European elm scale (EES) is a pest that normally attacks elm trees. Elm scale feeds by piercing leaves and bark and sucking juices from the tree. The eggs begin to hatch in late June and start feeding on leaves in mid-July. By autumn, the pest moves onto branches and twigs where they prepare to overwinter.


Damage symptoms are not readily apparent on Elm trees unless the infestation level is extremely high. Damage consists of:

  • Leaf discoloration and premature leaf drop.
  • Twig and branch dieback.
  • Sticky honeydew material falling as fine droplets from the tree canopy.

What you can do

The easiest way to prevent pests and diseases is to keep your trees healthy. One way to do this is to give your trees adeep watering on a monthly basis, beginning when the tree leafs out and ending when the tree drops its leaves in the fall.

If you detect an infestation, homeowners are encouraged to first gently hose the tree down with water. Another option involves applying dormant or horticultural oil in order to suffocate the pest. Early season control is applied shortly after tree bud-break, prior to the production of elm scale adults and crawlers. Late season control is undertaken just prior to leaf drop as elm scale move back to the branches. During peak crawler numbers, apply a soap and permethrin mix known as Trounce® to significantly reduce the infestation.

The City of Calgary is conducting an injection program to treat select, highly infested Elm trees with a systemic insecticide. This control approach permits the control of all parts of the pests lifecycle.

Elms in Calgary should only be pruned between October 1 and March 31 by law to prevent the spread of Dutch Elm Disease.


Learn more about tree pests and diseases in Calgary​

Elm scale