Early 20<sup>th</sup> Century Climate-Driven Shift in the Dynamics of Forest Tent Caterpillar Outbreaks

Abstract

Using tree-ring analysis, we show that the dynamics of forest tent caterpillar [Malacosoma disstria (Hbn.)] outbreaks in Alberta, Canada shifted at the turn of the 20 th century from cyclic, synchronous behaviour 1850-1910 to complex, asynchronous behavior 1910-1993. This shift in dynamics coincided with the emergence in 1910 of a latitudinal gradient in outbreak stability and periodicity reflecting a similar gradient in the periodicity of winter temperatures. We postulate that the synchronizing strength of winter temperatures has diminished as a result of climate warming, and that any synchronizing strength due to inter-population migration has been superseded by regionalized patterns of periodic forcing caused by weak low-frequency variability in winter temperatures. We speculate that a decrease in polar vorticity at the start of the 20 th century led to increased meridional jet stream flow and more frequent arctic weather anomalies, resulting in a loss of synchronous decadal periodicity in outbreak occurrence. These changes in insect disturbance probabilities, including rising uncertainty, have profound consequences for forest disturbance risk management.

Host association, environment, and geography underlie genomic differentiation in a major forest pest

“…Specifically, we have highlighted significant effects of summer temperatures and length of the growing season on genomic differentiation in M. disstria , which will help outbreak modelling efforts focus on a smaller subset of environmental parameters to explain regional variation in outbreak duration and synchronicity. This is particularly relevant for northern populations, which are expected to experience rapid shifts in local environmental conditions within the coming decades (Cooke & Roland, 2018 ; Schwartzberg et al, 2014 ). Finally, identifying specific genomic regions implicated in local adaptation will be an important next step to understanding specific functional traits that are subjected to environmental and host‐associated selection and mapping their distribution across the species' range.…”

Forest tent caterpillar (Lepidoptera: Lasiocampidae) across Canada, 1938–2001: I. Periodic outbreaks; episodic impacts

“…After 1995, survey responsibility in Canada was transferred to the appropriate provincial authority, and survey methods began to change, especially after 2001, with the advent of modern digital technologies, including global positioning systems and recording tablets. These records, up to 2001, have previously been analysed for Québec (Cooke and Lorenzetti 2006), for Ontario and Québec combined (Cooke et al 2012), and for Alberta (Cooke and Roland 2018), using formal pattern analysis methods that are objective and systematic; however, the full set of interprovincial data has never been collated and considered as a whole across Canada, from coast to coast.…”

Effects of larval host and natural microsporidian infection on adult life history traits of the forest tent caterpillar (Lepidoptera: Lasiocampidae)

“…The FTC is an efficient forager (Fitzgerald 1995), and larval feeding at outbreak densities can have economic and ecological consequences throughout its geographic range. Outbreak dynamics in FTC have been linked to numerous factors, including forest fragmentation (Roland 1993) and heterogeneity (Cooke and Roland 2000), climate (Cooke and Roland 2018), parasitism (Parry 1995), genetics (Miller 1996), host phytochemistry and budbreak phenology (Donaldson and Lindroth 2008), and pathogens (Fitzgerald 1995). Recent studies suggest FTC population cycling and outbreak dynamics may shift as a result of climate change (Dukes et al 2009, Uelmen et al 2016, Cooke and Roland 2018, and therefore understanding and identifying the driving forces behind its cyclical population dynamics is critical.…”