INTERACTION OF PRE-ATTACK AND INDUCED MONOTERPENE CONCENTRATIONS IN HOST CONIFER DEFENSE AGAINST BARK BEETLE FUNGAL COMPLEXES

Two pine species (Pinus resinosa, P. banksiana) responded to inoculati on with fungi carried by bark beetles by rapidly increasing monoterpen e concentrations at the entry site. Changes in total monoterpenes were more pronounced than changes in proportionate compositions. The exten t and rate of host response was affected by fungal species, the viabil ity of the inoculum, and host tree species. In general, host responses were highest to fungi that are phytopathogenic and consistently assoc iated with the major bark beetles in the study region. Simple mechanic al wounding cannot account for the observed allelochemical changes, as aseptic inoculations elicited only minor reactions. Similarly, inocul ation with autoclaved inviable fungi generally elicited intermediate r esponses, suggesting that both structural and metabolic fungal propert ies are important. Responses by jack pine, P. banksiana, were generall y more rapid and variable than those of red pine, P. resinosa. Dose-to xicity experiments with synthetic compounds demonstrated that monoterp ene concentrations present in vivo only a few days after simulated att ack are lethal to most beetles. Constitutive (pre-attack) monoterpene levels can also exert some toxicity. Because bark beetles engage in ph eromone-mediated mass attacks that can deplete host defenses, constitu tive monoterpene levels, while a necessary early phase of successful p lant defense, appear insufficient by themselves. Such interactions bet ween constitutive and induced defense chemistry may be important consi derations when evaluating general theories of plant defense.