To be or twice to be? The life cycle development of the spruce bark beetle under climate change

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We analyze the impact of climate change on the life cycle of the spruce bark beetle Ips typographus by means of a temperature-driven threshold model and temperature data from a network of more than 300 climate stations in Scandinavia. Using observed temperatures as well as climate model simulations, our model results exhibit univoltine behavior under current conditions, but predicts almost strictly bivoltine behavior for southern Norway in 2071–2100. The dynamics of this threshold phenomenon is investigated in detail. By logistic regression, the impact of regional warming can be described as a northward movement of bivoltinism by some 600 kilometers. Extension to two generations per year (bivoltinism) might increase the risk of devastating bark beetle outbreaks, although the impact of photoperiod-induced diapause in late summer and the ratio of soil or under-bark hibernations should be taken into account.