Oystershell Scale

In last few years, the cotoneaster hedges in Calgary are severely infested by Oystershell scale which causes die back of the hedge and makes dead patchy spots. Oystershell scales suck the fluids of cells underlying the bark, often killing the cells at the feeding site.

The eggs undergo overwintering in the shells of dead mother scales and they start hatching in spring. Crawlers emerge and move to find new sites and start sucking plant juices and secrete a protective shell. As summer starts, they gradually increase in size and become full-grown in midsummer. Eggs are laid in late summer and early fall and the mother scale dies at the end of the season. Eggs produced in late summer remain under the protective wax cover of the mother throughout winter.

Oysterscale is one of the “Armored Scales” in which adult has the outer shell that is impenetrable to traditional pesticide sprays. Therefore timing of treatment is very critical for controlling Oystershell scale. The best time to spray insecticides is immediately after babies, called “crawlers,” are hatched and have not developed their protective armoured coating. This is generally from June first week to third week in Calgary, depending on weather.

White Pine Weevil

The adult of white pine weevil is a small rust-colored weevil with a long snout-like beak. The larva is white with a distinct brown head, lives beneath the bark.

Both adult and larva feed the terminal buds of the leader tree but most damage is done by larvae generally in spring. The first symptom of weevil attack is resin oozing from feeding punctures. The presence of the insect is easily detected by the dropping, wilted appearance of the current year’s leader, which resembles a shepherd’s crook.

Leaf Minor

Birch leaf miner is one of the most common leaf problems in Calgary and surrounding areas between May and mid-September.

The newly hatched larvae feed within the leaf. Damage appears as a small brown or reddish-brown, irregular-shaped patch (a “mine”) on the upper surface of the leaf.

European Elm Scale

In last few years in Calgary, the problem of elm scale is getting bigger and bigger. The problem is even worst in public boulevard trees.

Crawlers begin feeding on leaves from May to June and later they migrate off of the leaves to overwintering sites on the twigs and branches. Elm scale feeds by piercing leaves and bark, and sucking juices from the tree. Heavy infestations may kill or weakened trees and cause branch dieback in healthy trees. Large amounts of honey dew are produced which eventually cause the leaves to be covered with grayish/black colored sooty mold. The sooty mold reduces the aesthetic appearance of trees and honeydew can become a nuisance as it coats patios, decks, and vehicles.

Spider Mites

Spider mites damage host plants by sucking plant fluid from needles as they feed. Both larvae and nymphs feed on foliage. Infested trees at first have a speckled, yellowish appearance, and lack rich green color. After prolonged feeding, needles turn rusty colored and may drop prematurely. Mites usually attack older needles located in the lower and inner parts of the plants.

Bronze Birch Borer

The larva bore into the phloem and cambium layers after emerging from their eggs on the bark. The borers’ tunneling weakens and kills trees by interrupting the flow of sap.

Common symptoms are death of the leader, canopy thinning and dieback in the top of the crown and raised, horizontal and zigzag ridges in bark.

Pear Leaf Blister Mite

This pest is most common in Mountain Ash in Calgary. They feed on the underside of the leaves and cause development of blisters on the upper leaf surface. These raised spots are generally light green, round, and look somewhat wrinkled. Older leaf blisters may turn brown. Severe infestations may result in premature leaf drop.

Gall Adelgid

Adelgids feed by sucking plant sap and cause characteristic cone-like growths or galls on spruces. Damage first occurs in late May when the new growth of the branch tips form into cone-shaped galls. The galls are green at first but later turn a reddish-purple color. The old galls dry out and turn a reddish-brown color and may remain on the branches for several years. By the time the galls turn brown, the adelgids have left.


Aphids feed on soft leaves of many species like Birch, Maple, Mayday, Hawthorn and many shrubs. Aphids can reproduce without mating in summer, so populations can rapidly increase.

Aphids cause damage by piercing the soft plant tissue and sucking plant sap.They excrete “honeydew”, which make leaves appear shiny. Sooty mould fungus grows on this honeydew, creating a black scum that can stick to cars, patio furniture, decks and sidewalks located beneath aphid-infested trees.

Leaf Rollers

Leaf rollers mostly affect green ash trees and arecommon pests in Calgary. Young caterpillars feed on new leaf and roll 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.

Yellow Headed Sawfly

The larvae of this insect are voracious eaters and strip needles from spruce, and those needles will never grow back. They attack new needle first, chew down almost to the base before the insect moves on to older needles. The bud at the tip is still alive, and will produce new needles next spring.

Cottony Ash Psyllid

Black and mountain ash are found attacking by this insect in Calgary. Green ash is not affected. They feed by piercing plant tissue and sucking sap. This feeding causes leaf puckering and curling, and early leaf drop. Inside the curled leaf there is a cottony material surrounding the insect.

Poplar BudgallMite

Budgall mites suck the sap from buds and unfolding leaf clusters of the lower branches in the spring which causes further twig growth to stop and cauliflower-like swellings (galls) to form. The mites live in the new galls, which are dark green, succulent, and covered with minute hairs. By late summer the galls have become hard and dry, and brick red in colour.



Black Knot

In Calgary, black knot fungus affects Mayday and Chokecherry trees. To manage black knot fungus, it’s essential to prune off infected branches 2 to 4 inches below each “knot” and dispose of them in a land fill. The fungus spores overwinter in the masses and reproduce in the spring. Pruning should be conducted when plants are dormant. The best time is during late winter when the abnormal “knotty” growths are clearly visible. The fungus is transported through air which carries the spores to new host. Fungal growth eventually chokes out the branch and kills it.

Bronze Leaf Disease

Bronze leaf is an air borne fungal disease that infects Swedish columnar aspens and tower poplars in Calgary areas. 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 become dark reddish-brown, chocolate brown or bronze although the veins and the leaf stem may remain green for some time. Infected leaves remain on the tree throughout winter.

Cytospora Canker

The fungus usually infects trees that are weakened by environmental stress, such as drought and freezing shock. Infection usually does not begin until trees are at least 10-15 years old.

The fungus generally enters through wounds. The infection usually starts on the lower branches and spreads upwards as spores of the fungus are dispersed by rain-splash.  Needles on infected branches first turn purple and then brown and drop, leaving the infected branches bare. Cankered branches are usually covered with white resin.

Septoria Leaf Spot and Canker in Poplar

This is commonly seen in stressed trees. As infected wood begins to die it often appears orange and may weep a brown liquid. Once the canker has gone completely around a stem, the rest of the stem quickly dies.

Apple Scab

Scab may occur on leaves, fruit, stems and green twigs but infections of the leaves and fruit are most common and obvious.

Infection on leaves first appears on the lower side. Young lesions are velvety brown to olive green with indistinct margins and later turn dark brown to black. Lesions on older leaves are typically raised, dark green to gray-brown with distinct margins, and cause cupping on the underside of the leaf. Leaves that are heavily infected with scab will curl, shrivel and fall from the tree. On fruits, small black spots develop at first and later become brown, corky and scabby. Heavily infected fruit becomes deformed and cracked when infected at an immature stage.

Cedar Hawthorn Rust

Cedar-hawthorn rust occurs on cedar, juniper, apple and crab apple, hawthorns etc. In order to survive, the fungus generally moves from one type of host to another (for instance, from juniper to hawthorn).

Small yellow spots first appear after infection in the spring. As the spots mature and enlarge, they take on an orange color and develop tiny black dots in the center of the lesion. By mid-summer, tubes are visible on the undersides of mature leaf lesions or within the lesions on fruit, petioles or twigs infections. With severe rust, hawthorn foliage may turn bright yellow and drop prematurely.

Needle Cast in Spruce

The common symptoms of needle cast include brownish purple discoloration and eventual death of older needles, while current-year needles show no symptoms. Needle cast diseases of spruce are treatable. Within a few years after treatment, an infested spruce tree can look beautiful again.

Fire Blight in Apple and Mountain Ash

Fire blight is one of the most destructive diseases of apple and pear trees. In Calgary, mountain ash, hawthorn and cotoneaster are also affected by blight. The bacteria can kill flowers, twigs and branches, and sometimes whole trees and shrubs. Mountain ash trees are badly affected by bacteria in Calgary and surrounding areas.

In spring, branch and trunk canker symptoms can appear as soon as trees begin active growth. The first sign is watery, light tan bacterial ooze that exudes from on branches, twigs, or trunks. The ooze turns dark after exposure to air, leaving streaks on branches or trunks.