Tree Care and Maintenance

Seasonal Tree Care Tips: Fall 2021
Healthy Trees are Defended Trees

By: Dr. Christine Buhl, ODF Forest Entomologist and Morgan Holen, Consulting Arborist

Most insects in our landscape are native, widespread and not pests. Some insects can become pests when their populations explode due to the availability of unhealthy trees (stressed by hot droughts, diseases, mechanical or storm damage, etc.) or a lack of natural controls (cold winters, natural enemies, etc.). It might be tempting to look for a silver bullet by blaming insects for tree damage or mortality, but in the urban environment, insect pests are typically not the primary cause of tree decline and their presence alone is generally not reason to remove a tree.

Since insect pests usually infest unhealthy or stressed trees, managing your trees for improved health and vigor can provide long-term protection for the tree. If your tree is already infested, the most effective treatment will be to determine and manage the primary cause of tree stress. The most common primary causes of tree stress are: hot droughts, planting of non-native species or any species not adapted to site conditions, poor site conditions, storm breakage or construction damage.    

Pay close attention to the progression of symptoms to determine what the causal agent is. Often, damage from the above primary stressors slowly weakens a tree and symptoms may include slow foliage loss, flagging (when a tree's twigs or branches scattered throughout the crown turn brown, wilt, or die), or topkill, for example. Stressed trees, particularly droughted trees, are most susceptible to bark beetle attack, therefore the slow progression of drought symptoms can then be followed by the sudden appearance of multiple piles of brownish frass (sawdust) kicked out by bark beetles infesting the trunk. Mass attack by bark beetles (our most common tree-killing insect group) can kill a tree and turn the foliage red within a year.

Signs of bark beetles and woodborers are not uncommon, especially in western redcedars that are stressed due to poor site conditions, summer drought or mechanical damage. These trees were struggling before beetle infestation, but they may persist for many years with proper care and maintenance including supplemental watering during periods of drought. 

In Lake Oswego, non-native ornamental pine trees are often attacked by sequoia pitch moth, which creates pitch masses near wounds from limbing performed in spring and summer. However, these insects are generally only an aesthetic issue and will not cause the tree to die. Keep in mind that excessive pitch is a normal and healthy wound response to pruning.

Visit the Oregon Department of Forestry’s Forest Health website to learn about insect and disease pests, including the examples pictured below.

Arborvitae (which is in the same genus as western redcedar) are often planted as “fences” in sun-exposed yards. This species prefers cool, moist habitat and will often struggle in yards especially during droughts and be finished off by woodboring beetles.

True fir species in the genus Abies (such as grand fir) prefer cool, moist habitat found at higher-elevations. Grand fir can persist in drier parts lower in the Willamette Valley, although they often reach a critical size that can no longer be supported by moisture on site and are finished off by fir engraver bark beetles (galleries under bark depicted on the right). Grand fir are also at risk from laminated root disease.

Birch prefer cool, moist habitat rather than yard and sidewalk locations and are often finished off by bronze birch borer beetle. Planting native water and paper birch varieties in clusters to provide each other shade, and adding mulch can prevent moisture stress and subsequent beetle infestation.

Douglas-fir is a moderately drought-tolerant tree but poor sites and persistent, intense droughts can cause needle then branch loss and finally the loss of the whole tree. Douglas-fir beetle often takes advantage of these trees when stressed by drought, storm blowdown and laminated root disease.

Even pine can become drought-stressed and lead to mass-attack by Ips beetles, especially if fresh pine slash is nearby to draw them in. Ornamental pines such as Scots and Mugo are often attacked by Ips. These pines are often attacked near pruning wounds by sequoia pitch moth, which cause large pitch masses all along the trunk (depicted on the right). Although this is unsightly, this insect is not a tree-killer.

 Please check back for more seasonal tree care and maintenance advice!

Link to Tree Care and Maintenance Archives


Special Edition: After the Storm

Winter 2021: What is an Arborist?

Fall 2020: Soil Management  

Summer 2020: Retaining and Creating Snags for Wildlife

Spring 2020: Mulch: Numerous Benefits and Easy Application

Winter 2020: Tree Planting: To Stake or Not to Stake?

Fall 2019: Insects and Diseases

​Summer 2019: Drought Stress Revisited

Spring 2019: Trees and Construction

Winter 2019: Topped Tree

Fall 2018: Autumn Leaves

Summer 2018: Emerald Ash Borer

Spring 2018: Pruning Young Trees

Winter 2017: Tree Related Storm Damage

Fall 2017: New Tree Selection and Planting

Summer 2017: Trees and Turfgrasses

Spring 2017: English Ivy Removal

Winter 2017: Recognizing Tree Risk

Summer 2016: Drought Stress

Fall 2016: Preparing Trees for Winter


Additional Tree Care Resources:

National Arbor Day Foundation The Morton Arboretum
Oregon Department of Forestry Tree Care Info
ISA Find an Arborist Alliance for Community Trees
Oregon Department of Forestry  


Right Tree in the Right Place

Use this handy guide for assistance in choosing the right species of tree based on the constraints of a site, such as overhead wires, narrow plant strips, and proximity to structures.

"Right Tree Right Place" - helpful information such as "Use this guide for assistance in choosing the right species of tree based on the constraints of a site, such as overhead wires, narrow plant strips, and proximity to structures."

"Master Plant List" - a guide to the plants that are acceptable for mitigation requirements in Lake Oswego.