Water can evaporate from the spray droplets, concentrating the product and causing it to burn foliage. If a spray mix is to concentrated, it may harm the plant that it's applied to. On the other hand, if it is diluted, it won't control the targeted pest. Read and follow the label directions. Generally, pesticides must come in contact with the pest in order to be effective.
Some materials are systemic and are translocated through the plant.
Thorough treatments are essential for success. Spraying the tops of a tree with a contact pesticide will not control the aphids that live on the undersides of the leaves. To get control you must know what you're fighting, and how to fight it. Mites on Alberta spruce require treating the inside of the plant. Some scale insects require treating the truck and branches in addition to the foliage.