Emerald Ash borer has been found for the first time in Missouri this summer. Wayne County is the location. One of the problems is that a tree may be infected by the borer for 2 to 3 years before it is detected. It will spread to other trees before it is detected in the first tree.
The borer starts at the top of the tree and works down. It leaves a D shaped hole in the bole of the tree where it enters. But because it starts at the top their infestation is not noticed soon enough.
Traps are set up in the tops of trees colored purple and added chemicals to attract the bug. This trap will show if the bug has entered an area or not.
Here is an article by the Missouri Dept. of Conservation that explains a little about the Ash borer.
Emerald Ash borer comes from Asia and was infesting forests in Michigan in 2002.
Web site on the treatment of the ash borer.
Emerald ash borer national info site.
Wisconsin's page shows how the borer leaves a trail inside the wood.