Bark beetles are major disturbance agents in forest ecosystems, and provide an ideal system for studying cross-scale interactions. They are ecosystem engineers at landscape levels, yet their populations are largely driven by microscale processes. In particular, their abilities to overcome tree defenses and colonize the relatively nutrient poor subcortical environment of the host tree rely upon mutualism with fungi. As parent beetles tunnel egg galleries in the tree phloem, they vector fungus propagules that germinate and penetrate the host. Brood feed on fungus-colonized phloem. This symbiosis relies on brood contact with fungus propagules after pupation for transport by emerging adults. Other, opportunistic fungi such as Trichoderma and Aspergillus are highly antagonistic to the beetles. We found that the bacterial assemblage associated with both bark beetles and the host tree. We set out to determine how these bacteria impact the growth of these symbiotic and opportunistic fungi using in vitro growth assays.
Results/Conclusions
Volatiles from one bacterium isolated from Dendroctonus ponderosae stimulated growth of its symbiotic fungus, Grosmannia clavigera, but only in the presence of host tree compounds. Volatiles from bacteria isolated from the bark beetles Dendroctonus valens and Ips grandicollis also stimulated the hyphal growth or spore production of their symbionts, Leptographium terebrantis and Ophiostoma ips. Growth of symbiotic fungi was not inhibited by the volatiles of any isolates. However, several of these beetle-associated bacteria inhibited the antagonistic fungi. Host trees play an important role in bacterial-fungal-bark beetle interactions by two mechanisms. First, their allelochemicals affect the strength and direction of the above relationships. Second, endophytic bacteria isolated from both constitutive and defensively induced tree tissues inhibited L. terebrantis and three opportunistic fungi. On average, bacteria isolated from induced tissues were more inhibitory than isolates from constitutive tissues. We hypothesize that bacteria associated with bark beetles provide a mechanism that improves the reliability of the beetle-fungus association. Conversely, endophytic bacteria may serve as an additional defensive mechanism against the beetle-fungus complex.
