No abstract

As climates change, thermal limits may no longer constrain some native herbivores within their historical ranges. The mountain pine beetle, Dendroctonus ponderosae Hopkins, is a tree-killing bark beetle native to western North America that is currently expanding its range. Continued eastward expansion through the newly invaded and novel jack pine (Pinus banksiana Lamb.) trees of the Canadian boreal forest could result in exposure of several species of novel potential host pines common in northeastern North America to this oligophagous herbivore. Due to the tightly co-evolved relationship between mountain pine beetle and western pine hosts, in which the insect utilizes the defensive chemistry of the host to stimulate mass attacks, we hypothesized that lack of co-evolutionary association would affect the host attraction and acceptance behaviors of this insect among novel hosts, particularly those with little known historical association with an aggressive stem-infesting insect. We studied how beetle behavior differed among the various stages of colonization on newly cut logs of four novel potential pine host species; jack, red (P. resinosa Ait.), eastern white (P. strobus L.) and Scots (P. sylvestris L.) pines, as well as two historical hosts, ponderosa (P. ponderosa Dougl. ex. Laws. var. scopulorum Engelm.) and lodgepole (P. contorta Dougl. var. latifolia Engelm.) pines. Overall, we found that beetle colonization behaviors at each stage in the colonization process differ between pine hosts, likely due to differing chemical and physical bark traits. Pines without co-evolved constitutive defenses against mountain pine beetle exhibited reduced amounts of defensive monoterpenoid chemicals; however, such patterns also reduced beetle attraction and colonization. Neither chemical nor physical defenses fully defended trees against the various stages of host procurement that can result in tree colonization and death.

Some subcortical insects have devastating effects on native tree communities in new ranges, despite benign interactions with their historical hosts. Examples of how insects, aggressive in their native habitat might respond in novel host environs are less common. One aggressive tree‐killing insect undergoing a dramatic range shift is the mountain pine beetle (Dendroctonus ponderosae Hopkins). Ongoing eastward expansion by the mountain pine beetle through the previously climatically unsuitable Canadian boreal forest may have large‐scale impacts on north eastern North American pine forests. No systematic studies have been conducted on potential reproduction of mountain pine beetle on pines common to north eastern North America. We report reproduction of mountain pine beetle in logs of novel pine species (jack, Pinus banksiana Lamb; red, Pinus resinosa Ait.; eastern white, Pinus strobus L.; and Scots Pinus sylvestris L.) compared to the two most common pine hosts in its historical range (ponderosa, Pinus ponderosa Dougl. ex. Laws. var. scopulorum Engelm. and lodgepole Pinus contorta Dougl. var. latifolia Engelm.) in a two year study. Successful reproduction of mountain pine beetle occurred in all novel hosts, demonstrating that constitutive defences pose no barrier to further range expansion. Despite the number of progeny in novel hosts on par with that of historical hosts, a greater number of adult brood in novel hosts died prior to emergence. Brood mortality was correlated with the number of brood that developed to adulthood prior to winter, particularly in red pine. Brood developed more rapidly in novel vs. historical pine hosts and, the summer after a warm fall, exhibited less synchronized emergence in novel hosts. Synthesis and applications. Outbreaks by an aggressive bark beetle may be possible outside its historical host range, but constrained by an interaction between host and seasonality. Our results suggest that pines common to north eastern North America are suitable hosts for mountain pine beetle and highlight the value of monitoring efforts and response preparations as the insect moves eastward.