Question
some type of insect is cutting limbs off my hickory trees.  they seem to saw them off smoothly, like a tubing cutter cuts pipe.  what are they and how bad are their effects?  thank you

Answer
These are a beetle called TWIG GIRDLER, Oncideres cingulata.  Twig girdlers are typical longhorned beetles that are 1/2 to 5/8 inch long. The body is cylindrical and generally grayish brown with a broad, ashy-gray band across the middle of the wing covers.

The adult beetles girdle twigs and small branches causing the injured portions to break away or hang loosely on the tree. It is not uncommon to see the ground under infested trees almost covered with twigs that have been cut off. Girdling affects the beauty and aesthetic quality of ornamental plantings. The cut by the twig girdler is the only one made from the outside by a beetle and is seldom complete, leaving a small central cylinder (this leaves a central jagged area when the twig breaks). The girdling extends through the bark and well into the wood in a complete circle around the stem and leaves only a thin column of the center wood attached. Since the twigs are girdled while the leaves are present, the severed twigs retain the brown leaves for some time.
The branches are apparently girdled by the female so that proper conditions will be provided for the development of the larvae. Eggs are laid during or after the cutting process, but never before the beetle makes part of the cut. They are inserted singly beneath the bark or slightly into the wood, usually near a bud scar or adjacent to a side shoot.
For control, the severed twigs on the ground as well as those lodged in the trees should be gathered and destroyed during the fall, winter, and spring when the eggs and grubs are in the twigs.

Here is a web link that will give you more information on twig girdlers
http://www.ext.vt.edu/departments/entomology/factsheets/twiggird.html