Lilac Ash Borer

Lilac Ash Borer- Cassey Anderson CSU Extension Adams County

Most of us are aware of the looming menace posed by Emerald Ash Borer to our ash trees along the Front Range in Colorado. However, we do have another pest of our ash trees, the lilac ash borer. Lilac ash borer can, like their name, impact both lilacs and ash trees. They are more of a concern in ash trees.  Fortunately they are usually not lethal to the tree.  Adult borers emerge from trees in the spring then lay egg on the bark .  The larva which develop then burrow into the tree where they spend most of their time in heartwood.  They then overwinter in the tree and emerge as adults the next year.

Pupal Skin Left Behind by Lilac Ash Borer (from CSU Extension Lilac Ash Borer fact sheet)

Distinctive round/melon shaped holes can often be found on the main trunk of the tree. During the spring and early summer it may even be possible to find the final pupal skins of the lilac ash borer left in the trunk as the mature insect emerges. The pupal skin may resemble a wasp. They are not related to wasps but rather are a type of clearwing moth which has some resemblance. 

Lilac Ash Borer Exit Holes

We had a severe wind event here in Adams County this spring and saw one of the unfortunate side effects of a tree that had been riddled with the insect.  While the damage caused by the larvae rarely kills trees because it does not feed extensively in sapwood it does weaken and makes them more prone to failure.   

This tree snapped a foot or so off the ground exposing the larval galleries both new and old from the feeding of the lilac ash borer. These trees had been planted in a very narrow strip between road and sidewalk, were not getting sufficient irrigation if you looked at past growth and the grass growing around the trees, several of the trees also had visible girdling roots which likely compounded their stress.

Lilac Ash Borer Galleries 

Generally speaking stressed trees are more susceptible to damage from lilac ash borer. Ensuring that existing trees are planted correctly, do not have girdling roots, and are watered appropriately can reduce infestations of lilac ash borer.  Treatment for this insect is not the same as treatment as for emerald ash borer. For treatment options and more information please see our fact sheet on lilac ash borer: https://extension.colostate.edu/docs/pubs/insect/05614.pdf