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.

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Demographic data from a 15-year outbreak of the spruce budworm, Choristoneura fumiferana (Clem.) (Lepidoptera: Tortricidae), in a boreal mixedwood forest in Ontario, Canada, are used to interpret stand-level ecological disturbance in terms of susceptibility and vulnerability (mortality) of balsam fir (Abies balsamea (L.) Mill.), white spruce (Picea glauca (Moench) Voss), and black spruce (Picea mariana (Mill.) BSP). All three host-tree species are highly susceptible for oviposition by the spruce budworm and all are suitable for completion of the budworm life cycle. Host-related differences in susceptibility arise from the degree of synchrony between spruce budworm phenology during the feeding stages and host-tree phenology. Spruce budworm density was highest on white spruce throughout the budworm's life cycle and over the course of the outbreak, but more rapid flushing and growth of current-year buds in white spruce reduced damage relative to that on balsam fir. Conversely, later flushing of current-year buds on black spruce led to a reduction in budworm density early in the season and a corresponding reduction in defoliation. The combination of high budworm densities and severe defoliation caused mortality first on balsam fir. By the end of the outbreak, 89% of the balsam fir component >10 cm DBH was eliminated compared with 49% of the white spruce in the same size class. The lower susceptibility of black spruce resulted in survival of all but the smallest size classes of that species. Nonhost species such as trembling aspen (Populus tremuloides Michx.) nearly doubled their basal area during the outbreak. The results link processes inherent in the insect–host relationship with the population ecology of the insect and the disturbance ecology of the forest.

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. Fig. 2. Density of spruce budworm populations ( n , • ) and survival in rearing ( φ t , ○ ) as functions of average instar ( ␣ t ) in the four study sites. Average instar ␣ = 1: egg stage; ␣ = 2: overwintered larvae. Lines are polynomial regressions of n t vs. ␣ ^ t (Table 2 ). 468 J. Régnière and V. G. Nealis

Abstract-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.

Baited pitfall traps were used to sample the Scarabaeinae dung beetle fauna in six habitats in South Texas. Habitats were defined according to their soil and vegetative cover characteristics. Most of the 19 scarab species collected displayed marked associations with a particular soil type (sand or clay) and (or) cover (shade or open). The result is the existence of distinct communities of coprophagous beetles, which differ in their species composition, species-abundance relations, and efficiency of dung removal. A discussion of the evolution of these habitat associations and the ecological implications for pasture ecosystems is included.

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