Once it is determined that treatment is necessary to control an insect or pathogen the method of treatment must be chosen.  The chemical and application method depend on the insect/pathogen and the plant being treated.  there are contact chemicals and systemic chemicals.  Contact chemicals must be applied directly to the insect/pathogen to work.  Systemic chemicals are absorbed by the plant and trans-located throughout the plant poisoning or killing the insect/pathogen feeding on the plant.  There are also several application methods available spraying, soil drench, soil injection, and trunk injection.  There are advantages and dis advantages for each application method.  Spraying is the most common application method it is generally fast and relatively easy to do.  The problem with spraying is the potential for chemical drift, over spray, and death of non target insects.  The larger the plant being treated the higher the potential for drift.  Soil drench is an alternative to spraying.  A systemic chemical is poured or injected into the soil around the tree.  The advantage to this application method is that there are no chemicals being dispersed into the air so there is little chance of drift or over-spray also less chance of killing non target insects.  The drawback to soil drench is that in order for the treatment to work the tree must absorb the chemical and then trans-locate it throughout the tree for it to work.  Another drawback is the potential for the chemical to leach in to ground water.  Trunk injection is gaining popularity for several reasons.  the chemical is injected directly into the tree so there is no drift and no leaching.  The problem is that the injection damages the tree so there is a limit to how many times a tree can be injected.  There are several injection methods available mauget, and arbor jet are two brand name injection systems on the market.  It is important to know that there are many options available for the treatment of plants.  The best option for treatment depends on the situation.  It is highly recommended that when having you plants treated you ask about your treatment options and the possible advantages and disadvantages of the recommended treatments.