In southwestern Ontario, there are 5 main categories of insect pests that affect our trees:
a. Leaf Chewers
b. Leaf Skeletonizers
c. Leaf Miners
- Sucking Insects
- Root Feeders
These pests feed on tree foliage – leaves of deciduous trees and needles of conifers. Because their effects are not systemic, the damage done by defoliators can usually be overcome by healthy, established trees. However, loss of foliage is still mild to moderately stressful on all trees, and if enough foliage is lost in one season, particularly in the new growth, it can spell disaster. The tree will have to use up significant amounts of reserve (stored) energy to make up for the loss of energy-producing foliage, and this loss of reserves can leave a tree vulnerable to many other stressors, both biotic and abiotic, especially if defoliation occurs several years in a row.
Defoliators are most commonly the larval (caterpillar or grub) forms of various moths, beetles, and sawflies. Because they are only feeding on foliage in the larval stage, their ability to damage trees is usually limited to a short time period in any growing season (2-6 weeks average). They are commonly broken down into 3 categories based on how they feed on foliage:
a. Leaf Chewers
The leaf chewing pests have the largest mouth-parts of defoliator larvae which allow them to eat large sections of leaf, sometimes devouring the entire leaf.
Common leaf chewers in southwestern Ontario include:
– Fall Webworm
– Gypsy Moth
Fall Webworm Damage
b. Leaf Skeletonizers
The leaf skeletonizer pests are able to eat the soft outer tissues of foliage while leaving a skeletal network of veins behind, producing a very distinct visual effect.
Common leaf skeletonizers in southwestern Ontario include:
– Oak leaf skeletonizer
– Japanese beetle
c. Leaf Miners
The leaf miner pests are very small, tunnelling unseen between the upper and lower surfaces of leaves, feeding upon the succulent interior tissues, again producing a distinct visual effect.
Common leaf miners in southwestern Ontario include:
– Birch leafminer
– Cedar leafminer
Cedar leafminer damage
The boring pests attack mainly the inner bark and wood, although all parts of the tree from roots to buds are affected. The main effects of boring insects are deformity and weakening of tissues, as well as serious damage to the transport tissues that allow trees to move water, nutrients, and sugars (cambium). If the transport tissues are sufficiently compromised, decline and death will occur quickly.
Common borers in southwestern Ontario include:
– Emerald Ash borer
– Bronze Birch borer
Emerald Ash borer damage
3. Sucking Insects
These pests have leaching mouthparts which they use to extract cell sap either from foliage or directly from transport tissue (cambium). Their effects are usually lack of vitality, wilting, or discolouration, but in extreme cases they are capable of causing serious stress, decline, and even death of plant tissues.
Common sucking insects in southwestern Ontario include:
– Oystershell scale
4. Gall Makers
These pests cause trees to produce distinct, abnormal growths which usually envelope the insect. Despite their conspicuous appearance, most gall makers affect the aesthetic value of trees as opposed to their actual health. Once the gall has formed around the insect, it is quite protected, making control or treatment very difficult.
Common gall makers in southwestern Ontario include:
– Ash flower gall
– Oak & Hackberry gall
5. Root Feeders
These pests live in the ground and feed on the succulent, non-woody portions of plant roots. They are seldom noticed and rarely cause the death of well-established trees. Seedlings or recently transplanted specimens with limited root systems are more susceptible to this type of injury.
Common root feeders in southwestern Ontario include:
– Black Vine weevil
– White Grubs
Tree Diseases and Disorders
Tree diseases may be defined as abnormal disruptions in the basic life processes of the plant, which may or may not be fatal. These disruptions are caused by parasitic agents (principally fungi but also bacterial or viral) or non-parasitic agents (nutrient imbalances, toxic chemicals, etc).
The presence of disease in trees is shown by the development of visible signs and symptoms. Signs are structures produced by parasitic agents (e.g. mushrooms, conks, fruiting bodies, etc). Symptoms are changes that occur as a result of injury (e.g. wilting leaves, discolouration, deformities, etc). Note that signs can only be produced by parasitic agents, while symptoms can be produced by both parasitic and non-parasitic agents.
The parasitic diseases of trees, of which most are caused by fungi, can be grouped into 2 main categories:
Foliar diseases attack the foliage of the plant and are rarely transported inside the plant to other tissues. Their effect is similar to the defoliator pests in that the damage is rarely serious and trees can usually recover.
Common foliar diseases in southwestern Ontario include:
– Powdery mildew
Systemic diseases are able to move within the plant, affecting not only the foliage, but also the cambium and roots. Because of their ability to attack the entire plant, systemic diseases are the most difficult to treat. Due to a lack of systemic fungicides that are approved for commercial use, pruning is often the only treatment available to help combat these diseases. Pruning of affected tissues can help slow the progression of disease but is often not enough to stop it.
The effects of systemic disease are often quite severe and can leave trees stressed and susceptible to attack by other organisms or lead to the outright death of the plant.
Common systemic diseases in southwestern Ontario include:
– White Pine blister rust
– Dutch Elm disease
Dutch Elm disease
In addition to pest and disease issues, trees can also be negatively affected by a host of other interacting biological and climatic factors. These include: air and water pollution, flooding, drought, soil compaction, grade changes, nutrient imbalances, adverse weather, temperature extremes, and mechanical damage, such as cutting of roots for construction projects.
When a tree is in decline it is rarely the result of one single issue. Quite often, a stressed tree has been assailed by many negative factors, both biotic and abiotic, that have left it with an energy deficiency from which it is too weak to recover. For example, a tree growing in poor, compacted soil with inadequate drainage will have difficulty overcoming defoliation by insect pests. In attempting to regrow its leafy crown, the tree will have to use up a significant amount of its energy reserves which will leave it vulnerable to attack by a countless number of fungal agents. The end result of all this stress is usually dieback of stems or branches. Trees in advanced stages of dieback rarely recover, typically succumbing to invasion by secondary pests and diseases.
Because of this spiraling effect of stress and decline, the best prescription for landscape trees is prevention, through efforts to maximize the health and vitality of the plant. A healthy tree is far less likely to be attacked by pests and diseases and if it is attacked, generous amounts of stored energy will help ensure the best chance of recovery.
Common non-parasitic diseases/conditions in southwestern Ontario include:
– Dessication (Winter Browning)
– Road Salt damage
– Dieback of Hardwoods
Road salt damage
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