INSECT DAMAGE IN THE NATIONAL PARKS, 7 



does not attack the pine, Douglas fir, or balsam fir. It flies in the 

 period from June to August and attacks the bark of the main trunks 

 of the older or matured trees. Its habits are similar to those of the 

 mountain pine beetle, except that it flies earlier in the sprmg. When 

 the trees begin to die the needles fade to a pale green and fall before 

 they change to yellow or brown, but the bare twigs present a grayish- 

 brown appearance. The infested trees are easily located in the fall 

 and early spring by the fallen needles and the bare twigs of the tops. 

 This species occurs from Britisli Columbia to Mexico and at 

 times is very destructive to the Engelmann spruce forests. It can 

 be controlled by felling the infested trees and removing the bark 

 from the main trunks during the period beginning with the Ist of 

 October and ending by the middle to last of May. 



THE BLACK HILLS BEETLE. 



The Black Hills beetle is by far the most destructive insect enemy 

 of the pine of the central and southern Eocky Mountains and the 

 Black Hills of South Dakota. Its habits are similar to those of the 

 mountain pine beetle, and the same methods are adopted for its- 

 control. 



FAVORABLE CONDITIONS FOR THE BEETLES. 



There are certain conditions in the administered as well as in the 

 natural forests which contribute to the multiplication and destructive 

 work of these Dendroctonus beetles. One of the most favorable con- 

 ditions is an extensive forest of matured and old trees of pine or 

 s[)ruce, because in the beginning of an invasion such trees are more 

 often the first to be attacked and killed. Trees in such a forest 

 injured by lightning or storms often form centers of infestation in 

 which the beetles increase to sufficient numbers to enable them to 

 kill a few trees, and then the invasion is started, year after year 

 increasing in force until a large percentage or all of the old timber is 

 killed. The beetles then attack the young trees and often waste their 

 energies on saplings, in which the broods fail to develop. 



Drought. — It is a common belief that severe droughts weaken the 

 trees and thus contribute to favorable conditions for the attack of the 

 beetles. AVe have made a very thorough investigation of this sub- 

 ject and are led to conclude that exceptionally dry seasons are more 

 unfavorable for the development of the beetles than are moderately 

 humid ones, and that, therefore, droughts do not contribute to their 

 multiplication. 



Forest fres. — Forest fires contribute, to a limited extent, to the 

 multiplication of certain species which breed in fire-scorched trees, 

 but as a rule forest fires kill more beetles than they protect. 



