Primary or secondary bark beetle? Behavior of endemic level populations of mountain pine beetle Dendroctonus ponderosae (Hopkins)

The mountain pine beetle Dendroctonus ponderosae (Hopkins) is credited as the most destructive primary bark beetle of pine forests in western North America. During outbreaks primary bark beetles aggressively attack healthy trees and typically overwhelm their host's defenses by a mass aggregation strategy. However, populations of mountain pine beetle undergoing an outbreak cannot be sustained indefinitely as host resources will be depleted and/or conditions may cease to favor population growth. Thus, populations of the beetle are more commonly present at endemic levels, where they rarely kill trees. Nevertheless, the behavioral characterization of endemic and incipient-epidemic level populations is limited. The present study aims to address this apparent knowledge gap. Seven lodgepole pine stands Pinus contorta (Douglas ex Loudon) in southern British Columbia, Canada were monitored for four years to examine populations of mountain pine beetle at endemic and incipient-epidemic levels. The locations and timing of colonizations 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 and linear regression. Our results suggest mountain pine beetle in its endemic phase behaves much like a secondary bark beetle and often attacks weakened trees previously colonized by other bark beetle species. Temporal partitioning is also evident and may minimize much interspecific competition. Understanding the endemic behavior of mountain pine beetle may provide information leading to more effective future management of economically important pine stands.