Combined chemical defenses against an insect-fungal complex

Abstract

This study considered how host plant allelochemicals may contribute to defense against insects and fungi that jointly colonize the subcortical tissues of trees, the relative roles of constitutive and inducible chemistry in these defenses, and how the actions of two different feeding guilds might be interrelated. Our model consisted of the coniferous treePinus resinosa, the root- and lower stem-colonizing beetlesHylastes porculus andDendroctonus valens, and their associated fungiLeptographium procerum andL. terebrantis, and the stem-colonizing bark beetleIps pini and its associated fungusOphiostoma ips. In a novel bioassay, extracts from reaction tissue elicted by wound inoculation withL. terebrantis were more repellent to beetles than were similar extracts from constitutive or mechanically wounded tissue. The effect on beetle behavior was more pronounced in nonpolar extracts, which contain mostly monoterpenes, than in polar extracts, which contain mostly phenolics. Synthetic monoterpenes at concentrations present in the various tissues exerted similar effects and were likewise repellent in dose-response experiments. Growth ofL. procerum andL. terebrantis was inhibited by polar extracts from constitutive and reaction tissue. Inhibition was higher in wounded than control tissue, but the inhibition response did not vary with the type of wounding. Synthetic monoterpenes strongly inhibited spore germination and mycelial growth of both fungi. Colonization of red pine roots byLeptographium spp. altered the subsequent effects of extracts of stem phloem tissue onI. pini. These effects varied with host condition. Beetles preferred extracts from constitutive stem phloem tissue of healthy trees to that of root-diseased trees. However, extracts from reaction tissues of healthy trees were more repellent toI. pini than were the reaction tissues of root-diseased trees. The implications of these results to plant defense against insect-fungal complexes and interactions among different feeding guilds are discussed.