Insects And Diseases

Identifying The Proper Treatment

Insects and diseases, or pests, can pose serious threats to the continued good health of trees. In most cases a pest attack will result in noticeable abnormalities in a tree’s appearance. Once irregularities begin to appear in a tree, it is imperative that a careful analysis be undertaken in order that the problem and its causes can be understood and a treatment, if required, implemented.

Analyzing A Problem

Before beginning an analysis, it is important to know beforehand what is normal for a particular species of tree. For example, cedar trees browning in the fall, called “senescence”, is normal and need not cause concern. Properly identifying a tree and knowing its growth characteristics should always be the starting point when analyzing any problem.

Once you have determined that a tree is behaving uncharacteristically, and have identified a pest, you must then look for the causes. Frequently you will discover that the pest is actually only the symptom of a larger problem. In fact, most insect and disease attacks in urban trees are the result of poor growing environments. Healthy trees are capable of fending off attacks from most pests. The problem is that many trees in the urban environment are not healthy and are therefore predisposed to pest attacks. The main cause of ill health in urban trees is stress. The most common types of stress in urban settings are:

  • Too much or too little water
  • Construction (particularly soil compaction and grade changes)
  • Nutrient deficiency
  • Mechanical injury (e.g. damage to bark from line trimmers)
  • Improper planting technique and transplant shock
  • Temperature extremes
  • Soil not suitable for species of plant
  • Salt injury
  • Plant provenance

For our purposes, it is important to realize that stress has the effect of weakening trees, thereby increasing their susceptibility to pest attacks. Therefore, before you treat an insect or disease problem, make sure that you are treating the real problem and not merely the symptom of a larger problem.


Diseases require three factors to develop:

  • Presence of a pathogen, or disease causing agent
  • A host plant that is susceptible to that pathogen
  • Environmental conditions that favour the establishment and development of a disease

Diseases can be classified into two categories: abiotic (non-living) and biotic (living). Abiotic diseases are usually caused by an excess or deficiency of moisture or nutrients, by temperature extremes, by mechanical injuries or by environmental pollution. Biotic diseases are caused by living organisms. Sometimes these are carried by insects: for example, Dutch elm disease is carried by the elm bark beetle. Other biotic diseases are caused by bacteria (e.g. fireblight), fungi (e.g. chestnut blight) or viruses (e.g. elm yellows).

Although abiotic diseases account for 70 – 90% of diseases in urban trees, the symptoms for both types of diseases are often similar. For the purposes of treatment it is therefore important to diagnose the problem properly. Proper diagnosis depends on knowledge of both symptoms and signs that can occur together and characterize the disease. Symptoms and signs must be differentiated: symptoms are the plant’s reactions to a disease, while signs are useful indicators of a disease type. For example, symptoms of root rot may include leaf discolouration and dieback in the crown. Signs of root rot may include mushrooms growing around the soil of a declining tree.

The following steps are helpful when attempting to correctly diagnose a tree disease:

  1. Accurately identify the tree. This is particularly important when diagnosing biotic diseases because many are plant specific.
  2. Look for patterns of abnormality. For example, if most trees along a road are displaying similar abnormal characteristics, deicing salts may be the culprit.
  3. Examine the land. Look at such factors as land contour (drainage), a history of the land, the number of species affected and the percentage of injured plants in an area.
  4. Examine the roots. In particular observe the colour of the roots. Brown, black or gray roots may indicate a disease problem.
  5. Check the trunk and branches. Wounds in the crown are entry points for disease.
  6. Note the position and appearance of affected leaves. Dead leaves in the top of the crown, for example, point to environmental or mechanical damage.
  7. Think about current and past management practices. Remember that sometimes a current problem is the result of something that happened to a tree much earlier.

Treatments for most diseases involve one or more of the following:

  1. Sanitation – remove infected branches or entire tree.
  2. Plant resistant species.
  3. Chemical treatments – application of fungicides.
  4. Remove disease carriers – some diseases, such as cedar- apple rust, require both hosts, juniper and apple trees, to survive. Removal of either host will halt the disease.
  5. Protect a potential host – maintain the health of a tree (prune, water, mulch).


In most cases insect problems are easier to diagnose than disease problems. This is because insects are usually present and visible, or because they leave distinct signs, normally visible damage. Once identified, you need to know the life history and habits of the insect in order to determine how and when it can be controlled.

Insects can be divided into three categories according to their method of feeding: chewing, sucking and boring. Chewing insects, which include beetles, wingless moths and caterpillars, eat plant tissue such as leaves, flowers, buds and twigs. Sucking insects extract the tree’s juices through leaves, twigs, branches, flowers, or fruit. Aphids, scales, and leafhoppers are included among sucking insects. Boring insects eat through a tree’s wood, forming tunnels in the process. Bronze birch borer and emerald ash borer are examples of this type.

In addition to causing direct damage (chewed or discoloured leaves, tunnels in wood, etc.) insects also cause indirect damage. First, insects frequently transmit disease. As mentioned previously, the elm bark beetle transmits Dutch elm disease. Second, most insect damage weakens a plant to some degree. This causes it to become susceptible to disease, secondary insect attacks or environmental stress. However, as mentioned earlier in this article, an insect attack that results in harm is very often a sign that a tree is already in a weakened state, most likely caused by a type of stress.

Once identified, information on the life history and habits of an insect can be gathered. This information can then be used to determine the best method of controlling an insect problem. Most insects can only be effectively controlled during one or two stages of their lifecycle. Scale insects, for example, are best controlled with insecticides when in the crawler stage. Habits of insects are also useful to know. As an example, gypsy moth caterpillars seek cool shade during hot afternoons. By tying a band of burlap around the trunk of a tree, you can create an area of shade where the caterpillars will shelter. The burlap can be removed and the caterpillars that have gathered can be destroyed.

There are a seemingly endless number of disease and insect problems that can harm trees under your care. If you are not sure how to diagnose a particular problem, contact us for a diagnosis.

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