Question
Most of our Oak tree is covered with a spider web like substance. It does not take any particular form but simple runs along the tree and branches. We have not seen anthing living in it but we have only seen it at the bottom. It appeared in just a few days and covers a good percent of the tree. We would really love to find out what it is and if it is harmful or not. This may or may not be of use but about 14 years ago the tree was struck with lightning and a second trunk of the tree split off and there is now a large hole in the side. Thanks for the help.

Answer
Hi there. I'm not sure but to repeat Jim Hylands answer; this may be your problem.
Sounds like bark lice.
Bark Lice are more properly called Psocids (pronounced "SOSS-sids"). They are also called tree cattle because of their habit of living as a group on the bark of hardwoods, particularly oaks, and have been reported locally on citrus trees.  Large numbers of adults and nymphs are occasionally observed on tree trunks often living underneath the fine silken webbing they construct.  These insects do not attack the tree, but feed on lichens, fungi, and dead animal and plant matter.
Usually, the first noticeable sign of the presence of this insect is the fine silken webbing. The bark lice secrete this as a protective covering on the bark.  This silken webbing has a silvery sheen and may cover parts of the tree trunk and large limbs.  When the webbing is pulled off, it is not uncommon to see hundreds of these insects.  This webbing is not found on the twigs or leaves, and looks different than webbing produced by destructive caterpillars.
Webbing bark lice are found  along the Gulf coast from Texas and along the Atlantic coast north to South Carolina.  They are distant relatives of the book lice, which are household pests that can consume wood and paper products.  These outside Bark lice are not a threat to either the inside environment or to  Yards.   Populations of these tree cattle rise and fall with the seasons - cooler weather kills off many in the winter, and as springtime temperatures rise, food sources increase, and more webbing appears.  The greatest populations are found in the fall before the onset of cooler weather.
No control measures are recommended for these insects.  If the property owner objects to the webbing of these insects, a strong blast of water from a hose will dislodge them from the tree.  But as scavengers, they perform a valuable function in consuming excess accumulations of lichens, dead bark, and other materials found on the outside of the tree.  On trees that are regularly sprayed with insecticides for other pests, psocids will rarely be seen.  If the homeowner does insist on removing these insects, a Pest Control Operator may be contracted to apply a legal insecticide.
Here is a web site that gives more information and pictures of bark lice.
http://www.ag.auburn.edu/enpl/bulletins/barklice/barklice.htm