Time of thinning is related to new or continuing beetle attack:

Freshly cut ponderosa pine slash and firewood are subject to attack by bark beetles.  The success of beetle attacks and production of young beetles are greatly influenced by when trees are cut.  Trees cut during the late summer and fall are seldom successfully attacked, because the inner bark dries during the fall and winter.  The inner bark of green trees cut from January to July remains moist and suitable for beetle habitat. An exception to this is the roundheaded pine beetle, which flies during the fall, and attacks trees at that time.  Roundheaded pine beetle activity is found near trees that have been thinned and chipped in October.
 

What can you do to limit new or continued beetle attack:

The slash and limbs of green pine trees should be buried or burned (according to safe conditions and laws, see note below about Thompson Ridge Slash Pit) within 30 days after a tree has been cut down.  The bole of the tree should receive the same treatment, unless it is needed for firewood or poles.  Then the material should be piled away from living pine trees and covered securely to the ground with heavy, clear plastic.  The plastic covering should be left on the pile at least 60 days if the trees are cut during the April though October period.  Heat builds up under the plastic and kills or traps the beetles. This treatment also dries the inner bark limiting the food for the beetles.  Trees cut in winter should be used in a fireplace or disposed of before April.  If not used, the firewood that is left at the end of March should be covered for 60 days.  If logs are to be used as poles they should be pealed as soon as possible.
 

The Thompson Ridge Slash Pit will be available to all area residents for the disposal of slash, pine needles, green Willows, and other bushes.  The pit is in good condition after a successful burn last year.  The road leading to the pit is in very bad condition, with no Forest Service money for maintenance.  Therefore, a 4 wheel drive vehicle with good clearance is required to navigate the non-maintained road.  Please, no stumps, no nails, no barbed wire, and no trash! Once a beetle build-up starts in the neighborhood all live pine trees are targets for an attack, but some trees are easier targets than others.  Trees that have been damaged or weakened by blasting, excavation, raising of the soil grade, lightning, disease, lack of water, septic systems, or other stresses are usually not able to withstand an attack as well as healthy trees.  An attack by the beetle may not be detected until the beetle has completed its life cycle and the tree starts to fade and die.  At this time, the beetle build-up can be stopped if the tree is immediately cut and the material treated within a few days as described above.

Use of insecticides is generally not recommended:

There are no practical and effective sprays or injections to stop an attack on green trees. Insecticides are not recommended once bark beetles have successfully attacked.  Un-infested trees can be protected from beetle attacks by spraying with insecticides.  When spraying, the entire trunk of the tree up to 4" diameter must be covered.  This is a protective measure only. Insecticide will not kill beetles once they enter the tree.  The available chemicals for this purpose can vary from year to year.  You must use a product that is especially formulated for bark beetles.  Typical home and garden insecticides will be ineffective.  Check with our local county Extension office for current recommendations on the proper insecticides to use as a preventive measure.
 

Watering increases trees resistance to attack, but.....:

Healthy trees offer some resistance to attack.  Watering during dry periods (when less than 2 in. of moisture per month occurs) can help keep trees healthy.  Thinning out dense stands of trees will make more water available for the remaining trees.  Insecticides can be sprayed on the bole of the tree to prevent initial attack by beetles.  This must be done prior to beetle emergence in the spring to be effective.  A fading crown and boring dust are the surest signs that a live tree has been attacked.  When the entire crown begins to fade, there is no hope of saving the tree, and it should be removed as quickly as possible to prevent emerging beetles from attacking other trees.  Once the entire crown has turned yellowish brown, it is generally too late to stop the spread because the beetles have already flown to attack nearby trees.