Current Literature. 4 l 7 



careful investigations conducted in recent years by Dr. Hopkins 

 and his assistants. Together with Technical Series No. 17, Part 1, 

 in which the species treated of in the present report are character- 

 ized in detail, it forms one of the most thorough and complete 

 treatises on a single genus of American forest insects that has come 

 to our notice. 



The genus Dendroctonus includes 23 species of which one occurs 

 in middle and northern Europe and in western Siberia while the 

 others are distributed over the whole of North America wherever 

 coniferous trees occur, and among them are the most dangerous 

 enemies of pine and spruce forests. 



Their destructiveness is due in large measure to their habit of 

 swarming or concentrating their attacks upon groups of trees in 

 the forest, and their ability to attack and kill living trees wherever 

 their numbers are sufficient to overcome the resistance offered by 

 the trees. Contrary to opinions hitherto held, in the case of certain 

 species especially the Black Hills beetle (D. ponder osae Hopk.), 

 an actual preference for living trees has been demonstrated, and 

 the widespread destruction caused by this species is mainly due to 

 this habit. 



The relative destructiveness of the different species also de- 

 pends upon the part of the tree attacked and upon the character 

 of the larval mines. Species attacking the middle and upper parts 

 of the trunk are more destructive than those which affect the lower 

 part, and those species whose larvae excavate transverse winding 

 galleries girdle and kill the tree more rapidly, though not less 

 surely, than those whose galleries are straight and longitudinal. 

 An immense amount of timber is killed in this way, and many large 

 denuded areas in the Rocky Mountains supposed to have been 

 killed by fire were primarily caused by Dendroctonus. 



Emphasis is laid upon the necessity of knowing the peculiarities 

 in the life history of each species as a basis for the successful con- 

 trol of their ravages. The time for directing operations towards 

 the control of a particular outbreak, depends e. g. upon the time 

 of hibernation and that of emergence of adults in the spring, the 

 number of broods in the season, and these features not only differ 

 in the different species but vary in any given species in different 

 parts of its geographical range, the variations depending upon 

 climatic conditions, chiefly those of temperature. 



