Insects and Diseases

Press Enter to show all options, press Tab go to next option

The City Forestry Division has 5 certified arborists on staff to diagnose insect and disease problems on city property. Common threats are summarized below with a brief description, symptoms/signs, and treatment options. If you would like to schedule an inspection of a city tree, please Contact Us.

Emerald Ash Borer

The Emerald Ash Borer (EAB) is a looming threat to Bozeman’s ash tree population. The non-native, invasive pest quickly kills entire trees and has the potential to wipe out ALL of our ash tree population. With the most recent outbreaks in nearby Colorado, the Forestry Division is making preparations. Bozeman City Commission recently adopted an EAB Action Plan to combat this devastating threat to our urban forest.

Description: The Emerald Ash Borer (EAB) is a non-native boring beetle. Adult beetles are a shiny, emerald green color. The adults deposit eggs underneath the bark of ash trees. The eggs develop into larvae that tunnel through conductive tissue, severing the flow of nutrients and water within the tree. Larvae develop into adults and emerge in mid-summer, usually July. 

EAB Adult                    EAB Larva (Cropped)

Image 1. The adult form of EAB.                                                         Image 2. The larvae feed under the bark.

Symptoms/Signs: Infested trees will start to die "from the top down". Look for thinning foliage and/or dieback in the upper canopy. This dieback is followed by lush new growth at the base of the tree. Severely declining trees will have "D-shaped" exit holes where adults have emerged. Removal of outer bark will expose "S-shaped" tunneling of the larvae.

            Canopy Dieback (Before)        

   Images 3-4. Heavy feeding from EAB will thin the canopy, starting from the top branches.

Treatment: We do NOT recommend ANY TREATMENT at this time, as EAB has not been detected in our area. It is best to wait until State and Federal entities declare an official outbreak. The most effective treatment is a trunk injection of chemical pesticide. This method provides direct uptake into the tree with minimal environmental or safety concerns for the public. This pesticide will need to be re-applied every 2 years until the EAB population is eliminated. Smaller trees (10" dbh or less) should be considered for removal and replacement.

Bozeman EAB Course of Action Plan

The Forestry Division has a plan for EAB's arrival, including early detection, treatment options, and necessary tree removals. To learn more about the Emerald Ash Borer, read the Course of Action Plan from the link below.

EAB Course of Action Plan - Bozeman


Fire Blight

Description:  Fire blight is a bacterial disease which affects members of the Rose (Rosaceae) family, including apple, pear, hawthorn, and mountain-ash. The disease targets new growth in flowers, fruits, and young branches. If allowed to progress, fire blight can lead to severe decline and eventually death.

           Fireblight Flagging Image 1              Fireblight Flagging Image 2

          Images 1-2. Fire blight is recognized by "flagging" - obvious dieback in smaller sections of new growth.

Symptoms/Signs:  Affected tissues experience wilting and turn brown or black. Opaque white or amber "droplets" may form under wet conditions, and infections spread to larger tree limbs, darkening the color of the bark.

            Fireblight Damage Closeup Image 1           Fireblight Damage Closeup Image 1

Images 3-4. Damaged tissue is easy to spot. Pruning to remove the infection and proper disposal off-site is key to managing fire blight.

Treatment: The fire blight bacteria is ever-present in North America. Chemical options are only preventative and not practical for our urban forest. The best way to manage trees that are infected is to prune out the damaged tissue during dormancy, and remove it from the site.  Caution should be used as the bacteria moves into the tree through open wounds or cuts, especially in warm, wet weather.  Susceptible trees can be replaced with cultivars that are resistant to fire blight.



European Elm Scale

Description: While we are fortunate to not have Dutch Elm Disease in our area, Bozeman's elms can be heavily affected by scale insects. These pests are soft-bodied and mobile when young, then they develop a tough armor coating into adulthood.

             Scale Outbreak on Trunk          Sooty Mold Staining Bark

Image 1. Scale insects are visible to the naked eye, and can cover branches on infested trees.

Image 2. Trunks are stained black from honeydew and the Sooty Mold fungus.

Symptoms/Signs: Scale insects feed on the leaves, twigs and bark of elm trees, sucking sap with their mouthparts. Heavy infestations will lead to reduced leaf production and yellowing. Scale excrete "honeydew", a sticky waste product that causes a nuisance for surfaces under the tree. The presence of honeydew promotes a secondary fungal issue known as Sooty Mold, which stains trunks to a dark brown-black.

Honeydew on Foliage

Image 3. Scale produce "honeydew", a sticky waste product that coats everything beneath the tree.


Treatment: Timing is critical for treating scale insects. Foliar sprays and soil drench applications of insecticides are most effective in the Spring (April-May). The 'Brandon' Elm cultivar is highly susceptible to this pest. Heavily infested trees should be considered for removal and replacement. There are several varieties of Elm that show high resistance to scale.


Cottony Ash Psyllid

Description: Cottony Ash Psyllid (CAP) is a major issue for Black ash (Fraxinus nigra). It does not affect other ash varieties. Psyllids are a type of insect with piercing, sucking mouth parts that feed on young leaves. Repeated infestations will stunt tree growth and eventually deplete resources, resulting in death.

Symptoms/Signs: Psyllids over-winter as eggs, hatching in the spring when trees leaf out. Feeding causes young leaves to curl along the edges. Heavy feeding results in a distorted leaf shape and a cottony material that protects the insect.

            Ash Psyllid Leaf Curling Image 1           Ash Psyllid Leaf Curling Image 2

Images 1-2. Heavy leaf "curling" caused by CAP feeding. It is best to remove affected trees.

Treatment: CAP often leads to tree death. There are effective chemical treatments, but they must be repeated annually and can be expensive. Most of Bozeman's black ash trees have already succumbed to CAP, and they will also be vulnerable to the Emerald Ash Borer. The best course of action is to replace affected trees with a new species, increasing the diversity of our urban forest.


Honeylocust Cankers

Description: Cankers are lesions of bark decay caused by fungal pathogens. They often enter the tree through wounds on the main trunk. Honeylocust trees are susceptible to several types of cankers, which often lead to severe decay and death.


Symptoms/Signs: Cankers are characterized by sunken, discolored areas of trunk tissue. Bark deteriorates to expose inner wood and heavy callous tissue. Heavily affected trees may show early fall color and leaf drop.


Treatment: Preventing wounds is critical, especially on the main trunk. Many honeylocusts are doomed by mower and string trimmer damage when young. Mulch rings and plastic trunk guards are essential to prevent cankers. Pruning branches should always be done with proper technique and clean, sharp tools. Healthy trees stand the best chance - ensure your honey locusts have a mulch ring, trunk protection, and adequate water. The 'Sunburst' cultivar is particularly vulnerable, while 'Imperial' and 'Skyline' cultivars are resistant to cankers.



Rough Bullet Gall

Description: Galls are portions of the tree that have been warped by a pest, in this case a small wasp. The Bullet gall wasp ONLY affects Bur oak (Quercus macrocarpa), forming rounded, bullet-shaped egg sites from young branches. In most cases, Bullet galls are a cosmetic issue for Bur oak, stressing the tree but rarely leading to death.

 Gall Massing 1         Gall Massing 2


 Galls are readily visible on affected Bur oaks. This pest damage is mostly an eyesore.

Symptoms/Signs: The brown, rounded galls can be easily spotted. Heavy infestations may reduce the growth rate of the tree. Secondary pests like bees and other wasps can be attracted in the fall by the developing larvae. Adults emerge in late October/early November, with females depositing new eggs into dormant buds.

 The Bullet Gall Wasp only affects Bur oaks.

Treatment: There isn't currently an effective insecticidal treatment for this pest. Start with good site conditions - ensure your oaks have a mulch ring, trunk protection, and adequate water. Developing galls can be removed by hand. Check for exit holes - there is little purpose in removing empty galls aside from aesthetics.

Old Galls with Exit Holes         Young Galls Developing

Exit holes show the wasp has already left. Removing old galls only serves an aesthetic purpose. Look for young, light-colored galls with no exit holes. Removing these by hand will also remove developing wasp larvae.