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Tree Diseases & Pests

Cottony ash psyllid (left) and forest tent caterpillar (right)

Positive Dutch Elm Disease case in Saskatoon

Saskatoon has confirmed a positive case of DED in an elm tree in Lief Erickson Park in the Westmount neighbourhood. This is the third case of DED in Saskatoon since 2015. As part of the City's DED Response Plan, the tree was immediately removed and disposed of at the City Landfill. Read more in the news release dated July 12, 2021.

For more information on DED and the City's DED Response Plan, visit Dutch Elm Disease.

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


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.   

Ash Leaf Cone Roller

Ash Leaf Cone Roller
Leaf affected by ash leaf cone roller

What is an ash leaf cone roller?
This insect 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 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 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, 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.

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.

Cottony Ash Psyllid

Cottony Ash Psyllid
Trees affected by Cottony ash psyllid

What is cottony ash psyllid?
Cottony ash psyllid (CAP) is a non-native pest that is impacting black and mancana ash trees throughout the city.  These trees are particularly susceptible to this pest and the combination of dry conditions and an insect infestation can eventually lead to tree loss.

What does cottony ash psyllid  look like?
CAP are very small (2.95-3.57 mm), light green to yellow-green pests. Because of their size, the presence of psyllids is most easily recognized by the damage they create:

  • White cotton curled within or along leaves.
  • Heavily infested trees will often be partially defoliated with some of the remaining leaves twisted into a corkscrew or cauliflower shape.

What 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.

Green ash, white ash, and mountain ash are not impacted.

How prevalent is the cottony ash psyllid in Saskatoon?
Saskatoon is currently experiencing a CAP outbreak.  In 2016, large numbers of CAP were discovered in trees planted within concrete cut-outs in our central business districts and the surrounding neighbourhoods. City-wide canopy inspections conducted in 2017 and 2018 found that most susceptible trees have some level of infestation. 

What is the City doing in response to this outbreak?
In 2018, the City removed approximately 1,600 ash trees from boulevards and parks that were identified as having 50% or less leaf cover.  An assessment of the remaining City-owned ash trees found an additional 2,900 trees as meeting removal criteria.  The 2019 budget allows for 2,100 boulevard tree removals.  The remaining 800 park trees will be removed in 2020.  

Is there anything that can be done to stop this insect from destroying ash trees?
Unfortunately, there is no quick or easy solution to stop this invasive insect. It is moving quickly and there are few proven or effective options for stopping it. Other prairie cities have found that while the investment in chemical controls has slowed its destruction it did not stop the loss of trees. Although certainly devastating, increasing the diversity of our overall forest by replacing these trees with non-ash species will be beneficial in the long term.

The best approach to keep tree(s) 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. 

Dutch Elm Disease

Dutch Elm Disease
Tree affected by Dutch elm disease

What is Dutch Elm Disease?
It is a vascular disease caused by two closely related species of fungi, Ophiostoma ulmi and Ophiostoma novo-ulmi, both of which are transmitted by elm bark beetles and through root grafts. It blocks water movement and eventually leads to death of the entire tree. It only impacts elm trees and no elm species is totally resistant. A tree can be infected by:

  • Elm bark beetles that transmit the disease
  • Through root grafts between trees
  • By pruning tools

How quickly can it kill an elm tree?
As quickly as a few weeks, or sometimes it may take several years.

What does a tree with Dutch Elm Disease look like?
Early symptoms include wilting or drooping of leaves on one or more of the major branches followed by flagging (leaves turn yellow and then brown and shrivel). As it advances branches may die and more of the tree becomes infected. If infected later in the summer the leaves may turn yellow and then droop and premature leaf drop can occur. This can be mistaken for early fall colouration which can happen as early as August. 

New Elm

Above Left: A healthy elm leaf; Above Right: Tree affected by DED (Courtesy: City of Regina) 

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 is the best way to prevent Dutch Elm Disease?

  • Do not store or transport elm wood – Provincial regulations prohibit the storing or transporting of elm wood. Elm wood must be disposed of at the City landfill.
  • Do not prune elm trees during the provincial elm pruning ban from April 1 to August 31.
  • Remove dead and dying tree limbs promptly as they are a breeding site for beetles that can act as a vector.
  • Sanitize tools after working on elm trees.
  • Keep elm trees healthy and rapidly remove infected trees.
  • Report any symptomatic trees to 306-975-2890.

For more information on DED and the City's DED Response Plan, visit Dutch Elm Disease.

Emerald Ash Borer

Emerald Ash Borer
Emerald ash borer, photo courtesy of Barry Lyons, Canadian Forest Service

What is the emerald ash borer?
The emerald ash borer is an highly destructive pest of ash trees. The larvae feed beneath the bark, killing ash trees. All ash (Fraxinus species) are susceptible.  Although the emerald ash borer is not known to be in Saskatoon, if established the impact would be considerable as ash are approximately 25% of the trees on City property. The emerald ash borer is difficult to detect and is often well established in a community once it is identified. 

What you can expect from us

The City of Saskatoon installs and monitors traps to help detect the emerald ash borer. Additional traps have been installed by the Canadian Food Inspection Agency.  

How you can help us

Emerald ash borer is spread through the movement of infested firewood. To prevent the movement of this destructive beetle, do not move firewood from emerald ash borer infested regions. 

    Forest Tent Caterpillar

    Forest Tent Caterpillars
    Forest tent caterpillars

    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 completely defoliate a tree but trees typically recover. With several years of heavy defoliation trees can decline.

    How do I control forest tent caterpillars?
    Typically outbreaks last 3-7 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.


    Tree affected by galls

    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.  

    Leaf Miner

    Leaf Miner
    Tree affected by leaf miners

    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.

    Spider Mites

    What is a spider mite?
    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?
    The City of Saskatoon does not have a control program for spider mites. Consult an arborist or local garden centre for options to control spider mites.