Assessing the effects of ecosystem disturbance on two insect vectors of pine wilt disease in the southern Rocky Mountains

Abstract

Ecosystem disturbances such as wildfire and bark beetle outbreak may structure the dynamics of forest tree diseases by altering vector behavior and population dynamics. As climate change intensifies, disturbances are increasing in both severity and intensity; however not all disturbance types have equivalent ecological effects: fire is abiotic and more homogenous in its spread, while bark beetle outbreaks are biotic and more spatially heterogeneous. Monochamus beetles are disturbance specialists, colonizing recently burned or stressed trees, and are important vectors of pine wilt disease, a lethal vascular wilt of conifers caused by the pinewood nematode (Bursaphelenchus xylophilus). Although pine wilt disease is globally destructive, studying its dynamics in the native range provides insight into how disturbance regimes influence disease risk under future climate scenarios. I examined how two major disturbance types- wildfire and spruce beetle (Dendroctonus rufipennis) outbreak—affect vector infection frequency, flight phenology, and population patterns in Monochamus clamator and M. scutellatus across forests of Colorado. In ponderosa pine forests impacted by recent wildfires, nematode infection prevalence in vectors was highest shortly after disturbance and declined over subsequent years. Infected individuals initiated flight earlier than uninfected beetles, and the relationship between infection probability and distance from the burn perimeter differed between species, indicating species-specific spatial responses to fire. These results suggest that wildfire can create short-term post-disturbance windows of elevated pathogen transmission risk. In contrast, in subalpine spruce–fir forests affected by spruce beetle outbreak, M. scutellatus abundance increased in outbreak stands, but infection probability and flight phenology did not differ between outbreak-affected and unaffected areas. Smaller beetle body size in outbreak stands suggests reduced host quality or increased competition, but infection dynamics remained stable across the disturbance gradient. Together, these studies demonstrate that disturbance type matters: wildfire, an acute abiotic disturbance, can amplify vector infectivity over short timescales, whereas bark beetle outbreak, a slower, chronic, and more heterogeneous disturbance, does not. These findings emphasize the need to distinguish disturbance regimes when predicting disease risk and highlight the importance of incorporating disturbance history into forest disease monitoring and management.