Do natural enemies explain fluctuations in low‐density spruce budworm populations?

Abstract

Understanding the causal pathways through which forest insect outbreaks are triggered is important for resource managers. However, detailed population dynamics studies are hard to conduct in low-density, pre-outbreak populations because the insects are difficult to sample in sufficient numbers. Using laboratory-raised larvae installed in the field across a 1,000 km east-west gradient in Québec (Canada) over an 11-yr period, we examined if parasitism and predation were likely to explain fluctuations in low-density spruce budworm (Choristoneura fumiferana; SBW) populations. Parasitism rates by the two main larval parasitoid species, Elachertus cacoeciae and Tranosema rostrale, peaked during different years. This suggests that temporal fluctuations in overall parasitism were partly buffered by compensatory dynamics among parasitoid species. Still, spatial covariance analyses indicate that the residual interannual variation in parasitism rates was substantial and correlated over large distances (up to 700 km). On the other hand, interannual variation in predation rates was not spatially correlated. Piecewise structural equation models indicate that temporal variation in parasitism and predation does not influence temporal variation in wild SBW abundance. Spatially, however, SBWs installed in warmer locations tended to show higher parasitism rates, and these higher rates correlated with lower wild SBW population levels. Overall, the results indicate that large-scale drops in parasitism occur and could potentially contribute to SBW population increases. However, during the period covered by this study, other factors such as direct effects of weather on SBW larval development or indirect effects through host tree physiology or phenology were more likely to explain large-scale variation in wild SBW populations.

Dynamics and Management of Rising Outbreak Spruce Budworm Populations

“…It therefore appears that endemic spruce budworm populations are kept in check by generalist parasitoids. Contrary to Theory 2 (Figure 1), our observations do not support the idea of a gradual relaxation of their impact over time, and thus populations do not rise after a gradual drop of mortality from natural enemies (JR, unpublished data; this study; [24]). But if budworm outbreaks do not occur as a result of a cyclical (regular) change in the impact of natural enemies, but rather in response to a sudden change in survival or apparent fecundity that shifts populations beyond the Allee threshold, how can a more-or-less regular outbreak cycle materialize?…”

“…We do not have an answer to this question. Fluctuations in the impact of natural enemies may be involved under some circumstances, but not generally [24]. It is possible that the likelihood of such a sudden shift in survival increases over time as forest stands recovering from a previous outbreak age and host-tree masting becomes more prevalent over a landscape [25]; masting events occur synchronously over rather large areas.…”

Low Non-structural Carbon Accumulation in Spring Reduces Growth and Increases Mortality in Conifers Defoliated by Spruce Budworm

“…During the 20th century, there were three SBW outbreaks (Blais, 1962;Morin, 1994). Since 2006, a new outbreak has been ongoing, affecting more than of 8 M ha with defoliation concentrated in stands dominated by balsam fir and black spruce (Boulanger et al, 2016(Boulanger et al, , 2017Bouchard et al, 2018). SBW lay eggs mainly in upper crown of trees in July.…”

Modeling Climatic Influences on Three Parasitoids of Low-Density Spruce Budworm Populations. Part 1: Tranosema rostrale (Hymenoptera: Ichneumonidae)

“…This spatial distribution of performance reflects the known decreasing survival and fecundity of the parasitoid with increasing temperatures [35], explained as an interaction between SBW's immune system and the parasitoid's polydnavirus [58,59]. Lower rates of SBW parasitism by T. rostrale occur in sites where temperatures are higher [22]. However, results of the present study indicate that the complex spatial patterns of parasitoid performance result from interactions of survival from egg to adult, realized fecundity, voltinism, and synchrony with the overwintering host.…”

“…Spatial and temporal differences in SBW parasitism by T. rostrale have been described for several sites in northeastern North America and have generally been linked to SBW population densities. The impact of T. rostrale decreases as SBW populations increase [4,23] and seems negatively affected by the abundance of competing parasitoid species [22,32,56]. In the present model, we used a simple description of density dependence in parasitoid attack, and there was no consideration of either intraor interspecific competition.…”