PLANT-INSECT INTERACTIONS Effects of Diterpene Acids on Components of a Conifer Bark Beetle–Fungal Interaction: Tolerance by Ips pini and Sensitivity by Its Associate Ophiostoma ips

Conifer resin and phloem tissue contain several phytochemical groups, composed primarily of monoterpenes, diterpene acids, and stilbene phenolics. The effects of monoterpenes and phenolics on stem-colonizing bark beetles and their associated microorganisms have been studied to some extent, but the roles of diterpene acids are largely unknown. Diterpene acids are known to have substantial feeding deterrent and growth inhibiting effects on a variety of insect groups and are known to inhibit a variety of fungi. We tested three diterpene acids present in red pine, Pinus resinosa, at various concentrations, on several life history components of the bark beetle Ips pini and the fungus Ophiostoma ips. No diterpene acid affected the host acceptance behavior or larval survival of Ips pini. In contrast, abietic acid and isopimaric acid strongly inhibited spore germination ofO. ips, and abietic acid strongly inhibited mycelial growth. The levels of inhibition observed were higher than with any previous assays of monoterpenes or phenolics in this system. These results support the view that conifer defenses against bark beetleÐfungal complexes are multifaceted, with all three phytochemical groups being important to P. resinosa, but each with varying relative activity against the beetles and fungi.