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
Abstract
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.