What are they?

What are the exterior signs of the presence of borers?
crown damage from two-lined chestnut borer
                                                    Crown damage on oak from two-lined chestnut borer

What kind of interior damage do they cause? Why are borers a serious threat? What trees are susceptible? How should this problem be handled? Where can I get more information?

Johnson, Warren T., and Howard H. Lyon.  1991.  Insects that Feed on Trees and Shrubs. 2nd edition, revised.  Ithaca NY: Cornell University Press. For other information, advice and help on this topic, call offices of your State Urban Forestry Coordinator or University Extension service, or visit urban forestry web sites.