The invasive ambrosia beetle Xylosandrus compactus as a vector of the novel fungal pathogen Thyridium lauri

Several species of invasive ambrosia beetles (Coleoptera: Curculionidae: Scolytinae) cause significant damage to agroecosystemsoutside their native range. Among them, Xylosandrus compactus (Eichhoff) is a highly polyphagous species that infests a widerange of trees and shrubs in orchards, forests, nurseries and urban areas. The beetle is primarily associated with the nutritionalmutualist Ambrosiella xylebori Brader ex Arx & Hennebert, which serves as a food source for its developing progeny. However,X. compactus can also harbor other fungal species, including potential plant pathogens. This study provides the first experimentalevidence that X. compactus vectors the novel phytopathogenic fungus Thyridium lauri Voglmayr, D. Aiello & G.R. Leonardi inbay laurel (Laurus nobilis L.) plants. We fulfilled Leach’s postulates through a series of experiments: (i) confirming the naturalpresence of T. lauri on X. compactus adult females collected from diseased plants under field conditions, (ii) demonstratingthat both beetle foundresses previously exposed to T. lauri and their F1 adult progeny can transmit the pathogen to susceptiblebay laurel plants. Subsequently, characteristic disease symptoms (e.g., stem blight and internal necrosis) developed in infestedplants under controlled conditions. Although our findings confirm a close association between T. lauri and X. compactus, nosignificant differences were observed in the brood size of beetle foundresses harboring both A. xylebori and T. lauri compared tothose associated solely with A. xylebori. The ecological role of T. lauri in X. compactus infestations remains unclear and furtherinvestigation is needed to understand how stable and widespread this beetle-fungus association is in populations of both its nativeand invaded range. Future research should also investigate whether X. compactus can transmit T. lauri to other host plant speciesin natural settings. Additionally, our findings highlight the need for integrated pest management (IPM) strategies targeting both thebeetle and its fungal associates.