Rapid Population Response of the Mediterranean Pine Engraver (Orthotomicus erosus Wollaston) Following a Severe Windthrow Event in Aleppo Pine Forest

Severe windthrow events can create highly favorable conditions for bark beetle outbreaks by generating large quantities of weakened breeding material. This study investigated the population response of the Mediterranean pine engraver, Orthotomicus erosus Wollaston, following a microburst windstorm that struck Marjan Forest Park, an urban Mediterranean Aleppo pine (Pinus halepensis Mill.) forest in Split, Croatia, in July 2025. Approximately 2000 m3 of damaged timber remained in the forest until February 2026, enabling assessment of colonization dynamics and outbreak development. Field inspections, laboratory analyses, and pheromone trap monitoring were conducted between July 2025 and April 2026. Insect sampling was conducted in the field through bark inspections and pheromone trap monitoring. For log inspections, bark sections were carefully removed to expose galleries and immature stages beneath the bark. Observations focused on the presence of entrance holes, boring dust, maternal galleries, larval galleries, pupal chambers, exit holes, adults, and immature developmental stages. In addition, ten pheromone traps were installed shortly after the windthrow event. Trap catches were collected regularly, and the captured insects were examined in the laboratory, where O. erosus individuals were identified and counted under a stereomicroscope. Colonization of windthrown material began within two weeks after the disturbance, and infestation rates reached 94% of examined logs by December 2025. At that point, 86% of infested logs already contained exit holes, indicating successful completion of beetle development. Pheromone traps captured a total of 131,588 adults, with more than 88% recorded during spring 2026 and a pronounced maximum recorded catch occurring on 8 April 2026. The appearance of newly attacked standing trees in spring (April) 2026 confirmed rapid population expansion from fallen material to living hosts. Results demonstrate the exceptional outbreak potential of O. erosus following extreme climatic disturbances and emphasize the importance of rapid sanitation measures and continuous monitoring in Mediterranean pine forests under climate change conditions. Delayed salvage logging after severe windthrow can rapidly trigger an outbreak of O. erosus.