There are two common insect problems found on birch trees. The bronze birch borer and the birch leaf miner . Thinning or dying birch tree crowns, which are the upper branches, probably mean the trees are infested with bronze birch borer. You can save infested trees by detecting the problem early. European birch tends to be more susceptible to borers than the native paper birch.
River birch is mostly borer free.
Borers are most likely to attack trees that have been weakened from a hot and dry location or by a lack of fertilizer. Vigorous, healthy trees are less attractive to borers and are better able to survive a borer attack. The site where you grow birches affects their borer resistance. Birch do best on a north facing slope or where plants or man-made structures keep soil cool, shaded and moist. The trees weaken in areas where they must endure soil compaction from foot traffic or a lack of moisture and high temperature conditions.