Insects & Disease Problems
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Insects and diseases can threaten tree health. As soon as you notice
any abnormality in your tree's appearance, you should begin a careful
examination of the problem. By identifying the specific symptoms of
damage and understanding their causes, you may be able to diagnose
the problem and select an appropriate treatment.
Stress
Basic elements that influence plant health include sufficient water,
light, and a proper balance of nutrients. Too much or too little of any
of these environmental conditions may cause plant stress.
Environmental stress weakens plants and makes them more
susceptible to insect and disease attack.
Trees deal with environmental stresses, such as shading and
competition for water and nutrients in their native environment, by
adjusting their growth and development patterns to reflect the
availability of the resources. Although trees are adapted to living in
stressful conditions in nature, many times the stresses they
experience in the landscape are more than they can handle and may
make them more susceptible to insects and diseases.
Diagnosis
Correct diagnosis of plant health problems requires a careful
examination of the situation. See picture at the bottom of this page.
1. Accurately identify the plant. Because many insects and diseases
are plant-specific, this information can quickly limit the number of
suspected diseases and disorders.
2. Look for a pattern of abnormality. It may be helpful to compare the
affected plant with other plants on the site, especially those of the
same species. Differences in color or growth may present clues as to
the source of the problem. Non-uniform damage patterns may indicate
insects or diseases. Uniform damage over a large area (perhaps
several plant species) usually indicates disorders caused by such
factors as physical injury, poor drainage, or weather.
3. Carefully examine the landscape. The history of the property and
adjacent land may reveal many problems. The number of species
affected may also help distinguish between infectious pathogens that
are more plant-specific as compared to chemical or environmental
factors that affect many different species. Most living pathogens take
a relatively long time to spread throughout an area, so if a large
percentage of plants become diseased virtually overnight, a
pathogen is probably not involved.
4. Examine the roots. Note their color: brown or black roots may signal
problems. Brown roots often indicate dry soil conditions or the
presence of toxic chemicals. Black roots usually reflect overly wet soil
or the presence of root-rotting organisms.
5. Check the trunk and branches. Examine the trunk thoroughly for
wounds because they provide entrances for pathogens and wood-
rotting organisms. Wounds can be caused by weather, fire,
lawnmowers, and rodents, as well as a variety of other environmental
and mechanical factors. Large defects may indicate a potential hazard.
6. Note the position and appearance of affected leaves. Dead leaves
at the top of the tree are usually the result of environmental or
mechanical root stress. Twisted or curled leaves may indicate viral
infection, insect feeding, or exposure to herbicides. The size and
color of the foliage may tell a great deal about the plant’s condition.
Make note of these and any other abnormalities.
Diseases
Three things are required for a disease to develop:
• The presence of a pathogen (the disease-causing agent)
• Plant susceptibility to that particular pathogen
• An environment suitable for disease development.
Plants vary in susceptibility to pathogens. Many disease-prevention
programs focus on the use of pathogen-resistant plant varieties.
Even if the pathogen is present and a susceptible plant host is
available, the proper environmental conditions must be present over
the correct period of time for the pathogen to infect the plant.
Diseases can be classified into two broad categories: those caused by
infectious or living agents (diseases) and those caused by
noninfectious or nonliving agents (disorders).
Examples of infectious agents include fungi, viruses, and bacteria.
Noninfectious diseases, which account for 70-90% of all plant
problems in urban areas, can be caused by such factors as nutrient
deficiencies, temperature extremes, vandalism, pollutants, and
fluctuations in moisture. Noninfectious disorders often produce
symptoms similar to those caused by infectious diseases; therefore, it
is essential to distinguish between the two in order to give proper
treatment.
Insects
Some insects can cause injury and damage to trees and shrubs. By
defoliating trees or sucking their sap, insects can retard plant growth.
By boring into the trunk and branches, they interfere with sap flow
and weaken the tree structure. Insects may also carry some plant
diseases. In many cases, however, the insect problem is secondary to
problems brought on by a stress disorder or pathogen.
It is important to remember that most insects are beneficial rather
than destructive. They help with pollination or act as predators of
more harmful species. Therefore, killing all insects without regard to
their kind and function can actually be detrimental to tree health.
Insects may be divided into three categories according to their
method of feeding: chewing, sucking, and boring. Insects from each
group have characteristic patterns of damage that will help you
determine the culprit and the proper treatment. Always consult a tree
care expert if you have any doubt about the nature of the insect
problem or the proper treatment.
Chewing insects eat plant tissue such as leaves, flowers, buds, and
twigs. Indications of damage by these insects is often seen by uneven
or broken margins on the leaves, skeletonization of the leaves, and
leaf mining. Chewing insects can be beetle adults or larvae, moth
larvae (caterpillars), and many other groups of insects. The damage
they cause (leaf notching, leaf mining, leaf skeletonizing, etc.) will
help in identifying the pest insect.
Sucking insects insert their beak (proboscis) into the tissues of
leaves, twigs, branches, flowers, or fruit and then feed on the plant’s
juices. Some examples of sucking insects are aphids, mealybugs,
thrips, and leafhoppers. Damage caused by these pests is often
indicated by discoloration, drooping, wilting, leaf spots (stippling),
honeydew, or general lack of vigor in the affected plant.
Boring insects. All pests in this category spend time feeding
somewhere beneath the bark of a tree as larvae. Some borers kill
twigs and leaders when adults feed or when eggs hatch into larvae
that bore into the stem and develop into adults. Other borers, known
as bark beetles, mate at or near the bark surface, and adults lay eggs
in tunnels beneath the bark.
Treatment
The treatment method used for a particular insect or disease problem
will depend on the species involved, the extent of the problem, and a
variety of other factors specific to the situation and local regulations.
Always consult a professional if you have any doubt about the nature
of the problem or proper treatment.
For Additional Information
This brochure is one in a series published by the International Society
of Arboriculture as part of its Consumer Information Program.
