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.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

Predicting insect distributions under climate change from physiological responses: spruce budworm as an example

“…Much has been written about the potential effects of weather factors (temperature, precipitation, wind) on spruce budworm population dynamics (Ives 1974;Royama 1978Royama , 1984Régnière and Nealis 2007). While of these factors play a role in determining the severity and duration of outbreaks (Gray 2008) as well as their regional synchrony (Williams and Liebhold 2000;Royama 2005), temperature is the one factor that has the most profound influence and is best understood.…”

Population studies of tree-defoliating insects in Canada: a century in review

“…Broadly speaking, we would highlight several practical advancements arising from Canadian population research for outbreak defoliators. Much of the research to date draws attention to the predominance of natural enemies in regulating populations, although recent work has reintroduced the potential importance of host quality as an indirect factor influencing population decline (e.g., Régnière and Nealis 2007;Moreau and Quiring 2011). This insight has contributed to several striking examples of successful biological control for invasive species (e.g., winter moth and European spruce sawfly) and to some extent native pests (e.g., larch sawfly and balsam fir sawfly).…”

Comparative ecology of conifer-feeding spruce budworms (Lepidoptera: Tortricidae)

“…During the last outbreak, observed defoliation in Canada began and ended in regions east of −80°longitude 10 years before these same events further west ( Fig. 1A, B; Régnière and Nealis 2007). Synchrony in outbreaks of western spruce budworm is limited to smaller geographic areas (Alfaro et al 2014) with occasional overlap among adjacent areas otherwise distinct Lynch 1989, 1993;Ryerson et al 2003).…”

“…But is it simply food supply? Certainly, foliage depletion can reduce survival and fecundity by depriving larvae of food but these are transient, first-order effects not associated with population collapse (Morris 1963c;Royama 1984;Nealis and Régnière 2004a;Régnière and Nealis 2007).…”