Most insects and mites can be categorized into one of five main groups according to the way in which they injure trees:
- defoliators
- borers
- sucking insects and mites
- gall-makers
- root-feeders.
Defoliators cause the functional loss of leaves, in whole or in part, through destruction of the chlorophyll-bearing tissues. Defoliators may be further classified as: leaf chewers which consume all of the leaf tissues, leaf skeletonizers which eat the soft outer tissues but leave a skeletal network of veins, and leaf miners which feed upon the succulent interior tissues while tunnelling between the upper and lower surfaces of the leaves.
Borers work mainly in the inner bark and wood, but all parts of a tree, from the buds to the roots, are subject to attack. The main effects of borer activity are deformity of the parts affected and weakening of the supporting fibres.
Sucking insects and mites extract the cell sap, causing a general devitalization, wilting, distortion, or discolouration of the parts attacked.
Gall-makers cause plants to produce abnormal growths as a reaction to irritations or stimuli such as feeding, stinging, egg-laying, or injection of toxins. Galls usually have a characteristic shape and location on the host, but in spite of their often conspicuous appearance most affect the appearance rather than the health of trees.
Root-feeders destroy the tissues whereby the plant absorbs the nutrients necessary for its sustenance and growth. Seedlings and recently transplanted stock with limited root systems are most susceptible to this type of injury.
Tree diseases may be defined as abnormal physiological conditions, or disruptions in the normal life processes, which may, or may not, be fatal. The disruptions are caused by both parasitic, or living, and non-parasitic agents. Parasitic agents of trees are principally fungi but, to a lesser extent, also include bacteria and viruses. Non-parasitic agents include nutrient imbalance, toxic chemicals, adverse weather, and other stress-producing conditions such as mechanical injuries and changes in soil and water levels.
As with insects, parasitic diseases also may be grouped according to the part of the plant attacked, namely below ground in the root, above ground in woody tissues (stems or branches), and in the foliage (leaves or needles).
The presence of disease in trees is shown by the development of visible signs and symptoms. Signs are structures produced by parasitic agents. Examples include mushrooms, conks on the trunk, and felt-like layers and “shoestrings” under the bark. Non-parasitic agents do not produce disease signs. Symptoms are changes that occur in trees as a result of injury by both parasitic and non-parasitic agents.
Examples include:
- wilted leaves
- leaf spots
- blotches and curls
- foliar discolouration
- premature leaf fall
- wounds and cankers
- swellings and galls
- bleeding and gum formation and;
- deformed, dying, and dead parts.
It is important to be aware that not all tree injuries are caused by insects and diseases. Damage may also be caused by birds (sapsuckers) and rodents such as mice, rabbits, and squirrels. Bird and rodent damage are not discussed in this publication.
Return to Table of Contents (Common Pests of Trees in Ontario)
