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Denver Tree Service Calendar For Pests and Diseases

April 4, 2018

Pests and diseases can damage or kill the beautiful trees in our environment. As a homeowner, one of your responsibilities is making sure common pests and diseases do not affect the trees in your yard. But, it can be difficult to keep an eye out for pests and diseases if you don’t know what time of year they are most likely to attack your trees. Make it easier by referencing this Denver tree service calendar for common pests and diseases:


Pests and diseases are typically not active during January and February, so March is the first month that many of them appear. The ash borer is one type of pest that makes its first appearance in the month of March. The ash borer’s larvae begin burrowing into the trunks of ash trees, slowly causing internal damage that can eventually lead to the tree’s death.  

The Dutch elm disease also is present in the month of March. This fungal disease can cause the leaves on elm trees to turn yellow and then brown before prematurely falling off. The fungus quickly spreads down the trunk of tree, causing many of the infected trees to die within one season. Learn more about Dutch elm disease.


The ash sawfly, which feeds on ash tree leaves, tends to appear starting in April. Feeding on the leaves of a tree may not sound harmful, but these pests can actually quickly devour all of the leaves on an ash tree, leaving it completely bare.

The crown borer is another pest that homeowners should watch for in April, but this pest usually only attacks fruit trees. Many people may not recognize the crown borer when they see it because it looks so similar to a bee or wasp at first glance.

The Zimmerman pine moth is yet another pest that often shows up in April. The larvae feed on the inner bark of pine trees, severely weakening the tree. Because the larvae typically target the areas around the branches, a Zimmerman pine moth infestation could cause the branches to die and separate from the rest of the tree. It’s important to take care of this pest infestation right away to prevent branches from falling unexpectedly and possibly injuring someone who is in the wrong place at the wrong time.

During the month of April, homeowners should also start to look for powdery mildew on their trees and grass. Powdery mildew is a fungus that causes grass to turn slightly gray and leaves to look as if they have been sprinkled with a light colored powder. Fortunately, treating powdery mildew is not as difficult as treating other diseases.


The bronze birch borer is one pest that typically starts to cause problems during the month of May. The bronze birch borer larvae dig into the trunks of the trees, where they feed on the inner bark. If left untreated, this pest can eventually kill the tree by cutting off its ability to transport nutrients.

The elm leaf miner is also known for making an appearance in May. Despite its name, this pest can affect many trees and plants besides the elm, including boxwood, hawthorn, and roses. Elm leaf miner larvae hatch within new leaves, leaving a small brown spot behind. If you notice these spots start to appear on tree leaves in your yard, call a tree service company right away.


The cottonwood borer may start to affect cottonwood, poplar, and aspen trees in your yard during the month of June. This pest drills deep into the bark, where it can prevent the flow of nutrients and slowly kill the tree. But, this pest usually only targets trees that have already been weakened by other pests or diseases.

The fireblight is a disease that is present in June, although it usually makes its first appearance much earlier in the year. This bacterial disease affects mountain ash, hawthorn, apple, pear, and peach trees and needs to be treated as quickly as possible. Compared to other diseases, the fireblight is rather aggressive, so it will spread through the tree quickly and cause a great deal of destruction.


Adult Japanese beetles are usually spotted for the first time during July, however the grub can appear months earlier. The Japanese beetle is one of the newest pests to come to the Denver area, and so far, it seems to target any type of plant or tree.

Another pest that is new to the area is the red-headed ash borer, which targets all types of ash trees. Similar to the cottonwood borer, the red-headed ash borer drills holes into the bark of ash trees, eventually causing enough damage to kill the tree. Unlike the cottonwood borer, the red-headed ash tree does not only target trees that have been weakened.


If you have walnut trees, one disease that you should look for in the month of August is the 1000 canker disease. This vicious disease is transmitted by the walnut twig beetle, which is a small brown beetle that has a pill-like shape. The beetle burrows into the bark, where it introduces this fungal disease to the tree. Exposure to the fungal disease will kill the surrounding tree bark tissue, leaving what is known as a canker. Walnut twig beetles can create a lot of these cankers in a short period of time, which is why this disease is known as the 1000 canker disease.

The cankers may not be immediately obvious due to their size and location. Many homeowners notice the yellowing of the leaves on the tree prior to recognizing the signs of cankers on the bark.

To learn more about pest and disease management, talk to the experts. Arbor Garden is a locally owned and operated tree services company that specializes in tree pruning, removing, stump removal, planting, and pest management. Our certified arborists are more than happy to answer your questions and care for your trees. Contact us today to learn more about our tree and integrated pest management services and receive a free estimate.