What’s bugging my trees?

by William Scott
Wednesday, April 30, 2008 10:24 AM MDT

City Forester

Insects and diseases can threaten tree health. 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 and light, and a proper balance of nutrients. Too much or too little of any of these environmental conditions may cause plant stress.

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, many times the stresses they experience in an urban landscape are more than they can handle and may make them more susceptible to insects and diseases.

Diagnosis

Correct diagnosis requires a careful examination of the situation. By following the following steps you can help yourself come to a successful conclusion.

1. Accurately identify the plant. Because many insects and diseases are plant-specific, this information quickly can 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.

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, or weather.

3. Carefully examine the landscape. The history of the property and adjacent land may reveal many problems. The number of species affected also may 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.

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, lawn mowers and rodents, as well as a variety of other environmental and mechanical factors. Large defects may indicate a potentially hazardous tree.

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.

7. Get help. A certified arborist or licensed tree care operation will have the education and experience to help you identify your specific problem and can provide proper recommendations for treatment and service, helping you ensure the health of your trees now and in the future.

Diseases

Three things are required for a disease to develop:

1. The presence of a pathogen (the disease-causing agent)

2. Plant susceptibility to that particular pathogen

3. An environment suitable for disease development

Diseases can be classified into two broad categories: those caused by infectious or living agents (disease) and those caused by noninfectious or nonliving agents (disorder).

Examples of infectious agents include fungi, viruses and bacteria. Noninfectious diseases, which account for 70-90 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.

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 also may 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 or boring. Insects from each group have characteristic patterns of damage that will help you determine the culprit and the proper treatment.

Always consult an arborist 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 often are 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 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, mealy bugs, thrips and leafhoppers.

Damage caused by these pests often is 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.

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 regulation.

Consult a certified arborist or licensed tree care operation if you have any doubt about the nature of the problem or proper treatment.

For specific treatment information, other questions regarding Casper’s trees, or to obtain a list of licensed arborists in Casper, please contact your city forester at 235-8309 or by email at wscott@cityofcasperwy.com.

Arborist certification information also can be found at www.isa-arbor.com.

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