270 PROTECTION AGAINST INSECTS 



(c) Special silvicultural measures to prevent the creation 

 of conditions favorable to injurious insects. 



Since most species of insects prefer unhealthy, weakened 

 trees in which to breed, it follows that efforts should be 

 directed toward maintaining the forest crop in a healthy condi- 

 tion. If possible thinnings should be made early and re- 

 peated frequently. All unhealthy trees should be cut. In 

 mixed stands, where there is a difference in susceptibility 

 among the species in mixture to the attacks of a dangerous 

 insect, present or likely to appear, thinnings and other cut- 

 ting operations can be directed toward the removal of the 

 species most liable to injury.^ This is the principal and most 

 practicable measure for controlling the gipsy moth in the 

 forest.13 ^^'^ ^* 



The fertility of the site should be improved if possible of 

 accomplishment. This is especially desirable on dry warm 

 sites, for in such localities insects thrive. Large clearings, 

 particularly when they are to be restocked with pure conifer- 

 ous stands, should be avoided. To reduce insect injury small 

 cutover areas and mixed stands are desirable. 



In foreign countries intensive methods of collecting and 

 destro>dng the insects in the egg, larva, pupa or final stages 

 by means of hand picking, various trapping devices, such as 

 bands of viscous substances encircling the tree trunks, the 

 use of trenches, lights, etc., have been employed. Similar 

 methods are already used on a limited scale in the United 

 States principally in connection with nursery management. 



The value of the timber protected must be high to justify 

 such methods of control in the forest. 



The development of effective control and preventive meas- 

 ures requires detailed knowledge of the local habits of the 

 injurious insects which is best secured by a forest entomologist. 



The local forester can quickly learn the essential facts for 



