Three centuries of insect outbreaks across the European Alps.

Knowledge of the persistence of regular larch budmoth outbreaks is limited in space and time. Although dendrochronological methods have been used to reconstruct insect outbreaks, their presence may be obscured by climatic influences. More than 5000 tree-ring series from 70 larch host and 73 spruce nonhost sites within the European Alps and Tatra Mountains were compiled. Site-specific assessment of growth-climate responses and the application of six larch budmoth detection methods considering host, nonhost and instrumental time-series revealed spatiotemporal patterns of insect defoliation across the Alpine arc. Annual maps of reconstructed defoliation showed historical persistence of cyclic outbreaks at the site level, recurring c. every 8-9 yr. Larch budmoth outbreaks occurred independently of rising temperatures from the Little Ice Age until recent warmth. Although no collapse in outbreak periodicity was recorded at the local scale, synchronized Alpine-wide defoliation has ceased during recent decades. Our study demonstrates the persistence of recurring insect outbreaks during AD 1700-2000 and emphasizes that a widely distributed tree-ring network and novel analysis methods can contribute towards an understanding of the changes in outbreak amplitude, synchrony and climate dependence.