Defensive response of evolutionarily naïve Pinus sylvestris to the mountain pine beetle fungal associate Grosmannia clavigera in comparison to Pinus ponderosa

Mountain pine beetle (MPB, Dendroctonus ponderosae) is a destructive pest of pine forests in western North America. This insect is currently expanding its range across the Canadian boreal forest towards eastern North America, where a suite of novel pine species will be encountered. One species of pine without prior association with MPB is Pinus sylvestris (Scots pine), which is native to Europe and naturalized in parts of central and eastern North America. Here, we take advantage of a unique opportunity in the Black Hills of South Dakota where an isolated, planted, and mature stand of P. sylvestris and native Pinus ponderosa (ponderosa pine) co-exist within the range of MPB. We conducted a punch-inoculation experiment to determine the chemical response of P. sylvestris from a blue-stain fungus associated with MPB, Grosmannia clavigera, and compared the response to that of P. ponderosa. We found that P. sylvestris had a higher localized monoterpene response than P. ponderosa in response to inoculation, but a lower sesquiterpene response. Among the significant monoterpenes associated with MPB behavior, limonene, 3-carene, and myrcene had a larger localized response in P. sylvestris than P. ponderosa; lower levels of 4-allylanisole were found in P. sylvestris. Fungal inoculation did not induce a stronger terpenoid response than mechanical wounding without inoculation, indicating that P. sylvestris responds to mechanical damage similarly as to fungal inoculation. Pinus sylvestris may provide one alternative plantation species for timber production in the Great Lakes Region following mountain pine beetle incursion, however, more evaluation is needed to determine the role of this species in future plantings.