Pests & Diseases

There are many different pests or diseases that can infest Calgary’s urban forest. Some are beneficial to the trees, but many are not.

  • Aphids – small pear shaped insects that feed on plant sap.
  • Ash Leaf Cone Roller – caterpillar/moth like insect that rolls tree leaves.
  • Ash Psyllid – small insects on tree leaves, resulting in curling leaves.
  • Black Knot Fungus – abnormal growths of bark that encircle the branch.
  • Bronze Leaf Disease (BLD) – affects certain poplar and aspen trees.
  • Elm Scale – tiny crawling insects on leaves, twigs and branches.
  • Fire Blight – bacteria that browns and kills leaves, flowers and branches.
  • Oystershell Scale – small clusters of oyster-shaped “shells” that cover bark.
  • Satin Moth – feeds on poplar tree leaves.
  • Yellow-Headed Spruce Sawfly – caterpillar like insect on spruce needles.

Aphids are tiny pear-shaped insects that are generally green in color; but some can be yellow, grey, black, brown and pink. Female aphids can reproduce without mating in the summer, so populations can rapidly increase.


Aphids feed by inserting their mouth into plant tissue. As the plant sap flows through its body, the aphid uses only a few of the nutrients in the sap and the rest is excreted as “honeydew,” which makes leaves on affected trees appear shiny. Fungus then grows on this honeydew, creating a black scum that can stick to cars, patio furniture, decks and sidewalks located beneath aphid-infested trees. This can be washed off with soap and water.

Losing sap to aphids can weaken a plant. When an infestation is severe, the leaves curl, the fruits and stems distort, and “honeydew” accumulates. Woody plants are usually able to compensate with new growth after an infestation, but flower and vegetable seedlings may not recover.

What You Can Do

  1. Assess the problem. Watch for aphids by looking closely at new shoots and underneath leaves. Early signs of plant damage are yellow or light red spotting, or curling leaves where the aphids are feeding.
  2. Wash away the aphids. Use a strong direct spray of plain water to remove aphids without harming the beneficial insects. Get plants dripping wet and spray into the middle of the plant where aphids may be hiding. Beneficial insects will fly away to dry off and return later to eat more aphids. Adding insecticidal soap will also help.
  3. Encourage beneficial insects. Predatory insects such as ladybugs will help to keep aphids under control.
  4. Trap the aphids. Sticky yellow cards are effective traps for aphids. They can be hung in or around plants affected by aphids.
  5. Keep trees healthy. Growing plants in a healthy condition in moist, fertile soil is the best way to avoid aphid infestations. Aphids can be a nuisance, as they cannot be fully eradicated, only controlled.

The ash leaf cone roller is a moth that is a relatively new pest in Calgary. It affects ornamental ash trees, principally Green and Manchurian Ash.

Young caterpillars feed within the leaf tissue. As the caterpillar grows, it emerges from the leaf, migrates to a new leaf, and rolls the leaf into a characteristic cone shape. The larvae will continue to feed within this enclosure until the pupal stage. Larvae feed until mid or late June and the adult moths will emerge from the leaf cone starting about mid-July.

Although the insect can be quite prevalent on individual trees, the damage to individual trees is minor and does not interfere with overall tree health. The rolled leaves remain attached to the tree and actual leaf damage is minimal.

This insect is an invasive pest and is new to Calgary. Black and Manchurian Ash trees are particularly affected.

What You Can Do

If your tree is infested with the cottony ash psyllid, proper timing of treatments is crucial. The soap / permethrin mix works well when applied during the insect’s early nymph stage, so treatment should occur in the first week of June and again in the first week of July. Homeowners should monitor trees daily beginning in spring and apply a soap / permethrin mix immediately upon detection of psyllid nymphs and / or the white, cottony material produced by this stage. More than one application may be necessary. Do this again beginning about the first week of July in preparation for the second generation. Once the psyllid adult stage is reached, it is too late and control measures are largely ineffective.

Controlling the Cottony Ash Psyllid

The most effective weapon against this pest is a soap / permethrin treatment known as Trounce®. Because of the lifecycle of this insect, Trounce treatments should commence about the first week of June and again about the first week of July. After that, as the insect matures, it becomes more difficult to treat the tree.

Why Cottony Ash Psyllids Are Bad For Our Trees

The Ash Psyllid feeds by piercing leaf tissue and utilizing plant juices. All of the damage is done by the immature (nymph) stage and damage can be severe due to feeding nymphs injecting a toxin into tree leaves. These insects feed are hatched in June and July and feed throughout the summer months, often until late August. Adults are highly active and resemble tiny cicadas.

Symptoms include curled or wilted (“cauliflowered”) leaves and premature leaf drop in summer. Feeding nymphs are surrounded by a cottony material enclosed within the leaf.

Is your Mayday, Chokecherry or Lilac showing a black, lumpy growth on its branches? It may be infected with black knot fungus – a disease that affects Prunus family trees. Although the fungal disease is rarely fatal, if left unchecked it will affect the health of your tree.


This fungal condition infects only Prunus species of plants, and may be recognized by the clumpy-looking, black masses of abnormal growths on the branches of your cherry trees.

What You Can Do

To manage black knot fungus, it’s essential to prune off infected branches 2-4 inches below each “knot” and dispose of them in a land fill. Between each cut, sanitize your tools with a bleach-water solution (25% bleach, 75% water).

The best time to prune is during late winter, as the fungus is dormant and the abnormal “knotty” growths are easy to see. Avoid pruning in the spring when the fungus is active. The fungus is transported by spores so the proper sanitization of pruning tools is very important to limit its spread from plant to plant.

For the health of the tree, we recommend citizens educate themselves in proper pruning techniques or hire the services of a certified arborist. Pruning a tree or shrub leaves a wound, requiring the tree to heal itself, and correct pruning provides the tree with optimal conditions for healing properly.

Bronze leaf disease is a fungus that infects various poplar species and hybrids, including Swedish columnar aspens and tower poplars.


Symptoms of bronze leaf disease typically appear in late summer or early fall and may only be on a few branches or leaves.

  • Infected leaves turn orange-brown to reddish-brown; colouration starts at the edges of the leaf and moves inward toward the base.
  • Dieback may occur on infected branches.
  • Browning may be observed under the bark of diseased branches.
  • Leaf veins often remain a bright green in stark contrast to the rest of the leaf.

What You Can Do

  • Prune out affected branches 20-30 cm into healthy wood while still attempting to maintain the tree’s shape. Often, this is back to the trunk. Between each cut, sanitize your tools with a bleach-water solution (25% bleach, 75% water). Removal of dead and or dying trees is important as they will infect nearby trees as well.
  • Pick up leaf litter from under the trees as soon as they drop. Fallen leaves and pruned wood should be bagged and disposed of in the garbage only. Do not bring infected leaves and wood to Leaf and Pumpkin Composting drop-off locations. Dispose of it in the garbage or take directly to the landfill.
  • If you are unsure about how/where to prune, we recommend hiring an International Society of Arboriculture-certified arborist.
  • If you suspect bronze leaf disease in a City-owned tree, contact 311.

Good sanitation is the best method of controlling the disease. Early detection and treatment is important.

European elm scale (EES) is a pest that normally attacks fruit and ornamental trees. Elm scale feeds by piercing leaves and bark and sucking juices from the tree. The eggs begin to hatch in late June and start feeding on leaves in mid-July. By autumn, the pest moves onto branches and twigs where they prepare to overwinter.


Damage symptoms are not readily apparent on Elm trees unless the infestation level is extremely high. Damage consists of:

  • Leaf discoloration and premature leaf drop.
  • Twig and branch dieback.
  • Sticky honeydew material falling as fine droplets from the tree canopy.

What You Can Do

The easiest way to prevent pests and diseases is to keep your trees healthy. One way to do this is to give your trees a deep watering on a monthly basis, beginning when the tree leafs out and ending when the tree drops its leaves in the fall.

If you detect an infestation, homeowners are encouraged to first gently hose the tree down with water. Another option involves applying dormant or horticultural oil in order to suffocate the pest. Early season control is applied shortly after tree bud-break, prior to the production of elm scale adults and crawlers. Late season control is undertaken just prior to leaf drop as elm scale move back to the branches. During peak crawler numbers, apply a soap and permethrin mix known as Trounce® to significantly reduce the infestation.

The City of Calgary is conducting an injection program to treat select, highly infested Elm trees with a systemic insecticide. This control approach permits the control of all parts of the pests lifecycle.

Elms in Calgary should only be pruned between October 1 and March 31 by law to prevent the spread of Dutch Elm Disease.

Fire blight is a serious bacterial disease affecting trees and shrubs in the rose family. It can ravage Calgary’s urban forests during humid and warm summer weather conditions. Generally, fire blight is very rare in Calgary since our summers are usually too cool and dry for disease development. However, severe thunderstorms and hail can lead to an outbreak of fire blight.

Fire blight affects primarily the rose family of trees and shrubs. Common members of this family include apple and crab apple, pear, mountain ash, cotoneaster, raspberry, flowering almond, and saskatoon.

Fire blight bacteria can spread a number of ways, including insect transmission, use of contaminated pruning tools and strong winds and rain. Hailstorms help spread the disease by wounding the bark and making the tree vulnerable to infection.


  • Diseased leaves appear red and fire-scorched. Blighted leaves eventually brown and die but remain attached to the tree.
  • New growth exhibits a dramatic downward wilting at the tips.
  • Clear amber liquid may be found oozing from diseased twigs. This liquid is highly charged with the bacterium, which causes the disease. Transferring even a small amount to healthy trees can generate new infections.
  • Some trees may develop secondary infections indicated by bark cankers, which appear as indented, discoloured areas on the branches and trunks. Bark cankers are usually more serious infections and can kill the tree.

What You Can Do

The first step to controlling the disease is to prune, remove and destroy all diseased wood. However, pruning can also be a means of transmission. It is critical that you sanitize your tools as you prune. After each and every pruning cut, the cutting blade must be sanitized with a 25% solution of bleach and water (1 part bleach + 3 parts water).

Take diseased branches and other wood to the landfill to prevent re-infection. Do not use fire blight-infected wood in mulch, compost or store as firewood or compost materials. Infected branches will continue to harbour the disease and serve as a potential source of re-infection.

Bacteria blight: A similar disease

Not all dead leaves and branches are caused by fire blight. A disease known as bacteria blight is commonly mistaken for fire blight. The symptoms of bacterial blight are somewhat similar to fire blight but can also affect lilacs, cherries, apricots, and other types of flowering trees. Treatment for bacterial blight is the same as for fire blight.

Oystershell scale is an introduced pest in Calgary. It has the appearance of small clusters of oyster-shaped “shells” that cover bark on shrubs and trees.

This pest only reproduces once per year, with the egg hatch occurring in early to mid June over an approximate ten day period. Once hatched, the “Crawlers” feed on fluids found in the twigs and branches; they then permanently attach to the branch, developing a hardened shell that protects them from predators, the elements and most pest-control methods.

Plants At Risk

Fruit trees, lilac, ash, maple, dogwood, poplar, and willow. Oystershell scale is considered a serious pest on Cotoneaster shrub- which is a popular choice for hedges in our city.


Symptoms of infestation don’t usually show up until your shrub or tree is heavily infested, making this a difficult pest to fight. Inspecting plants that are prone to Oystershell is a good preventative measure. Look for branches with oyster-shell shaped bumps, leaves that are turning yellow, and branch dieback or gaps in your hedges.

It’s important to act early in the infestation progression as once you can easily notice it, the infestation is usually quite severe. The best defence is to keep up on yard maintenance and inspect your plants for any signs of pests on a regular basis.

What You Can Do

Keeping your garden, including all plants, shrubs and trees healthy, watered and happy is your best first defence. A healthy plant can fight off many pests and rebound from infestations more easily.

When Oystershell scale eggs hatch in early June and become “crawlers”, they are most vulnerable to treatment methods. This stage only lasts for a week to ten days, so it’s important to act quickly:

  • Apply a Horticultural Oil (this helps suffocate the “shells”).
  • Natural pest control: ladybugs (lady beetles), lacewings and other natural predators are helpful in controlling outbreaks.
  • Once dead patches are easily spotted, pruning out affected stems or cutting the entire hedge to ground level is the most effective control. Cotoneasters with healthy root systems will quickly re-grow.

The Satin Moth (Leucoma salicis) is a non-native insect that was introduced to North America from Europe in the early 1920s. The satin moth population in the Calgary area has been generally non-problematic. This season a combination of environmental events have permitted Satin Moth populations to expand and become more noticeable. The caterpillar feeds primarily on the leaves of poplar species. They have also been known to feed on willow and oak trees.

Effects on Trees

Satin Moths can completely strip and defoliate large mature trees of all their leaves. They produce two occurrences of leaf eating caterpillars in a season and therefore can defoliate a tree twice. Mature healthy trees normally have enough resources stored to be able to recover from this type of damage. Repeated, severe defoliation over a few seasons can result in tree mortality. Rolled leaves containing pupae and silk webbing on stems and branches are signs of the Satin Moth pupae.

Identifying the Satin Moth

The mature Satin Moth caterpillars grow to be 3.5 to 4.5 cm long, and are pale to medium grey-brown, with a darker head and back. Their backs are black with a central row of white or light yellow markings. They can be confused with the tent caterpillar which has a white strip down its back bordered by two blue lines. The adult moths have pure white wings with a satin-like lustre. They have a wingspan ranging from 3.5 to 5.0 cm and can be distinguished from other local white species by narrow alternating black and white bands on their legs.

What You Can Do For Your Trees

Residents can help manage Satin Moth on trees on their property. How you manage them is closely related to the insect’s lifecycle. Satin Moth overwinters as a caterpillar in leaf litter, in the lower branches, and in the furrows of the bark of the tree. The Satin Moth caterpillars will begin their climb back up into the tree in May. Applying “sticky bands” is still an option for homeowners as they are able to more closely monitor Satin Moth activity and the effectiveness of the band.

The greenish egg masses concentrated on the lower tree trunk can be scraped off with a dull blade and destroyed before they hatch. High pressure washing does appear to be an effective management technique. These services can be secured through local tree care companies. These eggs masses appear around mid-July. Again, “sticky bands” can be applied to the tree trunk during leaf out in May and again in July to catch newly hatched caterpillars as they disperse into the canopy of the tree. These bands can be found at local garden centers and should be reapplied once they are saturated with the caterpillars.

The yellow-headed spruce sawfly (YHSS) affects young spruce trees in Calgary. The sawfly infests and feeds on spruce trees that are growing singly or on the edge of a group of trees. Infestations can begin when a tree is only three to five years old and if left untreated, can kill that tree within three years.


The larval form of the insect looks like a caterpillar and is a voracious feeder on spruce needles. This causes complete defoliation of branches and twigs with only a few chewed brown needle stubs remaining. The damage is likely to be noticed first on lower branches close to the ground.

What You Can Do

In late spring/early summer, homeowners should regularly inspect trees for signs of damage or colonies of feeding larvae. You can remove the crawlers by hand if there are only a few spruce trees and a low level of infestation. However, in situations where there are many trees, a high pressure blast of water is an effective non-chemical method of control.


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