Oyster Shell Spraying Starts Tomorrow, June 7th!

Hello Mount Royal Residents!

In recent years the City of Calgary has had an increasing problem with oyster shell scale (Lepidosaphes ulmiect). This insect is not native to North America and can decimate cotoneaster hedges and other species such as fruit trees, lilac, dogwood, maple, poplar, and willows. 

Treatment is highly time specific. The only opportunity to kill scale insects on contact with smothering oil is during the two week long crawler stage, right after they hatch.  


What to do

This year, we are advising a spray period of June 7-20 for those who are doing a Do-It-Yourself application of horticultural oil.  Commercial contractors who are spraying permethrin can start earlier since permethrin has some residual effect.

For small to medium hedges:

  • Apply a heavy spray of oil and water mixture to the stems and canes of your cotoneaster plants with the highest pressure applicator possible. This insect will be on the woody parts, not the leaves, so gtry to spray deep inside the shrub, not just on the outside.
  • Try to spray 2-3 times over the ten day period as it’s difficult to hit enough insects to achieve control unless you have a high pressure sprayer.
  • While most people use horticultural oil because it’s cheap and effective when mixed according to instructions on the bottle, other oils (linseed, neem) can be used.
  • Since the City of Calgary is not currently treating for scale on boulevard cotoneaster, you are wise to spray nearby cotoneaster to prevent spread to your private property shrubs.


For thick or tall hedges:

  • Property owners with big, mature hedges may want to consider a commercial application from a respected landscape company which can penetrate deep inside the hedge using a high pressure hose.


Additional Resources:

The City of Calgary Oystershell Scale

CBC News “Oystershell scale spreading through Calgary hedges” 

-Mount Royal Community Association