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Some Diseases and Insects that Affect Trees

Urban Forestry Home

This information applies to both Town and private trees.

Residents are asked not to interfere with trees on Town property. To request an inspection, please contact the Urban Forestry Section at (905) 771-8800, Monday to Friday between 8:30 a.m. and 4:30 p.m. or fill out the Public Property Tree Reporting Form. Upon inspection, information will be left for the homeowner detailing the course of action by Town staff.

Some diseases and insects that affect trees, include:

Black Knot of Cherry
Eastern Tent Caterpillars
Anthracnose, Drop Leaf
General Disease of Leaves
Birch Leaf Miner
Leaf Sucking Insects
Asian Long-Horned Beetle
Linden Borer
Gypsy Moth
Emerald Ash Borer

Black Knot of Cherry
Trees most susceptable, include:
  • wild and cultivated varieties of Cherry and Plum.
Black Knot on a major limb of an infected Plum tree.Signs and symptoms, include:
  • Dark green to black swellings on branches and stems, which start as small swelling approximately 0.5 to 1 cm long.
  • Swellings range from 2-15 cm, sometimes encircling entire branch.
  • By end of second year, the swelling turns woody in nature.
Control methods:
  • Control is limited to removal of infected branches.
  • Try to cut infected branches several centimeters below the visible infection, while maintaining the form of the tree.
  • If there are cankers on the trunks and large branches, removal of the tree may be the best long-term option.
  • Planting another variety of tree not affected by this disease would be appropriate. Seek advice from nurseries and garden centres.
  • Burn or bury infected branches and twigs.
  • If you are unsure if this disease is affecting your tree, bring a sample of the infected twig to a local garden centre.
For more information about Black Knot of Cherry and Plum, visit the Ministry of Agriculture, Food and Rural Affairs Website.

Eastern Tent Caterpillars
Trees most susceptable, include:
  • Flowering Crabapple;
  • Cherry varieties; and 
  • Apple varieties.
Signs and symptoms: 

Winter - look for egg masses at the ends of branches.
  • bronze metallic sponge-like formations around twig approximately  2.5cm long and 1.5cm diameter.
Eastern Tent Caterpillars form webs in the forks of branches.Spring - look for webs or caterpillars.
  • Webs start to develop just after leaves flush in the spring.
  • Grayish-white webs form in the junction of the branches.
  • Caterpillars will eat new growth outside of the web, along the branches.
Control methods:
  • The tent caterpillar is not considered a threat to a healthy tree, as the tree will re-foliate (put on new leaves) after this stage of the insect's life cycle
  • In winter - prune out the egg mass and dispose with household garbage.
  • In springtime - remove webbing from the tree with a stick or by hand, seal in bag and dispose of in household garbage.
    - Physically remove the caterpillars using gloves and dispose of them.
    - Contact a garden centre for other control measures.
Anthracnose, Leaf Drop
Trees most susceptable, include:
  • Green and White Ash;
  • Oak; and
  • Maple.
Anthracnose of Maple.Signs and symptoms, include:
  • dead spots or blotches on leaves and fruit,
  • development of cankers and dieback or death of twigs/branches; and 
  • spores which infect leaves during flush of new growth in the springtime under the right climatic conditions (high temperature and humidity).
Control methods: 
  • Control depends heavily on weather conditions during leaf emergence in the spring.
  • Wet conditions and low air movement with cool temperature increases the continued risk of fungal growth.
  • Keep leaves raked to minimize re-infection. Dispose of leaves during yard waste collection.
  • Ensure the affected tree receives adequate water to assist with growth of new leaves during the growing season. 
General Disease of Leaves

What are they?
  • Blight, mildew, scabs and spots caused by fungus, spread by spores.
  • Extent of the damage and infection relies heavily on weather conditions and plant location.
Signs and symptoms: 
  • Leaves can have a dusty powder layer to definite blotches of dead tissue.
  • It can be hard to differentiate from insect damage in a lot of cases, as symptoms and signs may be fairly similar.
  • Both result in deformed leaf growth and possible early leaf drop.
Control methods: 
  • The degree and number of infections rely on weather conditions. They can be spread by rain, sprinklers and humid weather.
  • Improving air flow around and through the tree, providing adequate water for the roots and clean up and disposal of fallen leaves and twigs is the preferred method of control
  • Avoid watering the leaves of the plant if the water will 'sit' on the leaf surface for an extended period of time. 

Birch Leaf Miner
Trees most susceptable, include:

  • All Birch trees.
  • White, Grey and European varieties are most vulnerable.
Birch Leaf Miner damage.Signs and symptoms: 
  • Infested trees will have leaves with "blistered areas" that are translucent, before turning brown and drying out.
  • More unsightly than harmful, however, it weakens the tree and makes it more susceptible to other insects or diseases.
  • If you hold the blistered leaf to the light, you may see the small worm-like insect still between the leaf layers.
Control methods: 
  • Keep trees healthy by introducing a watering and fertilizing program.
  • Contact a garden centre for control measures.
Leaf Sucking Insects

Trees most susceptable, include:
  • Honey Locust varieties
What do Leaf Sucking Insects do?
  • Insects suck on the leaf surfaces causing spotting.
  • Severe infestations can lead to leaf drop of the entire tree. 
  • This is generally only a problem during a four to six week period each year.
Signs and symptoms: 
  • larger than aphids (approximately 3-10 mm);
  • yellowish-green colour;
  • grasshopper shaped;
  • very active, quick hopper or runner;
  • stippling or spotting of leaf;
  • leaves may drop off if damage is severe;
  • damage ranges from a sparse tree to a leafless tree; and 
  • damage is usually short lived but may be severe.

Note: Gently shaking the lower branches will cause insects to fly around, making them easier to see.

Control methods: 

  • Due to a short life cycle, you can let nature take its course.
  • Totally defoliated trees will re-foliate within a few weeks.
  • Try and keep adequate water available to the tree during infestation and recovery.
  • High pressure water can disrupt insects and may cause some to vacate or become injured.
Trees most susceptable, include:
  • Maple and Catalpa trees. However, aphids may be found on most plants or trees.
What do they do?
  • suck on leaf surfaces, reproduce prolifically and excrete a honeydew substance, which tends to drip below the affected tree
Signs and symptoms:
  • Aphids are small, pear-shaped insects up to 6 mm in length and are a wide variety of colours (bright orange, green, red).
  • Aphids feed in groups and are rather slow moving.
  • Leaves or objects underneath become coated in a sticky clear fluid.
  • Damage is evident throughout the growing season.
Control methods:
  • Aphids are not usually a major concern.
  • The worst problem is the honeydew affecting cars and driveways beneath the tree and the attraction of honey bees, wasps and hornets.
  • Thunderstorms or heavy downpours can help control populations.
  • High water pressure spray from a garden hose will help disperse the colonies and wash away the sticky residue, reducing the number of lingering wasps and bees.
  • Insecticide will work, but usually is not required.
  • Many natural predators (ladybugs) tend to keep populations in check.
Asian Long-Horned Beetle
  • not yet sighted locally;
  • concerns are that they may exist, but populations are not yet high enough to be detected;
  • very destructive to most hardwood trees of this area including: Ash, Maple, Elm, Poplar and Willow, Birch, Hackberry and Horse Chestnut; 
  • insect is responsible for the removal of thousands of trees in New York State and in the Chicago area.
Asian Long-Horned BeetleWhat does it look like?
  • body up to35 mm in length and 12 mm in width;
  • antennae are longer than the body with white markings on them; and 
  • has a shiny black body with white dots in lines across the body and up the antennae.
Signs and symptoms, include: 
  • oval to round chew wounds on the bark of branches or trunk where eggs are laid;
  • may have sap leaking from these areas, attracting wasps, honeybees or butterflies;
  • large exit holes (9 to 11 mm) in diameter anywhere on the tree where the adults have exited the tree; and  
  • buildup of sawdust under the tree may indicate a feeding site or egg laying site.
Control methods: 

Linden BorerLinden Borer

  • attacks healthy, vigorous trees as readily as those under stress;
  • capable of weakening the tree to a point it succumbs to the damage in established shade trees; and 
  • insects bore into the layers under the bark, usually just above ground level or up to the lowest branch union on the trunk.
Signs and symptoms: 
  • The tree will usually start to appear sparse compared to others on the street in the summer.
  • There may be a swollen, corky appearance of the trunk just above ground level with sawdust and exit holes present.
  • Tree may weaken and die over a few growing seasons, or if the insect population is high enough the tree could die within the same growing year.
Control methods:
  • Control of this insect, like many borers, is difficult as a lot of the damage is usually done by the time the tree starts to show any symptoms of decline.
  • Monitor, water and fertilize trees that are affected through the summer months.
  • If necessary, remove the severely damaged trees between July and May of the next year, when larva are inside the tree to prevent spread of the insect. Affected area of the trunk should be either chipped up, bark peeled off and galleries plugged, buried or burned.
Gypsy Moth
Gypsy MothThe Town has been monitoring the Gypsy Moth's levels since the last serious outbreak in the early 1990’s. The survey results from this past winter indicate that the Moth is once again making a comeback with some fairly high numbers in the Bayview Avenue and Stouffville Road area. It is hoped that due to the low temperatures expreienced in the late part of the winter, there may be some egg mortality, which starts at temperatures below minus 15 degrees Celsius. 

The Gypsy Moth eats the leaves of both deciduous (leafed) and coniferous (needled) trees. If there are repeated annual defoliations on deciduous trees it is possible the tree will die after a few years. If coniferous trees are defoliated the tree will die after the first defoliation. It is important to identify the Gypsy Moth in its various stages, particularly the egg stage.

Trees most susceptible are:
  • Oak;
  • Basswood;
  • Poplar;
  • Willow; and
  • Maple.
Signs and Symptoms

The four stages in the life cycle, include:

  • Egg - the Gypsy Moth passes the winter months in the egg stage. Light beige coloured patches 2-3 cm long can be found on branches, trunks, rocks and walls.
  • Caterpillar (Larvae) - The eggs hatch into caterpillars when the trees are starting to bud. As they get larger, the caterpillars feed on the leaves for about 7 weeks, usually only leaving the veins. In the later stages of its development, the caterpillars are charcoal grey in colour with an easily recognizable double row of five blue dots and six red dots on its back, but some variation is known. Initially, small caterpillars hang from threads and let wind carry them to other trees to feed. High populations can defoliate entire trees from mid to late May until mid-July
  • Pupa - the Moth is now reddish brown, 2-3 cm long and cocoon-like from mid July until the end of July.
  • Adult - adult moths begin flying in late July or August. Female moths are approximately 30 mm long and are white in colour with zigzag markings on their wings. Females cannot fly; they die about one day after laying their eggs. Males are smaller, brownish in colour, and survive for about one week.
Control Methods

Manual Methods:

  • Scrape off the egg masses in late fall and through the winter months. Soak them in a soapy water solution and dispose of them in the garbage.
  • In summer, when caterpillars are active, place a band of cloth material around the trunk that is folded over about 15cm (6") to provide shelter during the heat of the day. Each afternoon, remove the resting caterpillars from underneath the material and soak them in a container of soapy water prior to disposing in the garbage.
Other Methods:
  • For severe infestations in larger areas of trees, a control agent application may be needed.
Trees that suffer more than 50 per cent leaf loss from this pest for two successive years may have a moderate to high mortality rate. Trees are weakened from trying to produce new foliage, which takes stored energy reserves and makes them more susceptible to other insects or diseases.

For more information about the Gypsy Moth, visit the Natural Resources Canada Website.

Emerald Ash Borer
The Emerald Ash Borer (EAB) is an non-native, invasive insect of Asian origin that attacks and kills ash trees. While EAB poses no health risk to humans or pets, ash trees of all species and sizes (with the exception of Mountain Ash) are susceptible to attack. EAB is currently posing a major economic and environmental threat to urban and forested areas across southern Ontario. EAB infested trees were discovered on King's Cross Avenue in Richmond Hill in March 2011. Its prescence has also been confirmed across York Region in the municipalities of Markham, Vaughan, Aurora and Whitchurch-Stouffville.

Richmond Hill's EAB Management Strategy 

Emerald Ash BorerWhat Does the Emerald Ash Borer Look Like?
The adult beetle has a shiny emerald or coppery green-coloured body. The eyes are large, bronze or black, and kidney-shaped. The body is narrow, about 3 to 3.5 mm wide and about 7 to 8 mm long.

Signs and symptoms of an infested tree:
  • vertical splitting of the bark;
  • epicormic shoots or sprouting from the trunk;
  • thinning crowns or dieback;
  • small, 3-4mm D-shaped exit holes on the trunk; and 
  • large bark sections peeling or falling off.
Control methods:
  • Currently there are no proven control methods.
  • Restriction on movement of ash wood and nursery stock, and firewood of all species is being used to slow the spread of the insect.
  • Survey, research and awareness programs by the federal government (Canadian Food Inspection Agency) are ongoing.
If you suspect the presence of the Emerald Ash Borer on your personal property, please call the Canadian Food Inspection Agency at 1-866-463-6017. If you suspect a tree on Town-owned land is infected with this insect, please call 905-771-8800. For more information about the Emerald Ash Borer, please visit the CFIA Website.

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