What hides beneath the bark? Associations between phytopathogenic fungi and emerging bark beetles in the Mediterranean region

Bark beetles are commonly associated with several fungal symbionts that play mutualistic, commensal or pathogenic ecological roles. In recent decades, reports of serious damage caused by both native and invasive bark beetle species, and the pathogenic fungi they vector, are rapidly increasing worldwide. In particular, new bark beetle–fungus associations are being reported in different contexts, e.g., forests, ornamental plant nurseries, urban areas and orchards. Due to heavy global trade and suitable environmental conditions for alien beetle establishment, the Mediterranean region has recently been invaded by bark beetle species originating from Africa, Asia, Australia and the Americas. Scolytus amygdali, S. rugulosus, S. mali, Cryphalus dilutus, Hylesinus vestitus, and Phloeotribus scaraboides are among the common bark beetles whose infestations are threatening cultivated trees in the Mediterranean area. However, their associations with symbiotic fungi and/or phytopathogenic fungi have been rarely investigated. This literature review aims to summarize knowledge on these species, focusing on the description of their fungal symbionts and also on their ecological roles. Understanding these associations of bark beetles with phytopathogenic fungi is the first step toward developing sustainable management strategies to reduce both beetle infestations and the spread of fungal infections.