Caution | Outdoor water restrictions in effect

Stage 3 outdoor water restrictions are in effect. Learn more about how City services are impacted and what you can do during this stage.

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Oystershell scale is an introduced pest in Calgary. It has the appearance of small clusters of oyster-shaped “shells” that cover bark on shrubs and trees.

This pest only reproduces once per year, with the egg hatch occurring in early to mid June over an approximate ten day period. Once hatched, the “Crawlers” feed on fluids found in the twigs and branches; they then permanently attach to the branch, developing a hardened shell that protects them from predators, the elements and most pest-control methods.

Plants at risk

Fruit trees, lilac, ash, maple, dogwood, poplar, and willow. Oystershell scale is considered a serious pest on Cotoneaster shrub- which is a popular choice for hedges in our city.


Symptoms of infestation don’t usually show up until your shrub or tree is heavily infested, making this a difficult pest to fight. Inspecting plants that are prone to Oystershell is a good preventative measure. Look for branches with oyster-shell shaped bumps, leaves that are turning yellow, and branch dieback or gaps in your hedges.

It's important to act early in the infestation progression as once you can easily notice it, the infestation is usually quite severe. The best defence is to keep up on yard maintenance and inspect your plants for any signs of pests on a regular basis.

What you can do

Keeping your garden, including all plants, shrubs and trees healthy, watered and happy is your best first defence. A healthy plant can fight off many pests and rebound from infestations more easily.

Check out What Goes Where for details on how to dispose of diseased trees and shrubs.

In spring (early June)

When Oystershell scale eggs hatch, and become “crawlers” they are most vulnerable to treatment methods. This stage only lasts for a week to ten days, so it’s important to act quickly:

  • Apply a Horticultural Oil (this helps suffocate the “shells”).
  • Natural pest control: ladybugs (lady beetles), lacewings and other natural predators are helpful in controlling outbreaks.
  • Once dead patches are easily spotted, pruning out affected stems or cutting the entire hedge to ground level is the most effective control. Cotoneasters with healthy root systems will quickly re-grow.

​​​Learn more about tree pests and diseases in Calgary​