COS 7-3 - Forest pests in a changing climate: Testing the effects of elevated CO2, O3, and humidity on mountain pine beetles and their fungal symbionts

Abstract: Mountain pine beetles (MPBs) are a major threat to pine forests, causing significant damage by feeding on and breeding in the inner bark of trees. Their unique ability to overcome host tree defenses and acquire nutrients through fungal symbiosis makes them particularly challenging to manage. However, how changing climate would modulate the biology of MPB, their fungal symbionts and their co-evolved interactions are still not well understood. To achieve this, male and female MPB pairs were inoculated into freshly cut lodgepole pine logs, which were then placed in climate chambers with manipulated climatic variables. Five separate chambers were used as follows: one with 1,000ppm CO₂, one with 100ppb O₃, one control without any variable modifications, and two with varying levels of relative humidity (66% and 33%). The beetle-infested logs were kept in their respective climate chambers with varying climatic factors for one month. After one month, the emerging offspring were collected for further analysis. Similarly, we treated three fungal symbionts of MPB either with 1,000ppm CO₂, 100ppb O₃, or varying humidity (66% and 33%) for five days and analyzed them. Our results showed that low humidity improved MPB reproduction success measured as larval gallery density, offspring number produced, and size of offspring, while also promoting the growth of symbiotic fungi. Elevated CO₂ accelerated larval growth and emergence, while it improved or restricted the fungal growth depending on the fungal species and isolate. Conversely, O₃ impaired MPB reproduction success, offspring emergence and deteriorated offspring fitness. However, O₃ also enhanced MPBs' immune response to entomopathogenic fungi. Our separate experiments on fungal biology showed that elevated O₃ restricts fungal growth and reproduction, while elevated CO₂ induces or restricts fungal growth and promotes ergosterol production. The observed effects of elevated CO₂, O₃, and humidity on MPB and their fungal symbionts suggest that ongoing shifts in climate may affect the dynamics of this complex co-evolved system. As such, this study provides valuable insights into the potential impacts of climate change on forest health and the need for adaptive management strategies. In conclusion, this study highlights the importance of understanding the effects of climate change on forest pests and their symbiotic relationships with microbes. By identifying how climate change affects MPB and their fungal symbionts, we can develop adaptive management strategies to mitigate the negative impacts of climate change on forest health.