Abstract
The sudden interruption of recurring larch budmoth (LBM; Zeiraphera diniana or griseana Gn.) outbreaks across the European Alps after 1982 was surprising, because populations had regularly oscillated every 8–9 years for the past 1200 years or more. Although ecophysiological evidence was limited and underlying processes remained uncertain, climate change has been indicated as a possible driver of this disruption. An unexpected, recent return of LBM population peaks in 2017 and 2018 provides insight into this insect’s climate sensitivity. Here, we combine meteorological and dendrochronological data to explore the influence of temperature variation and atmospheric circulation on cyclic LBM outbreaks since the early 1950s. Anomalous cold European winters, associated with a persistent negative phase of the North Atlantic Oscillation, coincide with four consecutive epidemics between 1953 and 1982, and any of three warming-induced mechanisms could explain the system’s failure thereafter: (1) high egg mortality, (2) asynchrony between egg hatch and foliage growth, and (3) upward shifts of outbreak epicentres. In demonstrating that LBM populations continued to oscillate every 8–9 years at sub-outbreak levels, this study emphasizes the relevance of winter temperatures on trophic interactions between insects and their host trees, as well as the importance of separating natural from anthropogenic climate forcing on population behaviour.
Highlights
Periodic larch budmoth (LBM; Zeiraphera diniana or griseana Gn.) outbreaks are a classic example of population cycles (Baltensweiler and Rubli 1999), with densities regularly oscillating every 8–9 years, ranging from ~ 1 to 20,000 larvae per host tree (Larix decidua Mill.)
We argue that even though widespread insect defoliation of subalpine larch trees across the European Alps has been absent for nearly 4 decades, LBM populations in this region most likely continued to oscillate every 8–9 years at sub-outbreak levels
The geographical extent of defoliation during the recent LBM outbreak is documented in the eastern (Grisons) and western (Valais) Swiss Alps (Figs. 1, 2a–i), though not yet reconstructed from tree-ring width chronologies (Fig. 2j–l)
Summary
Periodic larch budmoth (LBM; Zeiraphera diniana or griseana Gn.) outbreaks are a classic example of population cycles (Baltensweiler and Rubli 1999), with densities regularly oscillating every 8–9 years, ranging from ~ 1 to 20,000 larvae per host tree (Larix decidua Mill.). While the hiatus of three Alpine-wide outbreaks from circa 1990 to 2010 may have afforded aesthetic benefits to residents and possibly even benefitted tourism (Wermelinger et al 2018), the system’s failure has been interpreted as an example of an extraordinary breakdown of ecological behaviour in response to global warming (Esper et al 2007; Ims et al 2008) It remains unclear how, if at all, climate affects the intensity, frequency, and persistence of cyclic LBM population outbreaks. Despite numerous studies on a variety of biotic and abiotic factors that potentially influence the LBM system (Büntgen et al 2009; Hartl-Meier et al 2017; Johnson et al 2010; Kress et al 2009; Nola et al 2006; Rolland et al 2001; Saulnier et al 2017), the identity of climatic controls on the sudden ‘collapse’ of widespread LBM outbreaks after 1982 remains a mystery
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