The emerald ash borer is the most destructive tree pest to reach North America in modern history. Since its discovery in Michigan in 2002, it has killed hundreds of millions of ash trees across the eastern United States and Canada, steadily moving westward. It has been confirmed in Winnipeg since 2017, and its arrival in Alberta is considered a matter of when, not if. If you have ash trees on your Calgary property, now is the time to understand the threat and start preparing.
Why Emerald Ash Borer Is So Devastating
The emerald ash borer, commonly called EAB, is a small metallic green beetle native to Asia. The adult beetles cause minimal damage, feeding on ash leaves. The destruction comes from the larvae, which feed on the inner bark and cambium layer of ash trees, creating winding galleries that disrupt the tree's ability to transport water and nutrients. Within three to five years of infestation, most untreated ash trees die.
What makes EAB particularly devastating is that it kills virtually all species of North American ash. Unlike some pests that preferentially attack stressed trees, EAB kills healthy, vigorous trees just as readily as declining ones. There are no naturally resistant North American ash species, and mortality rates in untreated trees exceed 99 percent once the pest is established in an area.
How Many Ash Trees Does Calgary Have?
Green ash is one of the most commonly planted street and yard trees in Calgary. The City's urban forest inventory includes tens of thousands of ash trees on public land alone, and there are likely a similar number on private property. Many of Calgary's mature boulevard trees are green ash, and entire neighbourhoods planted in the 1970s and 1980s have ash as their dominant shade tree.
The potential loss of these trees would fundamentally change the character of many Calgary communities. Losing the ash canopy would remove shade, reduce property values, increase energy costs, and create a massive disposal and replanting challenge.
Identifying Your Ash Trees
The first step in preparation is knowing whether you have ash trees. Green ash, the most common species in Calgary, has the following identifying features:
- Compound leaves: Each leaf is made up of five to nine smaller leaflets arranged in pairs along a central stem, with one leaflet at the tip.
- Opposite branching: Branches and leaves emerge in pairs directly across from each other on the stem. This distinguishes ash from many similar-looking species.
- Diamond-patterned bark: Mature ash trees develop a distinctive pattern of interlocking ridges on the bark that resembles a diamond or net pattern.
- Seeds: Ash trees produce paddle-shaped seeds called samaras that hang in clusters and spin like tiny helicopters when they fall.
If you are unsure whether your tree is an ash, an arborist can identify it quickly during a consultation visit.
Signs of Emerald Ash Borer
Early detection is critical because treatment is most effective before significant damage occurs. Watch for these signs:
- Canopy die-back: Thinning and dying branches starting at the top of the tree and progressing downward over several years.
- D-shaped exit holes: Small, distinctly D-shaped holes in the bark about 3 to 4 millimetres wide where adult beetles have emerged.
- Bark splitting: Vertical splits in the bark revealing S-shaped larval galleries underneath.
- Increased woodpecker activity: Woodpeckers feed on EAB larvae and may strip bark from infested trees, creating blonde patches on the trunk.
- Epicormic shoots: New sprouts growing from the trunk or major branches, which the tree produces as a stress response when the upper canopy dies back.
Treatment Options
The good news is that ash trees can be effectively protected from EAB through trunk injection of systemic insecticides. The active ingredient most commonly used is emamectin benzoate, marketed under the trade name TreeAzin in Canada. When injected into the trunk, the product distributes through the tree's vascular system and kills EAB larvae as they feed on the inner bark.
Treatment must be performed by a licensed applicator and is typically done on a two-year cycle. Research from cities with established EAB populations shows that properly treated trees survive with minimal damage while untreated trees around them die. The cost of treatment varies based on tree size but generally runs from $150 to $400 per tree per treatment cycle.
The question for Calgary homeowners is when to start treatment. Preventive treatment before EAB arrives is an option but adds years of cost before the threat materializes. Most experts recommend beginning treatment when EAB is confirmed within 50 to 100 kilometres of your location or earlier if you want maximum protection for a particularly valuable tree.
The Removal vs. Treatment Decision
Not every ash tree is worth saving. Treatment is a long-term commitment, potentially spanning decades, and the cumulative cost over many treatment cycles can be substantial. For a small, declining, or poorly positioned ash tree, removal and replacement with a diverse species may be the better investment.
Treatment makes the most sense for large, healthy, well-positioned ash trees that provide significant shade, property value, and aesthetic benefit. A mature green ash that shades your entire backyard and would cost $3,000 to remove is worth protecting at $200 to $300 every two years.
What You Can Do Now
Even before EAB reaches Calgary, there are productive steps you can take. Identify all ash trees on your property. Invest in their health through proper watering, mulching, and pruning, because healthy trees respond better to treatment and resist pests more effectively. Start budgeting for future treatment costs. And consider diversifying your landscape by planting non-ash species so that your property is not entirely dependent on one species that may be threatened.
Most importantly, do not move firewood. EAB spreads primarily through the transport of infested wood. If you bring firewood from camping trips in eastern Canada or from outside Alberta, you could inadvertently accelerate EAB's arrival. Buy firewood locally and burn it where you buy it.