Trends in Outbreaks of Defoliating Insects Highlight Growing Threats for Central European Forests and Implications for Eastern Baltic Region

To identify general patterns in the effect of climate-driven changes in the outbreak frequency of forest defoliating species, we examined 60 years of records (1950–2010) of outbreaks of five defoliating species. Data on Lymantria dispar, Lymantria monacha, Bupalus piniarius, Panolis flammea, and Operophtera brumata from five Central European countries (Slovakia, Czech Republic, Austria, Hungary, and Germany), where the current climate is comparable with the projections of climate for the Eastern Baltic region by the end of the 21st century, were analyzed. Time series approach was applied to estimate the linkage between outbreaks and climate warming. Mean annual, summer, and winter deviations for the period of 1850 to 1900 were assessed as proxies of warming. To estimate the legacy effect, warming proxies were lagged by one year. Among those tested, warming proxies showed a linkage with outbreaks. Three significant outbreaks occurred in the analyzed period (at the beginning and end of the period). During the middle part of the analyzed period, the frequency and magnitude of outbreaks were low, implicating a higher insect outbreak risk with warming in Central Europe. In the latter part of the analyzed period, more frequent yet smaller outbreaks occurred, which supports the outbreak linkage with one-year lag, summer, and annual temperatures.