High plasticity in diapause responses benefits bark beetles in a changing climate

1. Numerous insects evolved diapause to cope with seasonally re-occurring adverse conditions, affecting multiple life-history traits, including reproduction, survival and voltinism. The spruce bark beetle Ips typographus-a major disturbance agent in spruce-dominated forests of Eurasia-enters reproductive diapause to survive harsh winters. Induction of facultative diapause is mainly regulated by short daylengths, but modified by warm temperatures, potentially postponing entry into diapause. Knowledge on the interplay of these cues under natural conditions is still lacking despite its importance for phenology and management in a changing climate. 2. We conducted a comprehensive field study over 3 years along an elevational gradient in South-West Germany to quantify diapause expression in I. typographus under a broad range of natural photoperiod and temperature conditions. 3. Most individuals entered diapause in mid-August (daylengths <14 h), regardless of temperature. However, some individuals still reproduced at warm temperatures (>22-26 degrees C) in late October (daylengths similar to 10 h), even following cold nights below 5 degrees C. However, fecundity of these late-breeding individuals was reduced. 4. In comparison to previous findings, we revealed a later diapause induction in I. typographus due to warm temperatures. Within-population variability in the timing of diapause induction is an effective strategy to increase the amount of late-breeding individuals and thus voltinism in a warming climate. Bark beetle phenology models should hence (i) include temperature as diapause-modifying cue in addition to photoperiod and (ii) consider potential differences between populations from diverse geographic origins.