Oystershell Scale

Posted by shane - February 6, 2011 - Insect Pests, Insects, Diseases & Other Problems - No Comments

Of all the pests that I work with in Calgary and Red Deer, the scale insects are doing the most damage.  And while European Elm Scale is now familiar to homeowners and tree professionals, oystershell scale is still largely unknown to people, and its potential for damage grossly underestimated by arborists, landscapers, and government bodies.  It attacks over 120 species of ornamental trees and shrubs, many of which are commonly planted on the Prairies.  Without proper recognition, this pest will destroy countless plant material in Alberta.

Oystershell scale on hawthorn in Douglasdale, Calgary, 2016. S. LePage photo.

Oystershell scale on hawthorn in Douglasdale, Calgary, 2016. S. LePage photo.

A fellow arborist first called my attention to oystershell scale in Calgary in 2006.  He had a number of clients in the Inglewood neighbourhood, with dying cotoneaster hedges.  We didn’t know what it was at that time.  I looked at the hedges, took a sample, and soon identified it.  Since then, oystershell has spread throughout the city, particularly severe in some neighbourhoods, such as Riverview and Douglasdale, and just getting established in others. Cotoneaster is the preferred host, but we have now seen it infesting crabapples, apples, hawthorn, mountain ash, green ash, common lilac, yellow-twig willow, saskatoon, dogwood, amur cherry, and ‘Schubert’ chokecherry.  Entire streets have lost their cotoneaster hedges in some neighbourhoods.  As of late 2015, the owner of another tree service in Calgary told me that virtually every yard he works in is now affected.


Infested cotoneaster hedges die-back in patches, leaving large voids in the hedge.  Close inspection of the twigs reveals thousands of adult oyster-shaped insects encrusting the stems.  In early to mid June, if an infestation is not obvious, take a piece of black paper or similar sized piece of black leather (I use a leather folder), and shake the branches over it.  Look closely.  If scale is present, you will see many tiny yellowish “crawlers”, which is the immature stage of the insect.  A few crawlers might not warrant control measures.  A heavy infestation will require a spray.  The main predators for oystershell scale are ladybugs and lacewings, but from what I’ve seen, not in sufficient numbers to control the scale.

Pest Control

Oystershell scale insects are straightforward to control using conventional insecticides.  They appear to have one generation per season in Calgary.  The crawler form is active in early to mid June, depending on weather conditions.  Inspect the leaves as described above.  If the infestation level is moderate to high, you can spray the entire canopy, thoroughly, with any contact insecticide.  I recommend hiring a licensed pesticide applicator, as they have an arsenal of different chemicals, as well as appropriate spray and safety equipment.  Dormant oil spray is largely ineffective during the dormant season, because the eggs are protected beneath the waxy dead adult shell.  Infested trees can be sprayed in the same manner as hedges.

In order to avoid the use of conventional insecticides, some municipalities and community associations are recommending the use of dormant oil spray in June.  The only product I know that is registered for such use is Purespray Green oil.  It is also effective to spray with oil later in June, when the crawlers are in the “furry” nymph stage between crawler and adult.

Most horticultural oils presently marketed are sufficiently refined to allow their use on plants when foliage is present. These ‘summer oils’ can be very effective for control of oystershell scale during early stages in their development, as post crawler sprays typically applied at some time in June. Young stages of oystershell scale, with minimally developed wax covers, can be effectively smothered with sprays of these oils.” – W.S. Cranshaw, Colorado State University (May 2013).

Starting in 2019, we will be using summer oil for two weeks as a post-crawler stage treatment in mid- to late June.

Renovation of Infested Hedges 

Heavily infested, or patchy hedges, are unsightly and will never recover as well as homeowners would like.  In this case, the best thing to do is 1) cut the hedge back to about 8″ above grade in late winter, before bud-break.  You may get away with cutting back sections of the hedge only; 2) in June, spray the sprouting stumps with a contact insecticide (oystershell will attack stems right to the base of the plant); and 3) as your new hedge grows, monitor pest populations in early June and spray with a contact insecticide, if necessary.  Through careful monitoring and appropriate spray treatments, this insect is easily controlled.  Exception:  If you hedge is “nearly-dead” or devoid of vigour, you would best remove, grind the stumps, and replace the plants.

To book pesticide applications, please call early in the season, as spray timing is very important.


We have just concluded our seasonal spraying for oystershell scale in Calgary, and this year has been, by far, the worst season for oystershell scale in the city.  Besides cotoneaster hedges, we have identified the insects on the following plants:

Apple, crabapple (especially purple-leaved rosybloom crabs; the ‘Dolgo’ crabs are much more resistant), hawthorns, common lilac, mountain ash, green ash (especially in Douglasdale and Inglewood), amur cherry (Douglasdale), and ‘Schubert’ chokecherry (Inglewood).

I expect to see a high rate of tree mortality among the ornamental species in the next 3-5 years.  The vast majority of plant material in Calgary goes untreated, and our control efforts are not even statistically significant.  To date, I have not seen or heard of a response by The City of Calgary.  The problem is getting so severe, that I’m not sure what the City would even be able to do about the problem at this point.  We’ve now seen scale in every neighbourhood in Calgary.

Our research indicates that oystershell has the potential to infest many more species, including elms (already infested by European elm scale), willows, and all poplar species.  I will update this blog as the situation progresses.


Unfortunately, many people are receiving poor advice about how to deal with oystershell scale.  Even more distressing is that the advice is coming from tree services, landscapers, and garden centres, whom people tend to trust.

The most common piece of advice is to cut down the hedge.  Well, ok, that’s fine if the hedge is half dead and looks terrible, but I’ve seen plenty of people cutting their beautiful hedges to the ground, when you can’t even tell that they have scale at all.  This is a mistake.  First of all, if you catch the infestation early, it can easily be controlled with properly-timed sprays.  Second, many people are cutting their hedges down when the plants are in full-leaf, which is a big mistake, because it robs the already weakened plants of stored energy.  If you must cut down your hedge, do it during the dormant season, from November until early April.

Ok, so you’ve cut down the hedge.  Now what are you going to do?  Well, nobody seems to have any advice at this point.  I can tell you from plenty of experience that a hedge that was cut to the ground in April, will be re-infested with scale by June, if proper control measures aren’t taken.  The scale infests the plants right to the base of the stems.

So cutting back a hedge is fine if required, but the cutting back does nothing to stop the advance of the scale.  At some point, if you want to keep your cotoneaster hedge, you will have to start a control program, which will involve at least one, and probably two applications of some kind of insecticide, in late May to mid-June.

Not many people want to talk about spraying anymore, and colleges and government agencies tend to avoid mention of chemicals altogether.  The fact is that proper insecticide use is part of a well-rounded integrated pest management program, a “tool in the tool box”.  Sometimes, sprays are necessary.  For many insect problems, they can be avoided.  For proper oystershell scale control, in an ornamental urban landscape, they are necessary.

(c) Shane LePage 2011/16/19, Wild Rose Tree Service & Pest Control, Red Deer, AB