Ecology of outbreak populations of the western spruce budworm

Abstract

We sampled outbreak populations of western spruce budworm, Choristoneura occidentalis (Lepidoptera: Tortricidae), between 1997 and 2016 in Douglas-fir forests in the interior of British Columbia, Canada. Annual rates of change in population densities were correlated with generation survival, modulated by egg recruitment via dispersal of moths. Most temporal variation in generation survival was the result of variation in survival of small, non-feeding larval stages. Survival rates of feeding larval and pupal stages determined the magnitude of generation survival but contributed little to temporal trend. Survival of small larvae was the product of density-related losses of larvae dispersing to and from hibernation sites and weather-related stress caused by warm temperatures in late summer and early spring. Overwinter survival improved in cooler years and at higher elevations. Survival of feeding larvae and pupae was associated with mortality by natural enemies, in particular a few species of dominant, ubiquitous endoparasitoids, and an episodic, virulent baculovirus. Recruitment of eggs indicated emigration of gravid moths independent of defoliation but proportional to density of local moths. During periods of increasing and decreasing population densities over a large outbreak area, a marked differential in adult densities among locations resulted in greater than expected per-capita egg recruitment to areas of relatively low density and vice versa, indicative of net exchange of gravid moths from high-to low-density populations. The result was homogenization of egg densities among locations, apparent synchrony of the outbreak at the large scale, and extended duration of the outbreak at some locations, despite declining generation survival. These results are compared with field studies of the closely related spruce budworm, Choristoneura fumiferana. We suggest our interpretation applies more generally and outbreaks of conifer-feeding budworms are maintained for several years by compensatory survival of sequential life-history stages and spatio-temporal smoothing of densities via dispersal of gravid moths. Parasitoids and pathogens can cause sudden declines in densities, but populations inevitably wane as a result of cumulative degradation of their resource. The interaction and temporal sequence of bottom-up and top-down factors explain the similarities and differences in outbreak characteristics within-and among-budworm species.

Warmer temperatures reduce the transmission of a virus in a gregarious forest insect

“…The winter moth ( Operophtera brumata ) has moved further north and west in Fennoscandia as the climate has warmed (Jepsen et al, 2013; Vindstad et al, 2022). Nealis and Régnière (2021) suggest that climate has caused the expansion of suitable habitat for western spruce budworm, Choristoneura occidentalis , in the Pacific Northwest, leading to prolonged and widely distributed population outbreaks. Similar range expansions have occurred with climate warming in the eastern spruce budworm C. fumiferana , and this is threatening the boreal forest (Pureswaran et al, 2015).…”

Abundance and Distribution of Foliage on Balsam Fir and White Spruce in Reference to Spruce Budworm Ecology and Absolute Population Density Estimation

“…A first sample was taken in June 1965, as the stand that had been damaged by spruce budworm during the 1950's was regenerating. This sample consisted of 42 young (10-15 years), open-grown balsam fir (23) and white spruce trees (19) with DBH ≤ 10 cm. The DBH and number of live nodes (annual stem growth) of each tree were noted.…”

“…The density of spruce budworm is most often expressed in relative terms, per branch, m 2 or kg of foliage. In recent work [23,31], the density expression used has been "insects per bud or shoot", which is more closely related to the insect's food resource. It is therefore useful to better understand the distribution of buds on host foliage over the live crown of young and mature, open-grown and closed-canopy trees of the main budworm host species.…”

“…Population density expressions derived from such samples are relative to the amount of foliage examined: buds or current shoots, branch surface area or foliage weight [19]. These relative density estimates are useful to forecast defoliation, or to measure density differences between locations or changes in the course of one or a few growing seasons [6,[21][22][23]. However, because they are always expressed relative to the quantity of foliage examined, they remain relative density estimates.…”

“…However, these relationships do not take growing conditions such as open versus closed canopies, and height-growth rate into account. Although Kleinschmidt et al [30] examined the distribution of foliage in the live crown of balsam fir, they did not provide relationships between tree size, foliage weight or number of growing shoots, common expressions of spruce budworm density [19,23,31]. No information on the relationship between tree size and the amount of spruce budworm resource is available for white spruce.…”