EXPERIMENT STATION BULLETINS. 



471 



all six years of age or older. Trees younger than six years, according to the 

 results of other experiments,* cannot be safely treated in the usual way and 

 should not be so treated until satisfactory methods are determined. 



Mechanical injury to trees. The unavoidable injury to the bark of trees 

 caused by the operation of digging out the borers is serious, especially on 

 badly infested trees. In contrast to this, the crowns of trees where the borers 

 are controlled by this method are maintained in sound condition. 



•Blakeslee, E. B., U. S. Dept. Agr. Bui. 796, and Peterson, Alvah, New Jersey Agr. Exp. Sta. Circ. 126. 



Fig. 21. — A tree from which several borers were removed by the usual "worming" process. The 

 bark was severely injured. 



