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European Elm Scale

​​What is European Elm Scale?

European elm scale is an insect that attacks a variety of elm trees. These insects feed on the trees and produce a large amount of honeydew which eventually covers the leaves and bark in a black coloured mold. The honeydew can also become a nuisance when it covers patios, decks and vehicles. Heavy infestation may kill weakened trees and cause branch dieback in healthy trees. Often a thin canopy in a tree is a symptom of heavy infestation.  

Signs & Symptoms

  • Visible mature insects (10mm long, oval and greyish-brown with a white waxy fringe) on the branches or where the branches intersect the trunk
  • Late leaf out in the spring
  • Stunted, yellowing leaves leading to premature leaf drop and die back
  • Honeydew secretions on leaves, branches, sidewalks, patios, vehicles, etc.
  • Black sooty mold growth on the leaves, branches and trunk

How Can I Help?

Healthy trees stand a better chance of surviving an infestation from European Elm Scale. The following are things that you can do to help keep your trees healthy:

  • Water regularly, especially during drought times (this includes mature elm trees)
  • Try not to damage the root systems of the tree when landscaping or doing other home/yard renovations
  • Be aware of overhead branches and try not to break or damage any elm tree branches
  • Avoid compacting the soil around the roots of the tree (driving or parking vehicles on the roots)
  • Mulch the area around the tree

If your elm tree is showing signs of stress from European Elm Scale we recommend contacting a qualified arborist that can administer a pesticide injection to combat the insect. Information about local arborists can be found in the Yellow Pages here.

It is estimated that there are 11,000 elm trees in Lethbridge (split evenly on public and private property) so your help is critical in preserving our urban elm forest.


Not sure if you have an elm tree on your property?

An elm is easily recognized.

To see good examples of American elms in Lethbridge, look at the trees that are growing around Civic Centre Park and around the Downtown Library. American elms have tall straight trunks. Branches start high and grow upward to form an arching shape like a vase or umbrella.

The bark of an elm tree is thick and rough, usually dark grey to greyish brown and made up of broad intersecting ridges.

The leaves are dark green, up to 9 cm (3.5 inches) long and 2.5-5 cm (1-2 inches) wide with double toothed edges. The leaf underside is rough because of raised veins and the two lobes next to the leafstalk are unequally rounded.


What is the City Doing?

After an aggressive campaign to fend off the Elm Scale infestation in 2016, the City of Lethbridge continues to be diligent in the protection of the City's elm tree population. In mid-June 2016​, City Council provided additional funding to treat approximately 3,600 public trees with TreeAzin, a natural insecticide, to combat European Elm Scale. How each tree responded to the treatment appears to be quite different and is dependent on many factors.

Elm trees treated in 2016 will not be injected again this year as per the recommendation of the TreeAzin manufacturer. The process involves drilling and injecting into the trunk which, if done repeatedly, could cause more damage the tree.

The City will continue to monitor public Elm trees and will implement a long-term rotating treatment schedule. It's estimated there are 11,000 elm trees in Lethbridge. Approximately 5,500 of them are on public property.

If you have an elm tree on your boulevard that needs attention please email trees@lethbridge.ca or call 403 320 3850. When submitting a tree please include your name, address (location of the tree) and contact information - not doing so may delay the assessment of the tree. These trees will be added to our database and evaluated throughout the summer.