Abstract
AbstractThe range of many Holarctic forest insects does not comprise the entire range of their hosts, as they are often limited to more southern latitudes by the adverse effects of cold temperatures. Global climate warming has led to the increased potential for forest insects to invade novel habitats of native hosts within the same landmass. The mountain pine beetle (MPB;Dendroctonus ponderosae) has recently expanded into higher‐latitude forests of the principal host, lodgepole pine (Pinus contortavar.latifolia), and the susceptibility of trees is greater in these systems compared to forests in the native range. We assessed the contribution of the induced defensive response of hosts to this elevated susceptibility, and whether these discrepancies are the result of coevolution with host populations within the historic native range of the insect. We challenged trees using paired treatments of a beetle‐attack simulation and a generic defensive response elicitor (methyl jasmonate) to mitigate variability in the induced response among trees within and among populations, from within and outside the historic range of the beetle. We then assessed the production of monoterpene chemicals by the trees in response to treatments using gas chromatography/mass spectrometry. The differential induction of monoterpenes in response to simulated beetle attack relative to the generic elicitor was highest in populations with the highest putative historic exposure to MPB. Elevated susceptibility and invasion potential of the beetle in novel systems is the proximate result of reduced defensive capacity, ultimately arising from a lack of coevolution with the beetle in novel systems. In forested systems with climate‐driven herbivore–host distributional asymmetry, continued warming will potentially exacerbate the impacts of aggressive insect herbivores as they invade defensively naïve host populations.
Highlights
Introductions of invasive insect species have been increasing exponentially across the world as a consequence of anthropogenic climate change and globalization (Mattson et al 1994, Liebhold et al 1995, Hulme 2009, Ramsfield et al 2016)
Terrestrial ectothermic organisms are susceptible to variations in weather and climate due to their sensitivity to temperature (Musolin 2007, Deutsch et al 2008, Jo€nsson et al 2009), and throughout the Holarctic region, the effects of climate warming on herbivorous insects have often resulted in range expansions into previously thermally unsuitable habitats (Parmesan et al 1999, Carroll et al 2004, Battisti et al 2005, Hickling et al 2005, 2006, Hagen et al 2007)
Total basal area, proportion basal area occupied by lodgepole pine and non-host species, and the diameter of experimental trees did not vary among historic climatic suitability classes (HCSCs) (P > 0.05; Table 2)
Summary
Introductions of invasive insect species have been increasing exponentially across the world as a consequence of anthropogenic climate change and globalization (Mattson et al 1994, Liebhold et al 1995, Hulme 2009, Ramsfield et al 2016). Within climatically and geographically suitable habitats, the ability of an invasive insect herbivore to establish is influenced by interspecific interactions, host-plant defense potential, and biological traits of the invading species (Davis 2009, Cullingham et al 2011, Liebhold et al 2013, Rochlin et al 2013, Raje et al 2016). Rapid range expansion in response to climate change may result in novel herbivore– host interactions, potentially exacerbating herbivore impacts and accelerating range shifts due to weakly coevolved or evolutionarily na€ıve host defenses (Braschler and Hill 2007, Cudmore et al 2010, Raffa et al 2013)
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