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. 

 AU of those studied, however, have demonstrated their ability to attack 

 healthy trees and hUl 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 



