Development of an aggressive bark beetle on novel hosts: Implications for outbreaks in an invaded range

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.