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Rapid monoterpene induction promotes the susceptibility of a novel host pine to mountain pine beetle colonization but not to beetle-vectored fungi
Chemical induction can drive tree susceptibility to and host range expansions of attacking insects and fungi. Recently, mountain
pine beetle (Dendroctonus ponderosae Hopkins; MPB) has expanded its host range from its historic host lodgepole pine (Pinus
contorta var. latifolia Douglas ex Loudon) to jack pine (Pinus banksiana Lamb) in western Canada. Beetle success in jack pine forests likely depends upon the suitability of tree chemistry to MPB and its symbiotic phytopathogenic fungi. In particular, how rapid
induced defenses of jack pine affect MPB colonization and the beetle’s symbionts is unknown. In the field, we characterized and
compared differences in rapid induced phloem monoterpenes between lodgepole and jack pines in response to various densities
of Grosmannia clavigera (Robinson-Jeffery and Davidson)—a MPB symbiotic fungus used to simulate beetle attack—inoculations.
Overall, lodgepole pine had higher limonene and myrcene, but lower α-pinene, concentrations than jack pine. However, myrcene
concentrations in jack pine increased with inoculation density, while that in lodgepole pine did not respond to density treatments.
We compared the growth and reproduction of MPB’s symbiotic fungi, G. clavigera, Ophiostoma montium (Rumford) von Arx and
Leptographium longiclavatum Lee, Kim and Breuil, grown on media amended with myrcene, α-pinene and limonene at concentrations reflecting two induction levels from each pine species. Myrcene and α-pinene amendments inhibited the growth but stimulated the reproduction of G. clavigera, whereas limonene stimulated its growth while inhibiting its reproduction. However, the
growth and reproduction of the other fungi were generally stimulated by monoterpene amendments. Overall, our results suggest
that jack pine rapid induction could promote MPB aggregation due to high levels of α-pinene (pheromone precursor), a positive
feedback of myrcene (pheromone synergist) and low levels of limonene (resistance). Jack pine is likely as susceptible to MPBvectored fungi as lodgepole pine, indicating that jack pine induction will likely not adversely affect symbiont activities enough to
inhibit the invasion of MPB into jack pine forests.
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