Volatile-mediated interactions between the Eurasian spruce bark beetle, Ips typographus, and its associated fungi

Bark beetle attacks on healthy Norway spruce trees currently cause the rapid decline of conifer forests in Europe. Our research focused on the interactions between the Eurasian spruce bark beetle (Ips typographus) and its associated fungi. During attacks, beetles inoculate their hosts with a conserved fungal assemblage, including Endoconidiophora polonica, Grosmannia penicillata and Ophiostoma bicolor. As I. typographus has no obvious structures to transport the fungi between hosts, it is not known how this association is maintained and how the beetle benefits from it. Bioassays revealed that the presence of associated fungi on artificial media with and without spruce bark stimulated the beetles to enter the media. The beetles were most attracted to fungi growing in spruce bark medium, while avoiding spruce bark medium without fungi. Analysis of volatiles emanating from fungus-colonized media revealed that the fungi produce a set of de novo synthesized compounds as well as a complex mixture of volatile detoxification products derived from host terpenes and phenolics. Electrophysiological measurements showed that I. typographus possesses dedicated olfactory sensory neurons for these metabolites. Beetles were attracted to some of these compounds at specific doses in walking olfactometer experiments, and the presence of symbiotic fungi enhanced the attraction of females to pheromones. Collectively, our results suggest that volatiles produced by fungal associates are used by bark beetles as cues revealing the presence of beneficial fungi, the defense status of the host tree and the density of conspecifics at potential feeding and breeding sites.