There are two main ways insects and diseases may harm trees: leaf attacking only, and systemic (causing the malfunction of the sap flow system). Of these, by far the more serious is the systemic.

An example of insects that damage the leaves, but not directly the rest of the tree, are the Japanese beetle and the Eastern Tent Catapillar. These eat leaf tissue often leaving a skeleton of veins. A disease which causes leaf loss, but does not get into the wood tissue is apple scab which often causes a tree to defoliate by mid-summer. These all cause damage by reducing the ability of the trees to produce sugar and starch for cell growth and defence. Also aesthetically, the trees can look alarmingly in danger of dying.

However, the trees will more often than not come back strong the following year. This kind of damage year after year can cause a gradual weakening of the tree, but leaf problems are considerably less harmful and easier to control than systemic problems.

Whereas leaf problems are almost immediatley noticable, systemic problems will go years without being noticed until a major section of the tree dies or even the whole tree dies. These are so difficult to notice because the destructive activity occurs out of site beneath the bark in the outer few rings of trunk growth.

I'm sure you've heard of borers, immature wormlike stages of some beetles and moths. They get under the bark and tunnel through vital sap conducting wood cells. After years this activity can become quite serious causing loss of major sections of the tree or even the whole tree.

Diseases which interfere with the function of the vascular system can be even more immediately serious. The most well known is Dutch Elm disease. Often the elm will completely die just weeks after infection. This desease is caused by a fungus that is spread by beetles that overwitner in diseased trees feeding on new growth. The chance that any given American elm or red elm will eventually get this disease is better than 50%. There is a preventiative treatment whereby a fungicide is injected directly into the sap flow system, but it must be done before any signs are showing. Vertilicillium wilt in maples and oak wilt in oak are similar diseases, but fortunately not as wide spread. All can be controlled by fungicide injection if you don't wait untill too late.

In general, insect and disease problems will occur less frequently and be less harmful when they do occur if trees are receiving the nutrition they need (see last article) and are not receiving improper pruning (see next article).

Rob Frothingham
Certified Arborist, Landscape Architect