Ash Trees, Ash Borer

Asked November 3, 2016, 5:07 PM EDT

What is your advice about keeping an ash tree (Autumn Purple Ash) safe from the Emerald Borer and any borer?

Larimer County Colorado trees and shrubs

1 Response


There are two borers that are a threat to ash trees--one is the lilac ash borer and the other is the emerald ash borer. Lilac ash borer is usually only a threat to younger trees or those that are stressed (drought stressed, improperly pruned, etc.). The best defense against lilac ash borer is to keep trees healthy, well watered and pruned properly. Planting properly is also important. Lilac ash borer rarely kills trees, unless it's prolonged attacks. In general, treatment for LAB is not necessary.

Emerald ash borer is a different situation...this insect is extremely destructive. However, it has NOT BEEN FOUND in Larimer County and treatment for this insect is not recommend at this time. It has been confirmed in Longmont and Boulder, as well as the Gun Barrel subdivision (between Longmont and Boulder). However, it is imperative that you make a decision on what you want to do for your tree. Not every tree is a candidate to treat or keep. Younger trees are more likely better replaced than to do treatments. If your tree has other issues, like storm damage, LAB damage, etc. then treatment may not be an option.

One option is to keep your current tree with the plan to remove and replace when necessary. Another is to plant a new species near your current tree, with the intent to remove. Treatment for emerald ash borer is also an option. There are various options. If the tree is small enough (less than 15" diameter at chest height), you can buy products yourself containing the chemical imidacloprid. You can also pay for a tree care company to do treatments. They may do a soil drench (imidacloprid) or trunk injections (emamectin benzoate, azadirachtin). Cost among treatments will vary. The trunk drenches and foliar treatments are not very effective. But again, treatment AT THIS TIME is not necessary...keep your ear out for advice as we get into the 2017 growing season.

For the most up to date information, as well as information on treatment options, please visit

There is also a very good publication from CSU Extension on treatment options: