Induction and Spread of a spruce bark beetle outbreak in the Wilderness Area Dürrenstein, Austria

Mass outbreaks of the spruce bark beetle (Ips typographus) currently represent the largest threat to spruce dominated forests in Central Europe. Besides the high reproductive capacity and the aggressiveness of the beetle itself, climatic conditions affect the number of possible generations per year, and thus the occurrence of high population densities of the insect. The aim of this study was to investigate the uninterrupted influence of microclimatic site and stand conditions on the dynamics of population densities of the spruce bark beetle in a valley end of the Wilderness Area Durrenstein (IUCN Cat.1). A retro perspective analysis of the areas infested by the beetle during the years 2003-2011 was done regarding climatic conditions (temperature, irradiation, and precipitation) as well as stand and site related predisposition factors. The results show that high amounts of trees killed by bark beetles only occurred after extreme events (e.g. avalanche or wind throw) and that there was no direct or delayed relationship between temperature conditions and new infestations. South facing slopes showed higher predisposition and were infested earlier than east, west or north facing slopes. The site variables geomorphology and snowbreakage, as well as the stand variables proportion of spruce, stand age and storm hazard were crucial for the study sites predisposition towards bark beetle infestations. In 2012, spruce bark beetles marked with fluorescent powder were caught in pheromone traps to examine the beetles’ dispersal (flight direction and distance). Most beetles were found in south-western direction and within 100 m (>50%), 93% within 500 m. Very few individuals (<0.2%) managed to fly over a 1500 m high mountain ridge and were found in pheromone traps up to 5 km away from their hatching site. The areas predicted temperature increase of +1.1° to +2°C (2021-2050) and +3° to +3.9°C (2050-2071) will severely influence the development of Ips typographus from currently a univoltine development at higher elevations to a mulivoltine development in future. This may have a significant effect on beetle’s population growth and trigger longer, more intense, and larger epidemics.