Identifying Ash Borer

Identifying Ash Borer

Identifying “clear winged” ash borer.

Although we’re now faced with the tenacious emerald ash borer, we’ve had our own Denver ash borer known as lilac /ash borer preying on our Colorado ash trees for some time.

While the following tips help you identify ash borer success in your ash tree, it’s more helpful to watch out for these symptoms in neighboring ash trees before they try to make a move on your tree.

Here’s what to look for.


Host plants (Denver):

  • ash trees
  • lilac bushes
  • Less common in alder, maple

What to look for?

  • Wasp-like insects about a quarter inch long, narrow front wings and shorter, wider hind wings.
  • Trunk injury and emergence holes.
  • Brown frass (sawdusty material) around bark cracks and bark wounds.
  • Thinning foliage in the crown of the tree.

Timing during the year

Ash/lilac borer and lesser peach tree borer adults begin to emerge when common lilac and Vanhoutte spirea bloom in early- to mid-May.  Ash borer deposits eggs underneath the bark, and the resulting larvae bore into the trunk -creating a tunneling system that damages the vascular tissue. The tunneling effect harms the tree by ruining its ability to transport water and nutrients from the roots to the branches and leaves.

Protecting your Tree from Ash Borer

The first line of defense for these pests is simply to maintain a healthy tree. Ash borers prefer stressed trees that have other problems such as:

  • lack of water
  • overwatering
  • lack of nutrients
  • being overgrown
  • root damage (typically caused by construction)
  • lack of room to grow (cement surrounding tree)
  • physical damage ( bad trimming, hail, lawn mower, etc).
image depicting a wound in a tree caused by Ash Borer

a characteristic Ash Borer wound

By identifying what may be stressing the tree, you may be able to implement a relatively easy fix that will make the tree more robust, allowing the tree to defend itself against attacks.

If the tree is nutrient deficient, a deep root fertilization can strengthen the tree against these pests. Ash trees have a tendency to become overgrown quickly, making it difficult for the tree to process energy. If this is the case, we recommend you begin with tree trimming.

A preventive Ash tree spraying is the only way to be sure your tree will not become infested. For the tree spraying to work, timing is extremely important. We recommend you spray in April and May when the ash borer is flying –  before it can drop its larvae in the trunk. Once the larvae bore into the trunk, they’re nearly impossible to reach with a contact spray.