Insects & Disease Problems
Insects and diseases can threaten tree health.
As soon as you notice any abnormality in your trees
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.
Basic elements that influence plant health include sufficient water and 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.
Correct diagnosis of plant health problems requires a careful examination of the situation.
- Accurately identify the plant. Because many insects
and diseases are plant-specific, this information can
quickly limit the number of suspected diseases and disorders.
- 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. Nonuniform 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,
- 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 becomes diseased
virtually overnight, a pathogen is probably not involved.
- 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.
- 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, lawn mowers, and rodents,
as well as a variety of other environmental and mechanical
factors. Large defects may indicate a potentially hazardous
- 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.
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
of infectious agents include fungi, viruses, and bacteria.
Noninfectious diseases, which account for 70 to 90 percent
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.
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 alsocarry 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,
or 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
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, mealy bugs, 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 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.
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.
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(c) 1998, 2004 International Society of Arboriculture.
UPDATED JULY 2005
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© 2007 International Society of Arboriculture.
UPDATED SEPTEMBER 2005