As a general rule, trees really do NOT require regular spraying to remain healthy.  Shot holes and a little insect chewing on your trees is normal natural and healthy.  In nature there is a beautiful give and take that happens between the trees and the insect and even the fungal and bacterial communities.  When we spray to control a particular insect we cannot help but affect other non-target insects that may be beneficial.  So when we spray, we view it as a last resort to insure tree health and as part of a whole tree health program.  We make sure we know what we are spraying for and that the damage warrants treatment, we insure that the material and application is such that we have the minimum amount of collateral damage, and we time the spray so that it is the most effective and efficient. Certain fungal disease sprays are one of the last resort situations that requires regular treatments to save the life of trees. (ref: Spraying Abstract)