Factors impacting management of pine bark beetles in ponderosa pine forests in northern Arizona

Bark beetles (Coleoptera: Curculionidae, Scolytinae) can be a serious damaging agent in most western forests. Although most beetles are native, and as such play a vital role in forest ecology and succession, current epidemics are thought to be exacerbated by global climate change and past forest management practices. The three studies I present here were designed to help forest managers in northern Arizona ponderosa pine (Pinus ponderosa Douglas ex. Lawson) forests mitigate bark beetle outbreaks. The first study addresses bark beetle seasonality, in particular, response to spring temperatures for initiation of bark beetle flight in southwestern ponderosa pine forests. Beetle flight in the spring was initiated when temperatures exceeded 15°C, although results varied slightly by species. These results have important implications for managers implementing slash removal and beetle monitoring projects. The next two studies address how bark beetle movement, attraction to pheromone lures and tree vigor differ across a range of basal area treatments. Stand thinning treatments are often advocated as a mechanism for improving forest health. In Arizona the goals of these treatments include increasing tree vigor, reducing the risk of catastrophic crown fire, restoring stands to pre-European settlement conditions, and increasing stand resistance to bark beetles. The objectives of my study were to examine a range of thinning treatments to determine if there was a threshold basal area above which bark beetle attacks were more likely to occur. When pheromone lures were not used, beetles and their predators were collected most often in traps located in stands with lower basal areas. However, when pheromone lures were used beetles were collected more often in stands of intermediate and high basal areas. More trees were successfully attacked by bark beetles in stands of higher basal areas. Measured characteristics of tree defense to bark beetles did not show a consistent relationship with stand basal area, however, we did find an overall pattern of less water stress and more resin flow in stands of lower basal areas. Additionally, mean resin flow was lower in trees successfully attacked by bark beetles than unsuccessfully attacked trees. Management implications for our studies are discussed.