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Tree Pests & Diseases

The information below will help you identify and control tree pests and diseases found in Saskatoon.  

Ash Leaf Cone Roller

What is an ash leaf cone roller?
This species is a newly introduced pest of ash trees in urban areas. The ash leaf cone roller is a small brown moth with a wingspan of 1.2-1.4cm. Eggs are laid on young ash leaves. The larvae, after hatching, are quite small, They feed between the upper and lower surfaces of the leaf. This type of feeding is also known as leaf mining. When larvae are finished feeding, they emerge from the leaf and use silk threads to drift to other leaves. At the new leaf, the larvae roll the leaf into a cone. Inside the rolled leaf the larvae pupate and the adult emerges. The adult moth emerges in summer and enters into a unusual state of summer sleep until the fall. Adults then seek out a place to survive the winter emerging the following year.

How do I control ash leaf cone caterpillars?
Damage created by the ash leaf cone caterpillar isn’t significant enough to warrant any control measures. It is important to keep your tree healthy through watering and pruning.

Ash Leaf Coneroller

Ash Plant Bug

What is an ash plant bug?
The adult insects are oval, 0.5 to 1.7 mm long, and are pale yellow to brown or black, with pink markings on the back. Nymphs, at the immature stage, are green with black spots.

What kind of damage is caused by ash plant bugs?
Ash plant bugs feed on ash tree leaves by piercing and sucking to obtain nutrients. In doing this, the surrounding tissue is killed, creating a stippled appearance. While the damage will not kill the ash tree, it is unsightly.

How do I control ash plant bugs?
For control of minor infestations, a hard blast of water will often remove the insects from the tree.

Cottony Ash Psyllid

What is cottony ash psyllid?
Cottony ash psyllid is a non-native 2.95-3.57 mm light green to yellow-green pest that was first identified in Saskatoon in 2006 on black and mancana ash.  This insect is native to central Europe but has made several appearances in North America including recent infestations in North Dakota, Minnesota and Alberta.

To date there is limited information on the biology of the cottony ash psyllid.  Stressed trees, especially those found in concrete cut-outs in Saskatoon’s downtown area, have been particularly susceptible.

What does cottony ash psyllid  look like?
Because of their size, the presence of cottony psyllids are most easily recognized by the damage they create:

  • White cotton curled within the leaflet margins and rolled under and towards the midrib.
  • White cotton along the midrib of an uncurled leaf is a sign of second generation psyllids.
  • Heavily infested trees will often be partially defoliated with the remaining leaves twisted into a corkscrew or cauliflower shape.    

​What other trees are impacted by psyllid?
The susceptible trees include:

  • Black Ash - Fraxinus nigra and the cultivar ‘Fallgold’ 
  • Manchurian Ash – Fraxinus mandshurica and the cultivar ‘Mancana’
  • ‘Northern Treasure’ and ‘Northern Gem’ which are a cross between Black Ash and Manchurian Ash.

To be clear green ash, white ash, and mountain ash (a different species) are not impacted.

How prevalent is the cottony ash psyllid in Saskatoon?
The first outbreak of cottony ash psyllid in Saskatoon was in 2006, which was followed by a crash in the population.  In 2016, the City started to notice large numbers of psyllids in trees planted within concrete cut-outs in our central business districts and the surrounding neighbourhoods. We are aware that most susceptible trees have some level of infestation and we are seeing some rapid decline of trees throughout the city. 

What is the City doing in response?
Insecticide injections were completed on selected trees in May in an attempt to reduce the infestation and data is being collected to determine whether injections may be an effective treatment.  

How many trees will be impacted?
The City does not know the extent of the loss or damage that will result from this psyllid infestation.  So far, the 2017 action plan includes removing and replacing (with a different tree species) more than 100 trees in the central business improvement districts and along 22nd Street.

However, there are approximately 7,000 susceptible City-owned trees on boulevards and in parks managed by Urban Forestry. We know other prairie cities have had substantial losses with cottony ash psyllid activity.   

What can you expect from us?

  • The Parks Division will conduct a canopy inspection of susceptible trees in July to assess the impact of the psyllid outbreak city wide.
  • Updates on the overall impact and emerging response efforts will be communicated to the public as information becomes available. 
  • Trees assessed for removal during the canopy inspection will be removed as resources permit. 

What can you do to help us?
Trees under stress are more susceptible to insect infestations.  The best approach to keep your tree(s) healthy and less vulnerable to insect infestations is to water between rainfalls, protect your trees from root or trunk damage, and avoid the use of herbicides or excessive salts in the soils near trees. The City does not recommend chemical treatments; however, homeowners can speak to a certified arborist or a local garden centre if they would like to know more about treatment options. 

Finally, please be patient.  This is an emerging pest issue and we do not have all the answers yet, but will let you know when we do.  

Dutch Elm Disease

What is Dutch Elm Disease?
Dutch elm disease (DED) is a serious disease caused by fungal pathogen Ophiostoma (novo) ulmi. The disease was introduced into North America in the 1930s, and has wiped out millions of elms across Canada and the United States. On July 21, 2015, the first case of Dutch Elm Disease (DED) was confirmed in Saskatoon.

What does a tree with Dutch elm disease look like?

The symptoms of Dutch elm disease are best detected from mid-June to mid-August. Typically, the leaves will start to wilt and turn yellow, then curl and turn brown. Pest Management does annual, systematic, surveillance of elm trees during this timeframe. 

New Elm

Above Left: A healthy elm leaf - Courtesy : Terri Smith Above Right: DED branch - Courtesy: City of Regina 

Dutch Elm Disease

Above Left: DED symptomatic elm - Courtesy : City of Winnipeg Above Right: DED symptomatic elm leaves 

How is Dutch Elm Disease spread?
In Saskatchewan, Dutch elm disease is spread by several species of elm bark beetles. These tiny beetles are able to fly up to two kilometres as the beetles search for elm trees. The Dutch elm disease fungus has tiny spores that stick to the body of the beetle. Bark beetles can carry these spores and infect other elm trees.

What is the life cycle of the disease?
Elm bark beetles spend the winters as adults burrowed into the base of elm trees. In the spring they emerge, flying to the crown of healthy elm trees where they feed. They then fly to elm trees that are sick or dying to breed and lay eggs. The eggs hatch and larvae feed on the inner bark of these trees. By the fall, the larvae turn into adults, emerge from the tree and fly to a new elm to over winter. If the tree they fly from has Dutch elm disease, there is a very good chance the adult will spread the disease to its new host.

What can you do?

Do not prune elm trees from April 1 to August 31. Provincial regulations prohibit pruning of elms during the time when elm bark beetles are most active. Outside the ban period, regular pruning helps keep trees healthy and better able to resist all types of diseases, including Dutch elm disease. Removing dead wood also makes your trees less attractive to elm bark beetles.

The provincial regulations also prohibts the storing, transport and use of elm wood. The only permitted movement of elm wood is to the City of Saskatoon Landfill which is the designated disposal site in Saskatoon.

Responsible tree maintenance protects your trees, and potentially the elm trees in your neighbourhood.

Please report any symptomatic trees to 306-975-2890.

For more information on Dutch Elm Disease:

SOS Elm Coalition

Canadian Food Inspection Agency - Dutch Elm Disease

Saskatchewan Ministry of Environment - Dutch Elm Disease

Forest Tent Caterpillar

What is a forest tent caterpillar?
Forest tent caterpillars are dark coloured with white spots down their back. Mature larvae are typically 5 cm in length.  The body is covered with long hairs, also known as setae. In June larvae will spin a cocoon and pupate. The adult moth is yellow or tan with a thick, furry body.

What kind of damage is caused by forest tent caterpillars?
Forest tent caterpillars feed on a large number of trees, including ash, poplar and chokecherry. In some cases these insects can completly defoliate a tree but trees will typically recover. With several years of heavy defoliation trees can decline.

How do I control forest tent caterpillars?
Typically outbreaks last 2-4 years and most trees do not decline. During an outbreak of forest tent caterpillars, the large number of insects can be a nuisance. These can be removed from the tree with a blast of water.

Forest Tent Caterpillar


What is an aphid?
Aphids are fragile, pear shaped insects measuring approximately 2mm in length. They are usually pale green in colour, but can be a variety of colours.

What kind of damage is caused by aphids?
There are many different species of aphids that attack many different plants. In Saskatoon's urban forest it is common to see aphids on American elm and Manitoba maple.  Damage caused by aphids does not typically impact plant health but can affect leaf shape and size.  Aphids can produce honey dew, a sticky substance that can adhere to sidewalks and vehicles.   

How do I control aphids?
Typically aphids do not require treating with insecticides.  Some are available and registered for aphid control.  Also read the label and use as directed.  In some situations ladybird beetles (ladybugs) can be used to control aphids.  

Box Elder Bugs or Maple Bugs

What is a maple bug?
Maple bugs are an insect that feeds by sucking the nutrients from the plant tissue. They are small, approximately 1.0 - 1.5cm in length, and black with red markings.

What kind of damage is caused by maple bugs?
Maple bugs will feed primarily on the seeds of Manitoba maple (box elder) and sometimes green ash. These insects are harmless, but they can become a nuisance when they congregate in large numbers on sidewalks, garages or sides of houses.

How do I control for maple bugs?
Maple bugs can most easily be dealt with by sweeping them into a garbage bag or by using a vacuum when they are abundant.


What is a gall?
Galls are malformations that develop on leaves, branches or roots. Galls are caused by nematodes, mites or insects, and to a lesser extent, by bacteria, fungi or viruses. Many galls are very different, depending on the plant material and the gall producer. For that reason, sometimes the organism making the gall can be identified using the shape and colour of the gall.   

What kind of damage is caused by galls?
Galls on trees can modify the shape of the affected tissue but typically not affect the health of the tree.  

How do I control galls?
Galls on trees typically do not require any treatment.  

Galls photo

Leaf Miner

What is a leaf miner?
A leaf miner is a generic name for insect larvae that feed between the two epidermal layers of a leaf. Birch leaf miner (sawfly) and the ash leaf coneroller (moth) are common in Saskatoon, but other tree species such as poplar, lilac, oak, and hawthorn are also susceptible to different leaf miners.

What kind of damage is caused by leaf miners?
Damage is caused by larvae feeding inside the leaves.  Feeding creates a meandering tunnel under the leaf surface. Typically, the damage does not affect the health of the tree.

How do I control leaf miners?
If only a few leaves are affected, leaves can be removed. Generally, no treatments are necessary for leaf miners.

Leaf Miner photo

Spider Mites

What is a spider mite?
Mites are more closely related to spiders than insects.  They are typically less than 1 mm in length and differ from insects as they have only two body segments and four pairs of legs. Spider mites in Saskatoon can attack spruce, fir, juniper and cedar.

What kind of damage is caused by spider mites?
Spider mites feed on conifers, causing the needles to turn yellow and fall off. Eggs lay over winter at the base of the needles and hatch in the late spring. Immature stages typically feed on the lower branches and inner-most foliage, causing the most damage.

How do I control spider mites?
Populations of spider mites are usually kept in check by wind, rain or a strong blast of water on infected trees. Organic pesticides may also be used to control mites.