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 
kill 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, 
