Most insects do not harm trees and shrubs. Many of them are beneficial. Indiscriminately applying insecticides can have adverse effects on these beneficial insect populations. While it is necessary at times to use insecticides to eliminate an insect pest from destroying a tree or shrub, it is only a tool, and should be used if there is no other alternative. It is critical to know that particular insect's life cycle and feeding technique in order to have the best chances of successfully treating the plant. Without that knowledge, an insecticide application may only kill non-target insects, and leave the actual pest unharmed. Along with the timing of the application, knowing how the pest feeds is critical for choosing the correct product.
Many times a landscape pest will be kept in check by other predatory insects, birds, or mammals. Ladybugs are one example of a beneficial insect in any home garden. Parasitic wasps are another. Some of these wasps will lay their eggs inside pests such as aphids. The wasp's larvae feed inside the aphid until it reaches maturity and emerges, leaving the dead aphid. Recognizing parasitized aphids on a plant is one way of knowing when not to use an insecticide, because that would also kill the beneficial wasp. These are just two of the many biological methods available for treating pests without the need for insecticides.
A typical I.P.M. program consists of two monthly visits during the growing season where I look for signs of insect pressure on susceptible plants. Most of the time, if a pest is found early, it can be eliminated without pesticides. If pesticides are needed, only what is needed to be effective is applied, minimizing the impact on beneficials, instead of spending hours spraying every plant, harming the environment, and wasting money.