Effects of symbiotic bacteria and tree chemistry on the growth and reproduction of bark beetle fungal symbionts

Bark beetles are associated with diverse assemblages of microorganisms, many of which affect their interactions with host plants and natural enemies. We tested how bacterial associates of three bark beetles with various types of host relationships affect growth and reproduction of their symbiotic fungi. Fungi were exposed to volatiles from bacteria in an arena that imposed physical separation but shared airspace. We also exposed fungi to vapors of the host compound, α-pinene, and to combinations of bacterial volatiles and α-pinene. Bacterial volatiles commonly stimulated growth of Leptographium procerum (W.B. Kendr.) M.J. Wingf. and Grosmannia clavigera (Rob.-Jeffr. & R.W. Davidson) Zipfel, Z.W. de Beer & Wingf., which are symbiotic fungi of Dendroctonus valens LeConte and Dendroctonus ponderosae Hopkins, respectively. They less commonly stimulated growth of Ophiostoma ips (Rumbold) Nannf., which is associated with Ips grandicollis Eichhoff. Some bacteria inhibited L. procerum, Ophiostoma montium (Rumbold) von Arx (another associate of D. ponderosae), and O. ips. Bacteria greatly stimulated spore production of symbionts of D. valens and D. ponderosae. α-Pinene strongly affected some of these relationships, causing amplification, reduction, or reversal of the interactions among the bacteria and fungi. Our results show that some bacteria associated with bark beetles directly affect fungal symbionts and interact with tree chemistry to affect fungal growth and sporulation. The strongest effects were induced by bacteria associated with beetles adapted to attacking living trees with vigorous defenses, and on fungal reproductive structures.