The spruce beetle (Dendroctonus rufipennis) is responsible for the death of more North American spruce trees than any other biotic agent. While Engelmann spruce is the main host in the lower 48 U.S. states, white spruce (Picea glauca) is the preferred host in the beetle’s northern range, including Alaska. Conifers defend themselves from bark beetles with both constitutive and induced levels of oleoresin, which consists mostly of a mixture of lower molecular weight volatile mono- (C10) and sesquiterpenes (C15) and larger non-volatile diterpenes (C20). Due to the toxicity of some compounds and its relatively high viscosity, tree oleoresin constitutes both a chemical and physical defense. The object of this study was to determine the role of constitutive phloem terpenoid chemistry of white spruce in resistance against spruce beetle. We took advantage of a plantation of white spruce on the Kenai Peninsula consisting of trees grown from seeds collected from different locations prior to the largest outbreak of spruce beetle in Alaska’s history (BEFORE) and seeds from the same locations from surviving trees (AFTER, presumed resistant trees). We collected phloem samples from 270 trees evenly divided between BEFORE and AFTER trees and measured tree DBH, phloem thickness, and terpenoid chemistry.
Results/Conclusions
Tree DBH was strongly positively correlated with phloem thickness (r = 0.49, P < 0.001), but neither of these parameters differed between BEFORE and AFTER trees. Four monoterpenes and two sesquiterpenes differed between BEFORE and AFTER trees, but five of these compounds occurred at very low levels. In contrast, β-phellandrene was the most abundant compound of all mono- and sesquiterpenes measured and was 22% higher in the phloem of AFTER (presumed resistant ) trees (P = 0.02). Other work with white spruce found that β-phellandrene inhibits response to the spruce beetle pheromone frontalin. In addition, β-phellandrene was found in greater quantities in the wound tissues surrounding the induced reaction zones of white spruce; further evidence of the importance of this monoterpene in defense against spruce beetle. Finally, our result with β-phellandrene is probably stronger than seen in these data, since BEFORE trees would be composed of both susceptible and resistant individuals containing both low and high constitutive amounts of β-phellandrene, respectively.