Tree defenses, host choice, and reproductive success of a native bark beetle under novel outbreak conditions

Bark beetles of the genus Dendroctonus are some of the most important disturbance agents in North American forests, having colonized conifers for millions of years. The selection pressure posed by tree-killing bark beetles pushed trees to develop an arsenal of defensive strategies to which beetles have adapted in their turn. Recent surges in bark beetle-related tree mortality have highlighted the potential of novel climatic and landscape conditions to push tightly calibrated relationships beyond historical norms. One such example is an unprecedented outbreak of the native eastern larch beetle (ELB), Dendroctonus simplex LeConte (Coleoptera: Curculionidae; Scolytinae), that has killed eastern larch (tamarack), Larix laricina (Du Roi) K. Koch, trees across more than 460,000 ha of forest in the Great Lakes Region since 2001. The ability of a bark beetle to attack healthy trees is dependent on sufficient local beetle numbers to overwhelm host defenses and a behavioral switch to target those trees that are avoided at lower population levels. ELB was not previously considered an aggressive tree colonizer, but extended growing seasons have contributed to recent eruptions in local populations of the species. We combined a dendrochronological analysis of tree cores with observational data collected from 2011 to 2013 in Beltrami Island State Forest, Minnesota, to understand tree defensive capacity and beetle outbreak dynamics in this understudied system. We found that preformed defenses visible in tamarack xylem were limited and did not determine host preference of ELB during our study. Beetles colonized the largest trees with the thickest phloem regardless of defensive capacity. Preformed resin defenses measured in tree phloem were correlated with reduced beetle reproductive success but were unrelated to resin metrics from tree xylem. With this work, the interaction between ELB and tamarack serves as a model to explore how climate change may alter species associations within native forest systems and the management challenges associated with underestimating historically benign pests.