Pest control should be initiated only after the pest has been identified accurately and its presence threatens either the aesthetic quality or the vitality of the plant. If a decision has been made to use a pesticide, timing of the application must coincide with a stage of the pest that is vulnerable to the application.
Many pests, including borers, armored scales and gall makers can be contacted with pesticidal sprays for only a short time during the growing season. For example, armored scales can be controlled best by attacking the newly hatched nymphs (called crawlers) are active or have recently settled on their host. Borer sprays must be applied either before egg laying begins (e.g., bronze birch borer and other flatheaded borers) or before egg hatch (e.g., dogwood borer and other clearwing moths borers). Pesticide applications at any other time during the life cycle of these pests will be ineffective and should not be implemented.
Many insects and mites complete only one life cycle (generation) each year. A single, well-timed, thoroughly covering spray with an effective pesticide should provide season-long control. Other pests, including aphids, mites, some scales and bark beetles complete two or more generations each year. These pests may require more than one spray during the growing season.
Most adult insects have wings and can fly. After spraying, new insects may fly in and reinfest a plant making it appear that the insecticide applied didn't perform well. For this reason, repeated applications are needed to protect some plants from incoming insects.
Although many insecticides/miticides are effective against a number of different kinds of pests, it is always important to choose a product that has been proven to provide excellent results against the pest you are trying to control. General purpose sprays and pesticides are not the best approach to pest control. Instead, consult the tables in this bulletin and choose a product that has been rigorously evaluated for its effectiveness against your target pest. Then, use it according to directions on the container label, using only the amount specified. Do not use adjuvants (i.e., spreaders and stickers) unless specified on the label.
Sprays should always be applied to dry foliage and bark when rain is not expected for several hours. However, as long as sprayed surfaces dry before rainfall occurs, reapplication is usually unnecessary. Sprayed plants should be monitored in five to seven days to determine treatment effects, especially if rain occurs soon after the application. If the treatment was not effective, and if the pest is still in a vulnerable stage, the application should be repeated. It is a good policy to spray when the temperature is between 50 and 90°F (10-32°C). Many pesticides are less effective below this range, and some products may cause plant damage above the upper limit.
Many insecticides/miticides may tend to lose their killing power over a period of time, once they have been opened. This process may be speeded up with improper handling and improper storage. Therefore, it is always best to buy only the amount of insecticide/miticide you expect to use in one season.
Pesticides can be used from one year to the next. Remaining product should be stored in a safe, dry place that does not experience freezing or extremely high temperatures. Refer to the label for specific instructions on long term storage.
Resistance is a general term which, in the broad sense, means pests that were previously killed by a pesticide have produced offspring that are no longer killed by it. To illustrate, let's suppose that an insecticide is applied and that it kills 95% of the insects in a population which contact it, but there are 5% of the insects which received the same dosage but survived the treatment. This 5% is considered resistant to the insecticide. They live to produce another generation, and this generation, having had resistant parents, passes on to its offspring the resistance factor. Most likely there will be a greater number of the new individuals carrying resistance to the insecticide compared to the first treated population. As repeated insecticide applications are made and more generations produced, it is only a matter of time before the majority of the insects in question will survive the insecticidal application. A possible explanation is that the insecticide has acted as a selecting agent, killing those members of a population which are susceptible to the chemical and leaving those which are resistant. Survivors breed and produce subsequent resistant generations. Resistance develops fastest in insects which have high rates of reproduction. This is another reason why pesticides should be used only when and where necessary to prevent damage to valuable landscape plants.
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Keith L. Smith, Associate Vice President for Ag. Adm. and Director, OSU Extension.
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