Common Tree Pests In Our Area
By far the best control for pests is a healthy tree.  Proper watering, fertilizing,
and pruning practices are essential in the overall health of your trees.  A
healthy tree has a much stronger natural immune system which it uses to fight
insects and disease.  With that being said here are a few things to look for.

Borers/Sapsuckers
I've grouped these together because they both leave about 1/8" to 1/4"
diameter holes in the trunk.  The
borer holes will be random and the
sapsucker or woodpecker holes will come in concentric rings up the trunk.  
Sapsuckers seldom do much damage but enough holes can inhibit the uptake
of moisture to the leaves of the tree.  The best method of control is to
physically scare the birds with things such as windsocks, scare tape, anything
shiny and has movement to startle the birds.
Borers, on the other hand, can be much more destructive.  The best control is
a borer spray/preventative applied at the proper time based on the type.  
Most sprays should be applied in April/May.

Bagworms/Webworms
Worms are grouped together because they all chew the leaves of the trees
sometimes to the point of defoliation.  
Bagworms seem to be the most
destructive.  The best time to control them is in the Spring as they just start to
form their bags.  There are several safe and effective sprays available.  By
late Summer it will be to late to spray because they will have sealed
themselves up in the bags and the sprays will not be able to penetrate them.
For
webworms first you must determine the extent of the damage.  Trimming
a branch or two may be the best way to treat.  Another method of treatment
can be to open up the webs and let the birds and wasps eat them.

Aphids/Scale
Aphids or scale rarely to enough damage to trees to warrant control except
for sometimes on young trees.  You will typically notice the sticky sappy
leaves followed by the black moldy stuff on the leaves and branches.  Again
several safe and effective sprays are available