Temporal and spatial proximity to wildfire is associated with species-specific patterns of phoresy by plant-pathogenic nematodes (Bursaphelenchus xylophilus) in populations of sawyer beetles (Coleoptera: Cerambycidae) in Colorado

Abstract Disturbances may shift forest ecosystem disease dynamics when they affect populations of insects that vector plant pathogens. Sawyer beetles ( Monochamus spp.) are common in conifer forests and beetles recruit to and colonize stressed hosts including those exposed or adjacent to wildfires. Monochamus spp. are also vectors of a lethal vascular wilt caused by pinewood nematode ( Bursaphelenchus xylophilus ), which is phoretic on beetles. Here we tested whether temporal and spatial proximity to wildfire events (2020 Cameron Peak and Cal-Wood Fires, Colorado, USA) predict detection of B. xylophilus in two sympatric Monochamus species ( M. clamator and M. scutellatus ) in the southern Rocky Mountains. Beetle flight phenology was also tested relative to B. xylophilus phoresy. Beetles were collected over three field seasons corresponding to 1, 2, and 4 years post-wildfire (2021, 2022, and 2024) using panel traps and subsequently tested for B. xylophilus . Detection of B. xylophilus was elevated in beetle populations 1 year following wildfire (29.8 and 11.8% for M. clamator and M. scutellatus , respectively) but declined to previously reported background levels by 4 years post-wildfire (0.8 and 3.4%, respectively). The occurrence of B. xylophilus in beetle populations had a species-specific relationship with distance to wildfire perimeters: B. xylophilus phoresy was more frequent in M. scutellatus near the fire perimeter, whereas the opposite was observed for M. clamator . On average, M. clamator flight initiated and peaked about 5 days before M. scutellatus , and beetles positive for B. xylophilus ceased flying ~14 days before those that without B. xylophilus . Collectively, these analyses suggest species-specific spatial and temporal patterns of B. xylophilus occurrence in vector populations relative to wildfire perimeters and also indicate that pathogen occurrence increased immediately after wildfire in this system, with potential implications for disease transmission across fire-impacted landscapes.