in a robust synchrony relationship between budworm and host plant over a wide geographic range in southern British Columbia, Canada.

Why western spruce budworms travel so far for the winter

1. Conifer‐feeding budworms (Choristoneura) hibernate in sheltered locations on their host trees from late summer of 1 year to spring of the next. During this period, they do not feed but rely on sustenance provided in the egg. Overwinter survival is dependent on the rate of consumption of these limited reserves.