Recent climate change strongly impacted the population dynamic of a North American insect pest species

Climate change is redefining the dynamics of forest ecosystems globally, particularly through its impact on forest pest populations such as the spruce budworm (SBW, Choristoneura fumiferana [Clem.]), a major defoliator in North American boreal forests. This study investigates the shifts in the population dynamics of spruce budworm across its range in response to recent climate change. We used a process-based, temperature-dependent ecophysiological model combined with the ERA5 reanalysis to assess changes in SBW phenology, reproduction rate, winter survival, and population growth rates from 1950 to 2022 across North America. Our findings demonstrate a pronounced northward expansion of suitable climate conditions for SBW, accompanied by earlier phenological events and increased reproduction rates in northern regions. Conversely, the southern parts of its range are experiencing increased winter mortality due to warmer temperatures. This study highlights the significant impact of elevated temperatures, particularly during critical developmental windows such as spring and summer, which are pivotal for spruce budworm survival and reproduction. Additionally, our results reveal that the observed shifts in pest dynamics are more strongly driven by climate change than by changes in landscape composition and structure. We estimated that suitable growth rates have shifted northward by over 68 km on average, but this shift reached more than 200 km in the easternmost portions of its range. Climate-induced shift in suitable conditions for SBW underscores the need for adaptive forest management strategies that consider the rapid ecological changes and the potential for increased forest vulnerability due to climatic and biotic stressors. This study provides vital insights that can inform adaptive management, ensuring the sustainability of forest ecosystems in the face of ongoing climate change.

Methods
The study employed a detailed individual-based spruce budworm (SBW) model to simulate development, survival, and reproduction rates under varying temperature conditions. This model integrates laboratory-derived development rates for SBW life stages and uses a non-linear development summation approach. Simulations, performed in BioSIM 11, utilized high-resolution ERA5 reanalysis temperature data from 1950 to 2022, spanning the United States and Canada. The model provided outputs on larval and adult phenology, reproduction, winter survival, and annual population growth rates. These were further analyzed using statistical methods to assess spatial and temporal trends in SBW ecophysiological parameters, focusing on the impacts of recent climate change.

Additionally, changes in landscape vulnerability were assessed using historical forest inventory data from Quebec’s commercial forests, which documented forest composition and age over multiple decades. Trends in ecophysiological parameters were compared with alterations in forest landscapes across areas heavily defoliated during the last two major SBW outbreaks (1967–1991 and 2007–2022). This comparison highlighted the relative influence of climate and forest changes on SBW dynamics, providing insights into spatial shifts in vulnerability and suitability for SBW reproduction and survival under changing environmental conditions.