What do emerald ash borers do to ash trees?

Emerald ash borer is a wood boring insect that feeds on the xylem and phloem of ash trees when it is in its larval stage. By feeding on these tissues, the larvae cut off valuable water and nutrient supplies to parts of the tree and cause die back. As the infestation progresses more of the tree dies back and, eventually the tree dies. It is really only the larval stage that kills trees.

It is important to note that it takes up to five years for the tree to die completely, so early detection is difficult. The best course of action if you are concerned for your tree is to treat it preventatively. There are many products available to do so, some of which you can do yourself.