Issue: March 10, 2007

Ash tree borers


I have a row of seven ash trees in my yard. They are either Modesto ash or mountain ash. I have been told that they have borers and spraying is the way to treat them. What insecticide is best and what time of year is best for treatment?

Judee B.


Proper identification of the tree makes a great difference in diagnosing problems. Modesto ash is a cultivar of Arizona ash and well adapted to the Southwest. It is in the olive family. It has an odd pinnately compound leaf. That means the leaf is composed of an odd number of leaflets arranged along a central axis. These leaflets are usually at least 3 inches long and pointed at the end. When the leaf falls from the tree, it often falls as a unit with most of the leaflets still attached. Its flowers are not showy. The Modesto ash also has a light tan bark on twigs and younger branches. The trunk develops a furrowed gray bark.

The mountain ash is a very different tree. It is in the rose family. It also has an odd pinnately compound leaf, but the leaflets are smaller, more numerous, and rounded at the end. It produces large clusters of small white flowers in the spring and clusters of red berry-like fruit that mature in the fall. It has a smooth, shiny bark on all but the oldest trunks.

The Modesto ash is common in New Mexico while the mountain ash is much less common. The mountain ash does not grow well in the lower, hotter regions of the state and does better in the higher elevations. However, it is sometimes planted at lower elevations. Nevertheless, it is less common than the Modesto ash in New Mexico landscapes.

The Modesto ash and its relative, the green ash, are often attacked by bark beetles and borers. The green ash is host to these pests more often than the Modesto ash. The mountain ash, while subject to borer attack, is more often affected by fire blight and drought problems. For all the reasons mentioned above, I suspect your tree is either the Modesto or green ash. However, without seeing it, I cannot be sure. Take a sample to your local Cooperative Extension office (even a winter sample of a twig with several nodes) to have it identified. Once the tree is identified, the County Extension agent can help you determine which pest is likely to be the problem and when and how to treat the tree to minimize the problem and reduce the chances the tree will be killed by borers.

Borers are difficult to control by spraying while they are inside the tree. Insecticide sprays can sometimes be used to prevent further attack by borers. The timing of the spray is very important to its success. Understanding which pest is present is critical to choosing the appropriate chemical to use and if there is a chance that the chemical will succeed.

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Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email:, office: 505-865-7340, ext. 113.


For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms and the NMSU Horticulture Publications page.

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