THE GENUS DENDKOCTONUS. 127 



ACTrVITY OP OVEKWINTEEED BROODS. 



The adults begin to emerge from the bark and fly during May or 

 June. They settle on felled or standing mjured or on healthy trees 

 and enter the living bark to deposit eggs (see fig. 78), which soon 

 hatch into larvae. The broods of larvae feed m the mner bark and 

 destroy that portion of it intervenmg between the egg galleries, and 

 thus completely girdle and cause the death of the trees which were 

 healthy when attacked. The medium to larger matured trees are 

 the ones commonly selected by the beetle, and it is capable of killing 

 all such timber within a forest. 



In the Black Hills of South Dakota, at an altitude of about 5,000 

 feet, Mr. Webb found an adult flying on June 10. On July 30 adults 

 were excavating galleries and depositing eggs, which hatched and 

 had developed into nearly fuUgrown larvse on October 14. It would 

 appear, therefore, that the adults of this species come out a httle 

 earlier than those of the eastern spruce beetle. 



ECONOMIC FEATUKES. 



This species, hke the eastern spruce beetle, attacks only the larger 

 or mature trees. It is evidently the most important enemy of the 

 Rocky Moimtain spruces, and from time to time has caused wide- 

 spread destruction. 



In October, 1905, the writer found evidence of its destructive 

 work (see figs. 79, 80) in the Pike National Forest, caused many 

 years ago. At the time the observations were made the indica- 

 tions were that the vast destruction of spruce in this reserve here- 

 tofore attributed to fire was primarily caused by this beetle. This 

 was particularly evident on the southern slopes of Pikes Peak, at 

 an altitude of about 10,000 feet, where nearly all of the timber had 

 been killed some fifty years ago. In the fragmentary patches of 

 living timber, old felled trunks of a primitive matured forest of 

 Engelmann spruce were found thickly covering the ground. On the 

 weather-beaten surface of these logs the characteristic markings of 

 the galleries of this beetle were so common as to leave httle doubt 

 that the trees had been killed during a destructive invasion by it — 

 indeed, very conclusive evidence of this was found in the presence 

 of dried resin in the egg-gallery grooves and on the siu'face of the 

 wood, which would not be found there if the trees had been attacked 

 after they were dying from other causes. 



This additional evidence, together with the known devastating 

 work of this class of insects, makes it clear to the writer that there 

 has been a most intimate interrelation of destructive barkbeetles 

 and forest fires in the denudation of the vast areas of once heavily 

 forested lands in the Rocky Mountain region, and that in very many 



