Western pine beetles’ infestation of ponderosa pine trees across the Boise National Forest has increased due to drought conditions.
Aerial surveys conducted by the U.S. Forest Service showed a three-fold increase in ponderosa pine mortality with approximately 5,900 acres impacted. The majority of trees affected occurred west of High Valley, with pockets of infested trees also in the Sage Hen Reservoir area.
Another area with an increase is Boise Basin in the vicinity of Placerville and Centerville. More ponderosa pine mortality is expected this year especially in areas of high tree density or with weakened trees due to dry conditions.
“During periods of extended drought, pine trees become water-stressed and their defenses are reduced giving beetles an upper hand,” said Joel McMillin, forest health protection Boise field office group leader. “Bark beetles will also exploit trees damaged in fires leading to increased tree mortality within the fire boundary and adjacent areas.”
Often there is a one- to two-year lag between the start of drought or fire occurrence and when beetle activity increases to the level that it becomes noticeable. However, once the drought ends and trees recover, beetle populations collapse and tree mortality diminishes.
Infested trees are typically first noticed when pine needles fade from green to reddish-brown in the summer and early fall. Tree trunks will often reveal popcorn-sized wads of pine resin before the needles begin to fade.
By the time all the pine needles have faded to brown, the beetles that killed a tree will have vacated and pinhead-sized exit holes can be seen on the bark surface.
“The best long-term approach to lessening bark beetle effects are to promote vigorously growing ponderosa pine in stands with a diversity of tree species and sizes,” McMillin added. “Typically this is achieved through thinning, which decreases competition for water among the remaining trees.”
The Forest Service said landowners seeking short-term protection against bark beetle attack on individual, high-value trees can treat them with an application of insecticide sprays registered for use on pines. They can contact their local Idaho Department of Lands or University of Idaho Extension office for additional information.
Landowners are reminded to not stack beetle-infested firewood or green logs against or near desired ponderosa pine.