Bark and ambrosia beetle fungi emit a greater diversity of volatile organic compounds than other fungi.

Bark and ambrosia beetles are associated with various filamentous fungi, whose roles range from mutualists to antagonists. Mutualists can provide various benefits and are typically selectively dispersed by these wood-boring beetles. However, it is currently unknown how such bark and ambrosia beetles distinguish their fungal mutualists from other fungal associates, including insect pathogens. We hypothesized that volatile organic compounds might help maintain the mutualisms between beetles and specific fungal partners. Here, we compared the volatiles emitted by 30 fungi known to act as mutualists, commensals or antagonists for their beetle hosts. GC-MS analysis showed that all the fungi had characteristic volatile profiles with beneficial fungi emitting a greater diversity of volatiles than other fungal species. The volatile profiles of beneficial fungi were dominated by aliphatic and aromatic alcohols and esters in terms of both diversity and abundance. Furthermore, we demonstrated that the larvae and adults of two ambrosia beetle species can distinguish their mutualistic fungi from others based on volatile cues. Our results highlight the importance of aliphatic and aromatic alcohols and esters as recognition cues for mutualist fungi by wood-boring beetles. Volatiles likely play key roles in maintaining beetle associations with fungi that are essential to their insect host.