Home on the (expanding) range: evaluating the effectiveness of a novel host's induced defenses against the mountain pine beetle-fungal complex.

The mountain pine beetle (MPB; Dendroctonus ponderosae) is one of the most destructive forest pests, responsible for the death of billions of coniferous trees from Mexico to Alaska (Bentz et al. 2009). Behavioral plasticity helps the beetle sustain endemic levels until the right conditions are met, thus releasing constraints on population growth and resulting in a population eruption (Raffa et al. 2008). These large-scale outbreak events have resulted in significant impacts on ecosystem function and negative effects on local and regional economies (Ayres and Lombardero 2000, Kurz et al. 2008, Edburg et al. 2012). While the mechanisms contributing to bark beetle outbreaks are complex (Raffa et al. 2008, Bleiker et al. 2014), predicted increases in mean annual global temperatures will influence insect population success and expansion both directly via changes in development (Parmesan 2006, Jamieson et al. 2012) and indirectly via altered host defenses (Lusebrink et al. 2016, Erbilgin et al. 2017, Jamieson et al. 2017). The role that plant secondary compounds play during an insect host expansion, however, is unclear, especially for insects that rely on associated symbionts, as is the case for the MPB and its associated blue stain fungi. Phloem-feeding bark beetles are intimately linked to the defensive chemicals of their hosts, particularly monoterpenes (C10). Monoterpenes play a critical role in beetle behavior, physiology, reproduction and survival by serving as precursors to aggregation pheromones, synergists of pheromones, as well as lethal defenses depending on the composition and vapor concentrations (Seybold et al. 2006). During outbreaks, MPB attack en masse to surpass both physical and constitutive chemical defenses of healthy well-defended trees, as well as the induced changes in the quantity and quality of a host’s terpene and phenolic chemistry (Franceschi et al. 2005). This means that a rapid induced chemical response can mean life or death for both the tree and MPB (Raffa et al. 2008, Boone et al. 2011, KeefoverRing et al. 2016). Thus, understanding how MPB attack induces changes in defensive chemistry provides important insights into successful beetle colonization and the propagation of mass attacks (Raffa et al. 2005). Just as individual monoterpenes can have varying effects on MPB, they can also differentially affect the fungal symbionts of MPB. Some host monoterpenes can impede the growth of fungi, which MPB relies on for nutrition, development and survival (Bleiker and Six 2007). Fungi not only provide a nutrient rich food source for bark beetles (Bentz and Six 2006, Adams and Six 2007, Bleiker and Six 2007, Cook et al. 2010, Goodsman et al. 2012), but can aid in overcoming tree defense (Klepzig and Six 2004, Hammerbacher et al. 2013) and metabolizing toxic monoterpenes (Wang et al. 2013, 2014). While the failure of the fungi would result in the demise of the beetle, the effect of rapidly induced monoterpene production on fungal function during colonization of novel hosts and its effect on the success and spread of the MPB into new habitats remains to be elucidated. In Canada, the MPB has expanded its range from lodgepoledominated forests through a lodgepole-jack pine (Pinus banksiana Lambert) hybrid zone into jack pine forests (Cullingham et al. 2011). The ecological and economical impacts of this range expansion have the potential to be disastrous (Ono 2003).