24 THE SCOLYTID BEETLES. 



LOW TEMPERATURE AND SNOW. 



While the severe cold at the high elevations in which most of the 

 western species prevail appears to have no detrimental effect on the 

 overwintering broods, we have a striking example of its effect on a 

 northern migration of a southern species, in the complete extermina- 

 tion of the southern pine beetle (No. 4) in the Virginias by the excep- 

 tionally cold winter of 1902-3. On the other hand, snows, when 

 sufficiently heavy to break down a large amount of timber, might 

 offer favorable conditions for the multiplication of some of the species 

 like the western pine beetle, the mountain pine beetle, and the 

 Douglas fir beetle. 



LIGHTNING. 



In certain sections of the country where a great many pine and 

 spruce trees are struck by lightning during the summer months these 

 trees furnish exceptionally favorable conditions for the perpetuation 

 and multiplication of the pine and spruce beetles. Although the 

 constant supply of such trees furnishes also favorable conditions for 

 the multiplication and perpetuation of the natural enemies of the 

 destructive beetles (insects and birds) , these enemies are frequently 

 not sufficiently numerous to serve as a natural check, and the living 

 timber is attacked by the broods of beetles which develop in the 

 lightning-struck trees. This is especially true in the Southern States, 

 where a pine tree struck by lightning attracts the beetles to the spot, 

 and they not only enter the injured tree but attack and kill a number 

 of those surrounding it. 



WINDSTORMS. 



Whenever a windstorm occurring during the period from June to 

 August is sufficiently severe to fell and break a large amount of pine 

 and spruce, favorable conditions may be presented for the multipli- 

 cation of certain of the destructive beetles, provided they are present 

 in the locality in sufficient numbers to infest the felled timber. This 

 has been demonstrated from time to time in Europe, where beetles 

 with much less aggressive habits than the Dendroctonus beetles have, 

 it is said, been thus enabled to multiply to sufficient numbers to 

 attack and kill the living timber and cause serious extension of 

 their depredations into the healthy forest. 



OTHER INFLUENCES AND CONDITIONS. 



FOREST FIRES. 



While some of the species find favorable conditions for their multi- 

 plication in fire-scorched trees, others, like the Black Hills beetle, 

 appear to prefer the uninjured trees. This is due, perhaps, to the 

 fact that if a fire be sufficiently severe to kill large pine trees, the bark 



