The fine‐scale population dynamics of spruce budworm: survival of early instars related to forest condition

Abstract

1. A lagged, density-dependent relationship between survival of early instars and host-tree condition is revealed during outbreaks of spruce budworm, Choristoneura fumiferana Clem. Persistent damage to hosts leads to deterioration of the stand.2. Resource limitation affects survival during early-instar dispersal of spruce budworm. Impediments to distinguishing these events with estimates of survival were overcome with a simple model that describes the dispersal and survival processes. The model was used to analyse a recent 15-year population series from Black Sturgeon Lake and two historical datasets from Green River, in Canada.3. Defoliation-induced damage to the trees resulted in increased losses of springemerging larvae that are dispersing in search of feeding sites. Losses were further exacerbated by biotic factors such as maternal fecundity, rates of infection by the pathogen, Nosema fumiferanae , and by weather-related effects on the foraging period.4. Survival of early-stage budworm larvae in persistent outbreaks declined and the likelihood of other density-related factors such as rate of mortality from natural enemies increased. These results may reconcile outstanding differences in interpretation of the role of the forest resource in spruce budworm population dynamics and point to a common process linking the dynamics of other well-known budworm species.

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

“…Eventually the negative feedback from damage will become more significant for these species as needles become sparse and needlemining more difficult, particularly for western spruce budworm (Nealis and Régnière 2009). It takes even longer for these negative feedbacks to affect spruce budworm where host foliage remains plentiful and palatable for several years and resource-related mortality does not become apparent until the vulnerable conifer component of the stand dies and becomes replaced by gaps and non-host species (Régnière and Nealis 2008). Thus delayed negative feedback from the host occurs in all species; it is just a question of how long it takes to become significant for population rates of change (Fig.…”

“…Field measurements show a high rate of loss of early-stage larvae in jack pine budworm and western spruce budworm (Nealis and Lomic 1994;Nealis and Régnière 2009). The comparable situation only occurs in spruce-fir forests once the stand has become thinned by prolonged defoliation and significant gaps develop that change the characteristics of the stand to one that resembles the more open condition of Douglas-fir and jack pine stands (Régnière and Nealis 2008).…”

“…The empirical evidence of large-scale outbreak dynamics of these species shows the same graded response in their defining parameters as the specific budworm-host ecological relationships summarised here. Spruce budworm, with its low frequency, long duration, and extensively synchronised cycles correlates to the slow negative feedbacks of the host to the effects of budworm density (Régnière and Nealis 2008). At the other extreme, the short-lived, relatively high-frequency, vaguely periodic and more spatially fragmented nature of jack pine budworm outbreaks correlates to the near immediate reduction of critical pollen cones with subsequent impact on jack pine budworm survival following the first episode of significant defoliation (Nealis and Lomic 1994;Nealis et al 2003).…”

“…The increasing negative, bottom-up feedbacks from feeding damage ultimately occurs in all budworm systems but at different temporal rates and spatial scales (Cooke et al 2007). Whereas the bottom-up feedback affects survival at the branch and tree levels within a few years for jack pine budworm (Nealis and Lomic 1994) and a little longer in western spruce budworm (Nealis and Régnière 2009), it does not become significant for survival of spruce budworm until the change in resources occurs at the stand level (Régnière and Nealis 2008) (Figs. 3, 5).…”

“…Continuous foraging during the spring emergence period may increase the likelihood that budworms will find fresh buds (McGugan 1954;Shepherd 1992;Nealis and Lomic 1994) but at the cost of an increased risk of dispersal at the stand level that also differs among systems (Régnière and Nealis 2008). Spruce budworm and twoyear cycle spruce budworm occupy boreal and cordilleran spruce-fir forests where their shadetolerant hosts characteristically develop full crowns forming dense stands with a high degree of canopy closure at maturity.…”