Thresholds in the life cycle of the spruce bark beetle under climate change

Using a simple temperature-driven multistage threshold developmental model, we analyse the impact of climate change on the life cycle dynamics of the spruce bark beetle Ips typographus. The completion of a full reproductive second generation within a single year (bivoltinism) might increase the risk of devastating bark beetle outbreaks. A network of more than 300 climate stations, comprising time series of air temperatures both observed as well as simulated using regional climate models, is exploited on the country scale. It is shown that current conditions strongly favor univoltine behavior, whereas climate scenarios predict almost strictly bivoltine behavior for southern Norway in 2071-2100. The dynamics of this threshold phenomenon is investigated in detail. Using a logistic regression, the impact of regional warming can be described as a northward movement of bivoltinism by some 600 kilometres. Thus, the risk for Norway spruce infestations in Scandinavia may be drastically increased over the next decades.