The Plight of Calgary Cotoneasters

Tuesday, May 29th, 2018

Cotoneasters are one of the more popular shrubs in Calgary. With glossy green leaves, scented summer blooms, blazing autumn colours and long-lasting winter berries, they provide interest all year round. They are an affordable, attractive and fast-growing plant with a dense growth habit. This makes them ideal for providing privacy, security, windbreak, noise and pollution reduction. They can be sheared into formal shapes or left natural and are also deer and drought resistant once established.

The one thing they are vulnerable to are scale insects, such as Oystershell Scale, also known as Apple Mussel Scale or Lepidosaphes ulmi . These insects attach their shell-like bodies to the bark and subsist by sucking the sap of the shrub, eventually killing the branch or entire plant. Populations can grow quickly as females will lay approximately 100 eggs before dying. After dying, their bodies harden into a shield protecting the eggs. Their waxy scales repel water and can survive temperatures below -30, making them a very resilient pest. Horticultural oils can be applied to suffocate adult populations to some effect. Only when the eggs hatch into “crawlers” in the springtime are they susceptible to pesticides.

Identifying Oystershell Scale

If you notice your Cotoneaster hedge is looking sparse, or there are bare, dry brown areas with crisp dried brown leaves, your hedge may be suffering from an Oystershell Scale infestation. Closely inspect the branches. Look for areas of crusty, rough looking bark. These are in fact mounds of the Oystershell Scale insects’ bodies covering the host plant. Individual insects are a little easier to spot – their bodies may be 1-4mm in length, slightly curved and brown in colour, resembling the shell of a mussel.

Oystershell Scale Calgary


Once established in Cotoneasters, Oystershell Scale can cause serious damage and may have to be treated by taking drastic measures; cutting your shrub down to the ground.

The best method of controlling this pest is prevention. By keeping plants healthy, they are more resistant to disease and pests. Regular maintenance, select pruning, fertilizing, mulching and regular watering will keep the shrub in optimal condition. Inspect your shrubs routinely to identify any signs of infestation and promptly treat or remove affected areas.

Calgary is currently experiencing a mass die-off of Cotoneasters due to this pesky insect. Infestations are affecting every part of the city, leaving no neighbourhood untouched. A community effort is needed to treat this problem on a mass scale, to retain these beloved Cotoneaster shrubs that adorn our beautiful city. If you are concerned about the health of Cotoneaster shrubs on your property, please get in touch to schedule a consult. As they say, “an ounce of prevention is worth a pound of cure.”