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In southwestern Ontario, there are 5 main categories of insect pests that affect our trees:

       1. Defoliators
               a. Leaf Chewers
               b. Leaf Skeletonizers
               c. Leaf Miners

       2. Borers

       3. Sucking Insects

       4. Gall-makers

       5. Root Feeders


1. Defoliators

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

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


2. Borers

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


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:

- Aphids
- 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


Emerald Ash Borer damage
Fall Webworm damage
Japanese Beetle damage
Cedar Leafminer damage
Aphid damage
Hackberry gall damage
White Grub feeding on roots