Hotter drought increases population levels and accelerates phenology of the European spruce bark beetle Ips typographus

Bark beetle-induced tree mortality has increased strongly in Europe in recent years. Bark beetle populations are highly sensitive to temperature, and drought weakens tree defenses against beetle attacks. Yet, the compound effects of drought and heat (termed hotter drought) remain poorly quantified, even though climate change increases their joint occurrence. Here, we analyzed data from a regional-scale network of pheromone-baited Ips typographus traps (158 traps across 7 Mha in southeast Germany, with 67.5·million beetles caught between 2015 and 2021), contrasting the unprecedented hotter drought period of 2018–2020 with non-drought years. Our objectives were (i) to assess the effect of hotter drought on bark beetle population dynamics, (ii) to quantify changes in spatial patterns during hotter drought, and (iii) to investigate how well trap data can explain observed tree mortality. Bark beetle population levels were strongly driven by temperature and drought, with an annual increase of approximately 2000 beetles per trap per °C under drought conditions (SPEI = −1). Furthermore, critical phenological thresholds were reached 7 days earlier for aggregation and 4 days earlier for peak swarming timing per °C increase in temperature. In drought years, I. typographus population levels were autocorrelated across hundreds of kilometers. Trap data explained between 37 % and 49 % of observed bark beetle mortality, highlighting that pheromone trap networks are a useful tool for monitoring and managing forest risk. We conclude that hotter drought intensifies and extends mass outbreaks of the European spruce bark beetle, suggesting the emergence of novel patterns of disturbance.