Consequences of distributional asymmetry in a warming environment: invasion of novel forests by the mountain pine beetle

Abstract

Abstract. The 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 contorta var. 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 na€ ıve host populations.

Priority Actions to Improve Provenance Decision-Making

“…Local adaptation can have both climate and many nonclimate components, including other abiotic factors such as soil and photoperiod (Savolainen et al 2013, Christmas et al 2016a, but biotic factors such as antagonists and mutualists are also important (Crémieux et al 2008, Gellie et al 2016, Gehring et al 2017, Urbina et al 2018. Some nonlocal provenances may have no history of exposure to local pests and pathogens (Potts et al 2016), and pests and pathogens may themselves be shifting their ranges (Burke et al 2017), potentially increasing the risk of maladaptation (Gellie et al 2016). Little is known about the relative importance of climate compared with these other factors for most plant species, but reciprocal transplant and experimental treatments (e.g., herbivore and pathogen exclusion or exposure trials) can help tease these effects apart.…”

Global Climate Change as a Driver of Bottom-Up and Top-Down Factors in Agricultural Landscapes and the Fate of Host-Parasitoid Interactions

“…Such changes in distribution patterns have been widely reported in Lepidoptera (Parmesan et al, 1999;Battisti et al, 2005Battisti et al, , 2006Wilson et al, 2005Wilson et al, , 2007Franco et al, 2006). Other examples include the northward spread of the mountain pine beetle Dendroctonus ponderosae (Coleoptera: Curculionidae) (Weed et al, 2015;Burke et al, 2017) and Dendroctronus frontalis (Coleoptera: Scolytidae) (Ungerer et al, 1999), all in response to winter warming. Such evidence in parasitoids is scant.…”

Single-generation effects on terpenoid defenses in lodgepole pine populations following mountain pine beetle infestation

“…In contrast, a significant induced defensive reaction from the MeJa treatment was not detected for the majority of terpenes. This may be because MeJa does not induce a defensive reaction in the same manner, or as extensively, as actual MPB infestation [ 20 ]. In addition, previous work has found that resistant trees undergoing induced defense had higher levels of both limonene and terpinolene than those that were less resistant [ 32 , 43 ].…”

“…Defensive reactions within lodgepole pines and other conifers can be artificially induced by the application of the phytohormone, methyl jasmonate (in the form of MeJa, the volatile derivate of jasmonic acid), which causes an induced stress response in plants via the regulation of gene expression, to reallocate energy from photosynthesis and growth to secondary metabolite production [ 13 , 16 , 19 ]. Further, this MeJa-induced response produces many of the same anatomical changes in conifers as that of mechanical wounding meant to mimic bark beetle attack [ 16 ], although the defensive monoterpene response from MeJa is more generalized and quantitatively different than that of simulated bark beetle attacks consisting of both mechanical wounding and fungal inoculation [ 20 ].…”