THE GENUS DENDROCTONUS. 29 



death of the trees. Therefore this interrelation between plant dis- 

 eases and insects must often be considered in our efforts to locate the 

 primary cause of a trouble. 



It has been conclusively determined, however, that when the beetles 

 occur in sufficient numbers, they are entirely independent of the aid of 

 other factors or the influence of their enemies, and that they attack and 

 Jcill perfectly healthy timber over extensive areas. 



SECONDARY ENEMIES OF THE TREES, AND DEPENDENTS, GUESTS, 

 ETC., OF THE DESTRUCTIVE BEETLES. 



As soon as the attack of one of the destructive beetles causes a 

 weakened or dying condition of a tree, such a tree becomes at once the 

 breeding place of many other species of barkbeetles and bark and 

 wood boring grubs which can not attack healthy trees. These sec- 

 ondary enemies of a tree are dependent on the more aggressive Den- 

 droctonus beetles or on other factors that may cause a similar weak- 

 ened or dying condition of the trees. Some of them render special 

 service to the destructive beetles by attacking the twigs, the branches, 

 and the unoccupied bark on the upper and lower portions of the trunk, 

 and thus aid in bringing about the certain death of the tree. There 

 are some insects which live in the galleries with the adult beetles, in 

 the relation of guests, others as scavengers, etc., so that it is always 

 important to distinguish which are the real primary enemies, which 

 are secondary, which are beneficial, and which are neutral in their 

 relation to an affected tree. 



GENERAL METHODS OF CONTROL. 



While the subject of control is treated under the special discussion 

 of each species, there are some general principles and features which 

 should be mentioned in this connection, especially such as relate to 

 the infestations of standing timber by the broods of the destructive 

 beetles. 



HABITS AND SEASONAL HISTORY AS SUGGESTING METHODS OF CONTROL. 



Any systematic plan or method for the destruction and control of 

 these beetles, in order to be least expensive and most successful, 

 must be based on a knowledge of the habits and seasonal history 

 and many other essential features relating to the particular species, 

 or group of allied species, involved in a given problem. The principal 

 facts of importance in this connection are as follows: (a) It is the 

 normal habit of all of the species to infest the bark on the main 

 trunk of the larger to medium sized trees; (b) in all species the devel- 

 oping broods of larvae live in the inner bark; (c) some of the species, 

 as in subdivision A, enter the outer dry bark to transform to adults, 



