Attack by Hylobius warreni on grafted lodgepole pine and its relationships with monoterpene composition and scion : rootstock diameter ratio

1 A lodgepole pine seed orchard at the Prince George Tree Improvement Station (PGTIS), with up to 60% of grafted trees attacked by the Warren root collar weevil Hylobius warreni was investigated to determine whether relative monoterpene composition or scion : rootstock interactions of grafts affected susceptibility to attack. 2 There was a significant relationship between relative levels of α-pinene, β-thujene, β-pinene, δ-3-carene and limonene in scion and rootstock in unattacked trees, indicating a potential effect of the scion monoterpene composition on their composition in the rootstock. 3 Relative content of δ-3-carene and β-phellandrene differed significantly in root stocks of attacked and unattacked trees but, for individual clones, a significant difference was only detected for β-phellandrene in one clone. δ-3-Carene levels may have been too low in the examined trees to exert a strong effect. 4 Interestingly, attack status was significantly associated with two scion monoterpenes: α-thujene and α-terpinolene, both of which had higher levels in unattacked than in attacked trees. 5 Warren root collar weevils appear largely unaffected by monoterpene content, but further study is required to determine whether high levels of δ-3-carene imparts some level of resistance to attack, and to verify whether the observed effects of scion monoterpenes are real or artefacts of the analysis. 6 Hylobius warreni-attacked trees had smaller scion : rootstock diameter ratio (i.e. a large rootstock diameter relative to the scion diameter) than unattacked trees. This effect was consistent among clones, and was not due to the absolute diameter of the rootstock or the scion. Trees with increased diameter-growth at the root collar (e.g. some grafted trees) may have increased susceptibility to attack by H. warreni, or diameter-growth at the root collar is affected by the attack.