Oystershell scale spreading through Calgary hedges

A warm, dry spring is fuelling the spread of the oystershell scale — an insect that sucks the juice out of the cotoneaster plant, according to a Calgary arbourist.

Bugs 'suck juice' out of cotoneaster plant, says horticulturalist

A Calgary arbourist says oystershell scale is damaging cotoneaster hedges all over the city. (Danielle Nerman/CBC)

The oystershell scale insect is spreading through cotoneaster — a plant widely used in Calgary for hedges, shrubs and bushes.

"If you drive through Inglewood, virtually every hedge is dead," said Keegan Seward, owner of The Arborest Expert Tree Service. 

He says the bug has been infiltrating other riverside communities, such as Douglasdale and Riverbend, for the last five years — but is ramping up all over the city because of the warm and dry spring.

"It's not naturally being slowed down by weather. Usually cold and rain will slow it down," he said. 

Oystershell scale affects 120 species, many of which grown in Calgary — such as lilacs, hawethorn, green ash and crabapple trees. 


If areas of your cotoneaster suddenly go brown, it may be infested with the tiny vampires.

"The bugs suck all the juices out of the plant," said Corinne Hannah, a horticulturalist at the Calgary Zoo.

Adult oystershell scale have a protective shell with a waxy coating. (Danielle Nerman/CBC)

"If you look on the branches closely, you'll see all these little bumps and ridges that look kind of like oyster shells. That's the armor that those little bugs hide underneath," said Hannah.


If the oystershell scale has only hit a small area of your cotoneaster, Hannah says you can spot treat it with insecticidal soap.

But if it's really bad, you're going to have to mow that hedge down and spray the stumps.

Seward is not a fan of pesticides, but says if you don't bring in the "big guns" the oystershell scale will just crawl back up the new growth.

Fire blight has caused the leaves on this cotoneaster plant to die. (Jennifer Keene/CBC)

He says now is the time to take action because the babies, called "crawlers," are about to hatch and have not developed their protective armor.

Calgary's habitat management superintendent, Lincoln Julie, is aware of the problem and is "currently assessing city trees and developing a treatment plan."

Fire blight

"It's ugly. It's an oozing, ugly mess," said Hannah.

Fire blight is another disease that affects cotoneaster hedges and all plants in the rose family, but it's a bacteria — not a bug.

Seward says fire blight is not as prevalent in Calgary right now as oystershell scale, but it's still a problem.

A classic sign of fire blight on cotoneaster is when the plant's branches and leaves die back.

Hannah says you can treat it by using 10 per cent bleach on pruners and cutting the hedge to 30 centimetres past the affected area.

"You're basically doing a little amputation and sterilizing your tools."

Fire blight is also deemed a pest under Alberta's Agricultural Pest Act. According to the City of Calgary, homeowners are responsible for keeping it under control on their own property.


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