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The western ash bark beetle (Hylesinus californicus) attacks true ash trees.  It is native to the Prairie Provinces and present in most locations in low numbers.  Following successive years of summer drought, the odour produced by stressed trees can trigger an escalated breeding response in this pest, resulting in an outbreak.

Western Ash Bark BeetleAdults are oval-shaped, 2-3 mm in length and may vary from light reddish-brown to dark brown.

Plants Affected


Beetles bore into branches and their galleries eventually cut off sap flow to the wood, resulting in branch death. Successive years of feeding may eventually kill a heavily infested tree.


Control can be accomplished by maintaining tree health. A well-watered ash tree seldom has high populations of the beetle. Affected branches showing breather holes may be pruned out and destroyed prior to September.

Western Ash Bark Beetle Damage on Black Ash

Symptoms of Western ash bark beetle infestation are deadwood and stunted leaves.

Western Ash Bark Beetle Breathing Holes  

Below these stunted leaves you'll find a ring of breathing holes girdling the branch. Fresh holes often weep sap that stains the bark a dark color.


Last updated: 6/21/2017 9:35:34 AM