Lilac-ash borer, which flies from April through June, is the number one problem-borer of ash. Stressed ash trees are the most susceptible to damage from this pest. Stressors include improper siting, under- and over-watered trees, recent transplants and trunk damage. Trees in poor condition have less capacity to fight off borers and recover from attack.
The emerald ash borer has been recently found in Colorado. All ash trees in the Fraxinus genus are susceptible to this pest. Mountain ash is not a true ash and is not susceptible.
The best way to control the lilac ash borer is to apply insecticide to the bark when the adults are flying. Permethrin (Astro) is the most widely labeled insecticide for borer management.
Several treatment options exist for emerald ash borer. Because the management strategy is complicated, please see the following website which contains detailed information about the topic.
The best way to prevent or reduce borer damage is to maintain a tree’s vigor through consistent, year-round watering and to avoid bark injury.
For more information, see the following Colorado State University Extension fact sheet(s).