V 
THE GENUS DENDROCTONUS. 21 
HABITS. 
HABITS OF PARENT ADULTS AND OF IMMATURE STAGES. 
All of the species of Dendroctonus will breed to a greater or less 
extent in the living and dying bark of stumps and logs, and in injured 
and weakened trees. Some of them show a preference for trees in 
weakened condition, while others show a preference for healthy trees. 
All of those studied, however, have demonstrated their ability to attack 
healthy trees and kill them whenever the individuals of a species occur 
in sufficient numbers to overcome the resistance of the tree. The habit of 
swarming, or of congregating in one locality and concentrating 
their attack on groups of trees within a forest, is one of the more 
striking features in the habits of these beetles. The part of a tree 
selected for the attack varies somewhat in the different subdivisions 
of the genus. The species that are more destructive to the life of a 
tree attack the middle to upper portion of the trunk, while those 
that are less destructive attack the trunk toward the base, or even 
at the roots. The beetles' power to resist the repelling effects of the 
resin that flows into the freshly excavated entrances and galleries 
in the living bark and to dispose of it by forming pitch tubes at the 
entrances is most remarkable. This alone demonstrates the ability 
of these insects to overcome the resistance exerted by a living, healthy 
tree. The manner of excavating the egg galleries and the directions 
followed in their extension are quite different among the several 
species and have a different effect on the tree. The almost transverse, 
very winding, and closely arranged galleries of species 1, 2, 4, 5, and 6 
serve to quickly girdle and kill the trees, while the straight, longitu- 
dinal course and parallel arrangement of those of species 9, 10, 11, 13, 
and 14 result in a much slower, but none the less certain, death of 
the tree. 
RELATION OF HABITS TO SUCCESSFUL CONTROL. 
The habits of the broods of larvae are of special importance in indi- 
cating methods of control. 
In subdivision A the larvae of species 1, 2, 5, 6, 7, and 8 excavate 
their larval mines through the middle layers of the inner bark, so 
that they are rarely exposed in the inner bark. Those of species 3 
and 4 are exposed, but in all of the species of subdivision A the 
transformations from the larvae to the pupae and adults are almost 
entirely in the outer corky bark, so that in order to destroy the broods 
of the species of this subdivision the simple removal of the bark is not 
sufficient; it must be burned or otherwise destroyed. 
In the species of subdivisions B, C, and D the larvae excavate their 
mines in the inner layers of bark and also transform to pupae and 
adults in the inner bark, so that when the bark is removed from the 
