Ash borer program in Terwillegar

The ash borer moth, or lilac ash borer, is an insect native to North America. The larvae feed by laying their eggs within the bark of ash trees. The caterpillar chews into the bark, developing inside the tree for two years, then exits and leaves behind a round hole.

Because the caterpillars are buried within the tree and are only visible when they emerge, it can be hard to detect an infestation. This makes them difficult to target with many chemical or mechanical control methods.

The Terwillegar area has seen as many as 61% of ash trees affected since 2009, but neighbourhoods far to the north have also been affected.

Removal Program

In 2011, trees with large numbers of emergence holes were targeted for removal.  As the process developed, it became evident that the number of trees affected was too large for simple removal. Furthermore, evaluations showed that the healthiest trees often had the most emergence holes. There was little correlation between infestation rate and whether or not the trees died.

Instead of removing these trees, they were instead targeted for pesticide treatment. In 2012, nearly two thousand trees were treated with permethrin. Because of the two-year life cycle of the caterpillars, insects emerging this summer will have actually been laid in 2012.  While the full effectiveness of the pesticide program can’t be fully evaluated until 2015 or later, other areas have had success with similar programs.

Alternative methods to regular biological pesticides are also being tested.  Predatory nematodes that burrow into the chambers or ambush emerging moths are being trialed in some neighbourhoods. Researchers at the University of Alberta are also testing new pheromone attracticides, which draw in and kill emerging males before they can mate. Natural enemies, such as predatory insects and parasitic wasps are also being investigated and monitored

The Terwillegar area was built on marshy wetland, and soil quality seems to be a factor for infestation. Poor soil drainage leaves trees sitting in waterlogged soils, drowning their roots. In some cases, ongoing efforts to keep basements drained may be contributing to the problem, as excess water is pumped out of basements and onto boulevards.

What can homeowners do?

Plant care is the key to assisting these ash trees. Moderate use of fertilizer, soil aeration, and anything to improve drainage conditions will all benefit these trees. For more information on how you can protect your trees, call 311 or visit