Spring dispersal and reproductive potential Ips typographus

Diapause is a common strategy in insects to respond to reoccurring adverse
events. The European spruce bark beetle Ips typographus is an important
forest pest in Eurasia affecting spruce-dominated forests by eruptive
outbreaks, which are likely to become more frequent and severe in a
changing climate. It expresses a reproductive diapause to cope with harsh
winter conditions. As diapause also affects the activity and voltinism of
I. typographus, understanding its regulation by abiotic environmental
factors can help to mitigate outbreak risks, e.g. by coordinating
management measures or adapting silvicultural strategies. While diapause
induction in late summer and autumn has been shown to be mainly triggered
by photoperiod and modified by temperature, information on environmental
cues affecting post-diapause swarming in spring is scarce. In late
winter/early spring 2021 and 2023, we conducted a laboratory experiment
assessing spring swarming and reproductive potential of I. typographus
after diapause termination under various temperature and photoperiodic
conditions. We applied mean temperatures between 13 and 23 °C with daily
oscillations of ±5 °C and daylengths between 9 and 14 h (January to
April). Post-diapause temperature sums had the strongest effect on spring
swarming, modified by daily maximum temperatures, while photoperiods only
had a small and ambiguous effect. In contrast, reproductive potential of
dispersing beetles remained almost unaffected by temperature sum, daily
temperature, and photoperiod. Our results show that spring swarming can
potentially start right after cold temperatures in mid-winter have
terminated diapause. Ongoing climate warming will consequently extend the
reproductive season, likely increasing voltinism and population density of
I. typographus. Our findings can be implemented in phenology- and risk
assessment models to more accurately predict the temperature-dependent
phenology and infestation risk in a future climate, to ultimately
facilitate bark beetle management (e.g. sanitation and salvage logging)
and guide long-term silvicultural adaptations.