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April 14, 2015
Vol. 2, Issue 3
April 2015
Save the Date!  

April 27 - May 1 - Western Chapter of the International Society of Arboriculture's 81st Annual Conference & Trade Show, "Nature and Science of Arboriculture;" Tenaya Lodge, Yosemite National Park; CEUs are available.

July 14 - 16 - CFPC 2015 Golf Tournament (7/14) and Weed Tour (7/15-16), Weed; CEUs are pending.

July 22 - California Forest Pest Council Summer Insect and Disease Field Tour, Klamath National Forest, Happy Camp; Admission is free. DPR credits are pending.

Native Bark Beetle Attacks on Rise - Are Your Trees at Risk?   

Overstocked forests and the relentless drought in California have led to a surge in attacks by native bark beetles and wood borers across the state, leading to the death of hundreds of thousands of trees. Principal bark beetle species in California include the mountain pine beetle, fir engraver beetle, western pine beetle, Jeffrey pine beetle, and various pine engraver beetles. Many have a unique gallery pattern (found on the inside of infested tree bark) which can be used to identify the attacking beetle.


Look for pitch tubes (light grey in this photo) on tree boles, indicating bark beetle attack. By B. Bulaon, USFS. 

To better understand tree susceptibility to bark beetle attack, it is important to know which tree species and environmental factors are present in an area. Tree and environmental factors that can cause stress include human activity, imbalanced species composition, high stand density, advanced tree age, drought prone areas, and root disease. The existing beetle population also influences tree susceptibility, as large beetle populations may be able to overcome the defenses of relatively healthy trees. If dead and dying trees are nearby, it is a good indication of increased tree susceptibility and elevated beetle populations.


Once a tree has been colonized by bark beetles, there is nothing that can be done to save it, making prevention of beetle attack of the utmost importance. Extended periods of drought, such as now, present the most challenging situation for protecting trees, particularly on a landscape basis. For individual high-value trees, preventative insecticide treatment is an option. Removal of infested trees is also an option for controlling beetles, but this strategy is very difficult to effectively apply in practice. Dead and dying trees should be removed if they present a hazard to life or property. Stand thinning is the best long-term preventative measure as it will increase tree health and vigor and reduce the likelihood of bark beetle attacks. The best time to thin, however, is during non-drought years because this allows trees to adjust to and take advantage of added space before the next drought occurs. Thinning during drought will have little benefit for already stressed trees. For more information, go to "Bark Beetles in California Conifers - Are Your Trees Susceptible?"

Drought, Insects, and Trees Q&A  

What is the cause of conifer die off?  The root cause of die off is moisture stress.  Trees weakened by drought are most often attacked and killed by native insects or diseases.


Ponderosa pines killed by western pine beetle and
Ips, Lake County. By D. Owen, Cal Fire. 

Can insect-related tree mortality be prevented from spreading?  Area-wide treatments to prevent additional mortality are very difficult when trees are under severe stress and  insect populations are high. Treatments to protect individual trees have the highest probability of success.


If a tree is half dead, can it be saved?  In some instances, trees can die in part and still recover.  However, these trees have a higher probability of dying when drought persists as they are under increased stress. It takes time for the foliage of a dead tree to change color, so it is possible for a tree to appear to be half alive, but actually be dead.   Carefully monitor these trees for signs or symptoms of overall decline.    


Will more rain or watering help distressed trees?  It will help, but it is difficult to say how soon and how much it will help.  Drought-related tree mortality continues beyond the end of a drought because precipitation benefits are not immediate.  It takes time for trees to recover and for beetle populations to decline. Providing supplemental water to individual trees has been recommended, but there is little scientific guidance on how to do this.  Different tree species also have very different watering requirements. 


If we don't remove the already dying trees, will the rest be lost?  Beetles that kill trees can fly long distances, which makes area-wide control of beetle populations difficult.  To confound the issue, it is difficult to identify and remove beetle-infested trees before the beetles have left the tree.  If a tree's foliage has turned red-brown, many, if not most, beetles will have already emerged, moving onto other trees.  Removing such trees will have little impact on the beetle population.


Is bug-damaged wood good for high-end sale?  Wood from beetle-killed trees typically has lower value than wood from a tree that was alive when cut. Wood from such a tree may have staining or even insects, which can limit its marketability.  

Newsletter feedback and ideas are welcome.  Please submit comments to

Spring is here and camping season is just around the corner! Remember to always buy or gather firewood from local sources, helping to minimize the spread of pests and diseases - Buy It Where You Burn It


Katie Harrell (formerly Palmieri)
Communications Director  
California Forest Pest Council
California Forest Pest Council | (510) 847-5482 | |

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