Once the insect is identified, it can be determined if it is harmful to the tree, beneficial to the tree, or has no effect whatsoever.

Believe it or not, some insects and related critters are beneficial, because they control populations of harmful insects through predation or parasitism. It is in your best interest to keep them, so you want to avoid any treatments that take out the good bugs with the bad bugs.

If the insect is indeed harmful, how harmful is it, and is it worth treating? Most professional arborists operate on the philosophy of treating only when the environmental/economic risk from the insect has reached a certain threshold.

Many bugs are benign. For instance, there is a common misconception that carpenter ants cause decline in trees! Quite the contrary, the presence of carpenter ants indicates decayed wood is present - they only make their galleries in dead, decaying sections of the tree. The ants are warning you of a potentially hazardous situation. On the other hand, termites pose a threat to trees in the southern United States and other parts of the world. They are best treated by a professional.