The tree-ring program OUTBREAK was used to reconstruct WSB outbreaks by applying a set of user-defined criteria for identifying sustained growth reductions in each site chronology, and thus potential insect-outbreak periods (Holmes and Swetnam, 1996). Individual host chronologies, comprised of standardized ring-width series averaged per tree, from each site were corrected separately using the regional non-host chronology using the following criteria: (1) a minimum threshold of 8 years of below-average growth; (2) reduction CP-690550 in growth below −1.28 standard deviation (representing the lowest 10th percentile in growth); and, (3) inclusion of periods of growth

release prior to and after the maximum growth reduction, to allow for the potential of increased growth years at the beginning and ending years of an outbreak when larval populations may be fluctuating (i.e., declining and then surging) (Swetnam et al., 1995 and Ryerson et al., 2003). Similar threshold parameters were previously used to identify WSB outbreaks (Swetnam and Lynch, 1989,

Swetnam and Lynch, 1993, Swetnam et al., 1995, Campbell et al., 2005, selleck Campbell et al., 2006 and Alfaro et al., 2014). WSB reconstructions were developed with both the regional ponderosa and lodgepole pine non-host chronologies over the common period (1775–2011) and correlated to ascertain the degree of fidelity between the two reconstructed outbreak histories. Evaluation of historical WSB outbreaks at each site required a minimum sample-depth. Accordingly each outbreak reconstruction was truncated

at a minimum of four trees. Outbreak number, duration and return intervals were summarized for each site, and averaged across sites. Return intervals were calculated from the start of one outbreak to the start of the next outbreak. Three thresholds were used that correspond to light, moderate and severe defoliation: dipyridamole (a) at least 15% of trees recording an outbreak (light), which minimizes noise but is more inclusive of lower intensity outbreaks; (b) at least 50% of trees recording an outbreak (moderate); and, (c) at least 75% of trees recording an outbreak (severe). To evaluate the robustness of the reconstructed outbreak history we compared those occurring in the latter half of the 20th century with documented outbreaks in the southern interior of BC (Harris et al., 1985 and Erikson, 1992) and with those identified in recent provincial aerial overview surveys (Westfall and Ebata, 2000–2011). Our reconstructions were also compared to previous multi-century WSB outbreak reconstructions at sites in the southern BC interior (Campbell et al., 2005, Campbell et al., 2006 and Alfaro et al., 2014) and in the northwestern US (Swetnam et al., 1995 and Flower et al.