Temporal dynamics of spruce budworm (Choristoneura fumiferana) populations during the rising phase of an outbreak

Background/Question/Methods

The spruce budworm (Choristoneura fumiferana) is among the most destructive forest disturbances in Eastern Canada. Its periodic outbreaks cover several million hectares and cause extensive damage to economically important tree species. Outbreaks are typically studied using defoliation maps, and environmental conditions, such as temperature and hardwood species proportion, have been shown to influence defoliation patterns. Although spruce budworm outbreaks have been studied extensively, there is still no consensus on what trigger outbreaks start. A key phase is the transition of populations from endemic to epidemic densities; the rising phase. This stage is difficult to study using defoliation data, and has never been examined at large-scales using population data. Our objective is to develop a model that provides early forecast of increasing populations densities – prior to noticeable defoliation – in order to facilitate the use of proactive mitigation measures. As a first step, we modeled population growth rates in >1000 time-series of spruce budworm larvae counts and identified the impacts of environmental conditions. Population data were collected annually through the outbreak extent by the MFFP (Ministère de la Forêt, de la Faune et des Parcs). The dataset includes counts in both the in-between outbreaks (pre-2006) and the ongoing outbreak (post-2006).

Results/Conclusions

Our results show spatial variability in growth rates, with Northern locations exhibiting higher growth rates. These locations were associated with lower annual degree-day >5°C, lower proportion of hardwood species in the plot, and increased slope, which we identified as the main drivers of populations’ growth rates. Plots with a higher proportion of hardwood species are known to host a greater parasitoid diversity, which could explain its impact on populations’ growth rates. These results will be used to further develop a predictive model that can accurately predict when and where local spruce budworm populations are at risk of switching to epidemic densities. To provide adequate forecasts, a detailed understanding of populations dynamics between low and rising phases is necessary. Understanding how environmental conditions influence population dynamics over large areas is crucial to adapt our management strategies to a changing world.