COMBINED CHEMICAL DEFENSES AGAINST AN INSECT-FUNGAL COMPLEX

This study considered how host plant allelochemicals may contribute to defense against insects and fungi that jointly colonize the subcortic al tissues of trees, the relative roles of constitutive and inducible chemistry in these defenses, and how the actions of two different feed ing guilds might be interrelated. Our model consisted of the coniferou s tree Pinus resinosa, the root- and lower stem-colonizing beetles Hyl astes porculus and Dendroctonus valens, and their associated fungi Lep tographium procerum and L. terebrantis, and the stem-colonizing bark b eetle Ips pini and its associated Fungus Ophiostoma ips. In a novel bi oassay, extracts from reaction tissue elicted by wound inoculation wit h L. terebrantis were more repellent to beetles than were similar extr acts from constitutive or mechanically wounded tissue. The effect on b ee tie behavior was more pronounced in nonpolar extracts, which contai n mostly monoterpenes, than in polar extracts, which contain mostly ph enolics. Synthetic monoterpenes at concentrations present in the vario us tissues exerted similar effects and were likewise repellent in dose -response experiments. Growth of L. procerum and L. terebrantis was in hibited by polar extracts from constitutive and reaction tissue. Inhib ition was higher in wounded than control tissue, but the inhibition re sponse 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 by Leptographium spp. altered the sub sequent effects of extracts of stem phloem tissue on I. pini. These ef fects varied with host condition. Beetles preferred extracts from cons titutive stem phloem tissue of healthy trees to that of root-diseased trees. However, extracts from reaction tissues of healthy trees were m ore repellent to I. pini than were the reaction tissues of root-diseas ed trees. The implications of these results to plant defense against i nsect-fungal complexes and interactions among different feeding guilds are discussed.