A rationale for chemical defense mixtures in spruce oleoresin: most monoterpenes are highly toxic to either bark beetles or to their symbiotic fungi, but not both

Monoterpenes in conifer oleoresin frequently occur as complex blends of 15-30 components, but the underlying ecological reasons for these mixtures are not yet known. We tested the toxicity of 12 individual Norway spruce monoterpenes and their mixtures to adult Eurasian spruce bark beetles, Ips typographus, and three major symbiotic fungi in the vapor phase. Individual monoterpenes had generally opposite effects on bark beetles compared to symbiotic fungi, with the compounds most toxic to beetles being the least inhibitory to fungal growth and vice versa. These results support the interaction diversity hypothesis, which posits that defense mixtures are maintained because the individual components target different attackers. Among the symbiotic fungi tested, one (Grosmannia penicillata) exhibited high tolerance to monoterpenes, and its growth was even stimulated by the monoterpenes most toxic to the beetle. Interestingly, spruce bark responds to G. penicillata inoculation by accumulating higher concentrations of specifically fungistatic monoterpenes.