Temperature affects the development of the pine bark beetle Ips acuminatus and the growth of its most common fungal associates

Abstract Bark beetles play a key role in shaping forest ecosystems, and some species are among the most significant disturbance factors in coniferous forests of the Northern Hemisphere. Their ability to successfully infest living trees is closely linked to mutualistic relationships with microorganisms that help to overcome host defences or provide essential nutrients. For a deeper understanding of the effects of climate change-related temperature alterations on these mutualistic interactions, the present study investigated how temperature affects the development of the pine bark beetle Ips acuminatus and the mycelial growth of its most common ophiostomatoid fungal associates. Key thermal performance parameters of fungi and beetles were determined using laboratory experiments and temperature-dependent development and growth rate models. Temperature had a significant effect on insect development and fungal growth. All fungal species tested on malt extract agar had lower temperature optima (~ 22–28 °C) and thermal thresholds (lower thresholds: ~ 3–6 °C; upper thresholds: ~ 27–33 °C) than I. acuminatus tested in logs (lower threshold: 8.1–9.6 °C; optimum temperature: 30.3 °C; upper threshold: 36.2 °C). Tolerance to high temperatures varied among fungal species. Particularly, the proposed key fungal symbiont, Ophiostoma macrosporum , suggested to be essential for the nutrition of larvae and beetles, showed strong growth reduction at 30 °C, close to the beetle’s optimum temperature. Conversely, two other associated fungi, Graphilbum acuminatum and Ophiostoma cf. clavatum , were more tolerant to higher temperatures, suggesting that these species may be ecologically important as well and facilitate functional redundancy within the beetle’s fungal community.