Stand-level spatial interactions of secondary bark beetles with endemic Dendroctonus ponderosae populations may facilitate outbreaks

The mountain pine beetle Dendroctonus ponderosae (Hopkins) and its outbreak interactions with suitable hosts has been under investigation for more than a century. However, characterization of populations when beetle numbers are low, specifically at endemic levels, has been quite limited. Seven lodgepole pine stands in British Columbia, Canada were monitored for four years to examine mountain pine beetle populations from the endemic level to the incipient-epidemic phase. The locations of all trees attacked by mountain pine beetle and various secondary bark beetles including Pseudips mexicanus (Hopkins), Orthotomicus latidens (LeConte), Ips pini (Wood and Bright), Hylurgops porosus (LeConte), and D. murrayanae (Hopkins), were analysed using spatial point process models. In the stands that experienced mountain pine beetle population eruptions, a significant positive association between the locations of O. latidens and P. mexicanus galleries was found. Furthermore, trees colonized by P. mexicanus were often strip-attacked by mountain pine beetle the following year. These strip attacks frequently led to mass attack. Our results suggest that secondary bark beetles, particularly P. mexicanus and O. latidens, interact in conjunction to facilitate establishment of D. ponderosae in lodgepole pine hosts at endemic phases, and are thus important species promoting endemic to incipient-epidemic phase transitions by mountain pine beetle.